From 8cc8fe4a14383c7de91ab7bde7897c5d9656b892 Mon Sep 17 00:00:00 2001 From: Troy Liu
Date: 2016年8月17日 10:48:00 +0800 Subject: [PATCH 01/65] ADD: java-inner-class-and-static-nested-class.md --- ...ava-inner-class-and-static-nested-class.md | 44 +++++++++++++++++++ 1 file changed, 44 insertions(+) create mode 100644 contents/java-inner-class-and-static-nested-class.md diff --git a/contents/java-inner-class-and-static-nested-class.md b/contents/java-inner-class-and-static-nested-class.md new file mode 100644 index 0000000..e6d08c2 --- /dev/null +++ b/contents/java-inner-class-and-static-nested-class.md @@ -0,0 +1,44 @@ +## Java内部类和嵌套静态类 + +### 问题 +Java 当中的内部类和静态嵌套类有什么主要区别? 在这两者中有什么设计或者实现么? + +### 回答 +嵌套类分为两类: 静态和非静态. 用`static`装饰的嵌套类叫做静态类, 非静态的嵌套类叫做内部类. + +静态嵌套类使用外围类名来访问: +```java +OuterClass.StaticNestedClass +``` +例如, 实例化一个静态嵌套类的对象就要使用这种语法: +```java +OuterClass.StaticNestedClass nestedObject = new OuterClass.StaticNestedClass(); +``` + +内部类对象的存在需要依靠一个外部类的对象. 看看下面的类: +```java +class OuterClass { + ... + class InnerClass { + ... + } +} +``` +内部类对象只有当外部类对象存在时才有效, 并且可以直接访问他的包裹对象(外部类对象)的方法以及成员. + +因此, 要实例化一个内部类对象, 必须先实例化外部类对象. 然后用这种语法来创建内部类对象: +```java +OuterClass.InnerClass innerObject = outerObject.new InnerClass(); +``` +参考: [Java Tutorial - Nested Classes](http://download.oracle.com/javase/tutorial/java/javaOO/nested.html) + +提醒一下, 还有一种不用外部类对象来创建内部类对象的方法: [inner class without an enclosing ](http://stackoverflow.com/questions/20468856/is-it-true-that-every-inner-class-requires-an-enclosing-instance) +```java +class A { + int t() { return 1; } + static A a = new A() { int t() { return 2; } }; +} +``` +在这里, `new A() { ... }`是一个定义在静态上下文的内部类对象, 并没有一个外围对象. + +stackoverflow链接: [Java inner class and static nested class](http://stackoverflow.com/questions/70324/java-inner-class-and-static-nested-class) From 63083bfc942599b665479d5260bf93bbe68d1eee Mon Sep 17 00:00:00 2001 From: yu Date: 2016年8月17日 15:21:06 +0800 Subject: [PATCH 02/65] owen1190 --- contents/comparing-java-enum-members-or-equals.md | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/contents/comparing-java-enum-members-or-equals.md b/contents/comparing-java-enum-members-or-equals.md index 867246c..a83be17 100644 --- a/contents/comparing-java-enum-members-or-equals.md +++ b/contents/comparing-java-enum-members-or-equals.md @@ -44,10 +44,10 @@ public final boolean equals(Object other) { 答案是肯定的,因为枚举有着严格的实例化控制,所以你可以用 == 去做比较符,这个用法,在官方文档中也有明确的说明。 >JLS 8.9 Enums ->An enum type has no instances other than those defined by its enum constants. ->It is a compile-time error to attempt to explicitly instantiate an enum type. The final clone method in Enum>ensures that enum constants can never be cloned, and the special treatment by the serialization mechanism ensures>that duplicate instances are never created as a result of deserialization. Reflective instantiation of enum types>is prohibited. Together, these four things ensure that no instances of an enum type exist beyond those defined by>the enum constants. ->Because there is only one instance of each enum constant, it is permissible to use the == operator in place of the>equals method when comparing two object references if it is known that at least one of them refers to an enum ?>constant. (The equals method in Enum is a final method that merely invokes super.equals on its argument and ?>returns the result, thus performing an identity comparison.) +一个枚举类型除了定义的那些枚举常量外没有其他实例了。 +试图明确地说明一种枚举类型是会导致编译期异常。在枚举中final clone方法确保枚举常量从不会被克隆,而且序列化机制会确保从不会因为反序列化而创造复制的实例。枚举类型的反射实例化也是被禁止的。总之,以上内容确保了除了定义的枚举常量之外,没有枚举类型实例。 +因为每个枚举常量只有一个实例,所以如果在比较两个参考值,至少有一个涉及到枚举常量时,允许使用"=="代替equals()。(equals()方法在枚举类中是一个final方法,在参数和返回结果时,很少调用父类的equals()方法,因此是一种恒等的比较。) #### 什么时候 == 和 equals 不一样? As a reminder, it needs to be said that generally, == is NOT a viable alternative to equals. When it is, however (such as with enum), there are two important differences to consider: 通常来说 == 不是一个 equals的一个备选方案,无论如何有2个重要的不同处需要考虑: @@ -73,8 +73,8 @@ if (Color.BLACK == Chiral.LEFT); // DOESN'T COMPILE!!! Incompatible types! Bloch specifically mentions that immutable classes that have proper control over their instances can guarantee to their clients that == is usable. enum is specifically mentioned to exemplify. 具体来说,那些提供恰当实例控制的不可变类能够保证 == 是可用的,枚举刚好符合这个条件。 -> Item 1: Consider static factory methods instead of constructors -> [...] it allows an immutable class to make the guarantee that no two equal instances exist: a.equals(b) if and only if a==b. If a class makes this guarantee, then its clients can use the == operator instead of the equals(Object) method, which may result in improved performance. Enum types provide this guarantee. +考虑静态工厂方法代替构造器 +它使得不可变的类可以确保不会存在两个相等的实例,即当且仅当a==b的时候才有a.equals(b)为true。如果类保证了这一点,它的客户端可以使用"=="操作符来代替equals(Object)方法,这样可以提升性能。枚举类型保证了这一点 总而言之,在枚举比较上使用 == , 因为: 1. 能正常工作 From 9e9aa6e687a75cc19b1505f3bc02f571229422dd Mon Sep 17 00:00:00 2001 From: AcceptedBoy <972192420@qq.com> Date: 2016年8月17日 17:34:41 +0800 Subject: [PATCH 03/65] add->whats-the-difference-between-component-repository-service-annotations-in.md --- ...onent-repository-service-annotations-in.md | 77 +++++++++++++++++++ 1 file changed, 77 insertions(+) create mode 100644 contents/whats-the-difference-between-component-repository-service-annotations-in.md diff --git a/contents/whats-the-difference-between-component-repository-service-annotations-in.md b/contents/whats-the-difference-between-component-repository-service-annotations-in.md new file mode 100644 index 0000000..be6945a --- /dev/null +++ b/contents/whats-the-difference-between-component-repository-service-annotations-in.md @@ -0,0 +1,77 @@ +## @Component, @Repository, @Service的区别 + +#### 问题 + +在spring集成的框架中,注解在类上的`@Component`,`@Repository`,`@Service`等注解能否被互换?即这些注解的区别是什么? + +#### 回答1 + +引用spring的官方文档中的一段描述: + +在Spring2.0之前的版本中,`@Repository`注解可以标记在任何的类上,用来表明该类是用来执行与数据库相关的操作(即dao对象) + +在Spring2.5版本中,引入了更多的Spring类注解:`@Component`,`@Service`,`@Controller`。`Component`是一个通用的Spring容器管理的单例bean组件。而`@Repository`, `@Service`, `@Controller`就是针对不同的使用场景所采取的特定功能化的注解组件。 + +因此,当你的一个类被`@Component`所注解,那么就意味着同样可以用`@Repository`, `@Service`, `@Controller`来替代它,同时这些注解会具备有更多的功能,而且功能各异。 + +最后,如果你不知道要在项目的业务层采用`@Service`还是`@Component`注解。那么,`@Service`是一个更好的选择。 + +就如上文所说的,`@Repository`早已被支持了在你的持久层作为一个标记可以去自动处理数据库操作产生的异常(译者注:因为原生的java操作数据库所产生的异常只用了几种,但是产生的原因却有很多种,这样对于数据库操作的报错排查造成了一定的影响;而Spring拓展了原生的持久层异常,针对不同的产生原因有了更多的异常进行描述。所以,在注解了`@Repository`的类上如果数据库操作中抛出了异常,就能对其进行处理,转而抛出的是翻译后的spring专属数据库异常)。 + +| 注解 | 含义 | +| ------------- |:-------------:| +| @Component | 最普通的组件,可以被注入到spring容器进行管理 | +| @Repository | 作用于持久层 | +| @Service | 作用于业务逻辑层 | +| @Controller | 作用于表现层(spring-mvc的注解) | + +#### 回答2 + +这几个注解几乎可以说是一样的:因为被这些注解修饰的类就会被Spring扫描到并注入到Spring的bean容器中。 + +这里,有两个注解是不能被其他注解所互换的: + +* `@Controller` 注解的bean会被spring-mvc框架所使用。 +* `@Repository` 会被作为持久层操作(数据库)的bean来使用 + +如果想使用自定义的组件注解,那么只要在你定义的新注解中加上`@Component`即可: + +```java +@Component +@Scope("prototype") +public @interface ScheduleJob {...} +``` + +这样,所有被`@ScheduleJob`注解的类就都可以注入到spring容器来进行管理。我们所需要做的,就是写一些新的代码来处理这个自定义注解(译者注:可以用反射的方法),进而执行我们想要执行的工作。 + +#### 回答3 + +`@Component`就是跟``一样,可以托管到Spring容器进行管理。 + +@Service, @Controller , @Repository = {@Component + 一些特定的功能}。这个就意味着这些注解在部分功能上是一样的。 + +当然,下面三个注解被用于为我们的应用进行分层: + +* `@Controller`注解类进行前端请求的处理,转发,重定向。包括调用Service层的方法 +* `@Service`注解类处理业务逻辑 +* `@Repository`注解类作为DAO对象(数据访问对象,Data Access Objects),这些类可以直接对数据库进行操作 + +有这些分层操作的话,代码之间就实现了松耦合,代码之间的调用也清晰明朗,便于项目的管理;假想一下,如果只用`@Controller`注解,那么所有的请求转发,业务处理,数据库操作代码都糅合在一个地方,那这样的代码该有多难拓展和维护。 + +#### 总结 + +* `@Component`, `@Service`, `@Controller`, `@Repository`是spring注解,注解后可以被spring框架所扫描并注入到spring容器来进行管理 +* `@Component`是通用注解,其他三个注解是这个注解的拓展,并且具有了特定的功能 +* `@Repository`注解在持久层中,具有将数据库操作抛出的原生异常翻译转化为spring的持久层异常的功能。 +* `@Controller`层是spring-mvc的注解,具有将请求进行转发,重定向的功能。 +* `@Service`层是业务逻辑层注解,这个注解只是标注该类处于业务逻辑层。 +* 用这些注解对应用进行分层之后,就能将请求处理,义务逻辑处理,数据库操作处理分离出来,为代码解耦,也方便了以后项目的维护和开发。 + +#### Stackoverflow链接: + +[http://stackoverflow.com/questions/6827752/whats-the-difference-between-component-repository-service-annotations-in](http://stackoverflow.com/questions/6827752/whats-the-difference-between-component-repository-service-annotations-in) + +#### 拓展 + +1. [Spring注解@Component、@Repository、@Service、@Controller区别](http://www.cnblogs.com/JAYIT/p/5593169.html) +2. [Spring注解@Autowired、@Resource区别](http://www.cnblogs.com/leiOOlei/p/3713779.html) \ No newline at end of file From 13fd9c69a9c9e5d71716c77d545265f470618014 Mon Sep 17 00:00:00 2001 From: giantray <51961070@qq.com> Date: 2016年8月17日 22:22:01 +0800 Subject: [PATCH 04/65] =?UTF-8?q?=E6=9B=B4=E6=96=B0=E5=B7=B2=E7=BF=BB?= =?UTF-8?q?=E8=AF=91=E9=97=AE=E9=A2=98?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- README.md | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/README.md b/README.md index 93bb18c..6688723 100644 --- a/README.md +++ b/README.md @@ -67,6 +67,7 @@ stackoverflow-Java-top-qa * [如何计算MD5值](/contents/how-can-i-generate-an-md5-hash.md) * [Java中软引用和弱引用的区别](/contents/what-is-the-difference-between-a-soft-reference-and-a-weak-reference-in-java.md) * [JSF, Servlet 和 JSP (三种技术)有什么区别](/contents/what-is-the-difference-between-jsf-servlet-and-jsp.md) +* [Java内部类和嵌套静态类](/contents/java-inner-class-and-static-nested-class.md) > 编程技巧 @@ -118,7 +119,6 @@ stackoverflow-Java-top-qa - [Why is executing Java code in comments with certain Unicode characters allowed?](http://stackoverflow.com/questions/30727515/why-is-executing-java-code-in-comments-with-certain-unicode-characters-allowed) - [Convert a String to an enum in Java](http://stackoverflow.com/questions/604424/convert-a-string-to-an-enum-in-java) - [Android SDK installation doesn't find JDK](http://stackoverflow.com/questions/4382178/android-sdk-installation-doesnt-find-jdk) -- [Java inner class and static nested class](http://stackoverflow.com/questions/70324/java-inner-class-and-static-nested-class) - ['Must Override a Superclass Method' Errors after importing a project into Eclipse](http://stackoverflow.com/questions/1678122/must-override-a-superclass-method-errors-after-importing-a-project-into-eclips) - [Fastest way to determine if an integer's square root is an integer](http://stackoverflow.com/questions/295579/fastest-way-to-determine-if-an-integers-square-root-is-an-integer) - [How to fix: Unsupported major.minor version 51.0 error?](http://stackoverflow.com/questions/10382929/how-to-fix-unsupported-major-minor-version-51-0-error) From 8fffae44b2b507d42c67510980c4e8c7be3cc980 Mon Sep 17 00:00:00 2001 From: giantray <51961070@qq.com> Date: 2016年8月17日 22:28:29 +0800 Subject: [PATCH 05/65] =?UTF-8?q?=E6=9B=B4=E6=96=B0=E5=B7=B2=E7=BF=BB?= =?UTF-8?q?=E8=AF=91=E9=97=AE=E9=A2=98?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- README.md | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/README.md b/README.md index 6688723..defedb3 100644 --- a/README.md +++ b/README.md @@ -68,6 +68,7 @@ stackoverflow-Java-top-qa * [Java中软引用和弱引用的区别](/contents/what-is-the-difference-between-a-soft-reference-and-a-weak-reference-in-java.md) * [JSF, Servlet 和 JSP (三种技术)有什么区别](/contents/what-is-the-difference-between-jsf-servlet-and-jsp.md) * [Java内部类和嵌套静态类](/contents/java-inner-class-and-static-nested-class.md) +* [@Component, @Repository, @Service的区别](/contents/whats-the-difference-between-component-repository-service-annotations-in.md) > 编程技巧 @@ -139,7 +140,6 @@ stackoverflow-Java-top-qa - [Hibernate hbm2ddl.auto possible values and what they do?](http://stackoverflow.com/questions/438146/hibernate-hbm2ddl-auto-possible-values-and-what-they-do) - [How can I pad an integers with zeros on the left?](http://stackoverflow.com/questions/473282/how-can-i-pad-an-integers-with-zeros-on-the-left) - [Sort ArrayList of custom Objects by property](http://stackoverflow.com/questions/2784514/sort-arraylist-of-custom-objects-by-property) -- [What's the difference between @Component, @Repository & @Service annotations in Spring?](http://stackoverflow.com/questions/6827752/whats-the-difference-between-component-repository-service-annotations-in) - [Efficiency of Java "Double Brace Initialization"?](http://stackoverflow.com/questions/924285/efficiency-of-java-double-brace-initialization) - [Why does this go into an infinite loop?](http://stackoverflow.com/questions/3831341/why-does-this-go-into-an-infinite-loop) - [decompiling DEX into Java sourcecode](http://stackoverflow.com/questions/1249973/decompiling-dex-into-java-sourcecode) From 85619235f92d2b2d14012c92c2e0ba01bb972d8d Mon Sep 17 00:00:00 2001 From: AcceptedBoy <972192420@qq.com> Date: 2016年8月17日 23:24:57 +0800 Subject: [PATCH 06/65] add->iterating-through-a-collection-avoiding-concurrentmodificationexception-when-re.md --- ...concurrentmodificationexception-when-re.md | 69 +++++++++++++++++++ 1 file changed, 69 insertions(+) create mode 100644 contents/iterating-through-a-collection-avoiding-concurrentmodificationexception-when-re.md diff --git a/contents/iterating-through-a-collection-avoiding-concurrentmodificationexception-when-re.md b/contents/iterating-through-a-collection-avoiding-concurrentmodificationexception-when-re.md new file mode 100644 index 0000000..47195c4 --- /dev/null +++ b/contents/iterating-through-a-collection-avoiding-concurrentmodificationexception-when-re.md @@ -0,0 +1,69 @@ +## 遍历集合时移除元素,怎样避免ConcurrentModificationException异常抛出 + +#### 问题: + +在遍历集合的过程中,不会总出现`ConcurrentModificationException`异常的抛出,但是在下面的代码块中: + +```java +public static void main(String[] args) { + Collection l = new ArrayList(); + + for (int i=0; i < 10; ++i) { + l.add(new Integer(4)); + l.add(new Integer(5)); + l.add(new Integer(6)); + } + + //遍历的过程中移除部分集合元素 + for (Integer i : l) { + if (i.intValue() == 5) { + l.remove(i); + } + } + + System.out.println(l); +} +``` + +运行之后,结果显而易见,总是会抛出异常: + +```java +Exception in thread "main" java.util.ConcurrentModificationException +``` + +所以,遍历集合时移除元素,怎样避免ConcurrentModificationException异常的产生?有什么好的解决办法? + +#### 回答: + +`Iterator.remove()`是线程安全的,所以你的代码可以这样写: + +```java +List list = new ArrayList(); + +for (Iterator iterator = list.iterator(); iterator.hasNext();) { + String string = iterator.next(); + if (string.isEmpty()) { + + // 从迭代器中移除集合元素,集合中相应的集合元素也会安全地被移除 + // 在这里,如果继续调用的是list.remove(string),那么仍会抛出异常 + iterator.remove(); + } +} +``` + +在遍历集合时修改集合的结构或内容的情况中,`Iterator.remove()`是唯一线程安全的方法。 + +#### 问题原因: + +fail-fast, 快速失败机制,是java集合类的一种错误检查机制。当有多个线程同时对集合进行遍历以及内容或者结构的修改时,就有可能产生fail-fast机制。这意味着,当它们发现容器在迭代的过程中被修改时,就会抛出一个ConcurrentModificationException异常。 + +迭代器的快速失败行为无法得到保证,它不能保证一定会出现该错误,但是快速失败操作会尽最大努力抛出ConcurrentModificationException异常,这个异常仅用于检测bug。这种迭代器并不是完备的处理机制,而只是作为并发问题的一个预警指示器。 + + +#### 拓展阅读: + +[fail-fast机制的原理解析](https://github.com/AcceptedBoy/backstage-vacation-plan/blob/master/chapter1/concurrency/fail-fast.md) + +#### StackOverFlow地址: + +[http://stackoverflow.com/questions/223918/iterating-through-a-collection-avoiding-concurrentmodificationexception-when-re](http://stackoverflow.com/questions/223918/iterating-through-a-collection-avoiding-concurrentmodificationexception-when-re) \ No newline at end of file From 5ebf82154ce017de5d4cfd114acb97a1ecc9d930 Mon Sep 17 00:00:00 2001 From: giantray <51961070@qq.com> Date: 2016年8月18日 08:57:56 +0800 Subject: [PATCH 07/65] =?UTF-8?q?=E6=9B=B4=E6=96=B0=E5=B7=B2=E7=BF=BB?= =?UTF-8?q?=E8=AF=91=E9=97=AE=E9=A2=98?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- README.md | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/README.md b/README.md index defedb3..807429a 100644 --- a/README.md +++ b/README.md @@ -91,6 +91,7 @@ stackoverflow-Java-top-qa * [如何在整数左填充0](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/how-can-i-pad-an-integers-with-zeros-on-the-left.md) * [在调用 instanceof 前需要进行null检查吗](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/is-null-check-needed-before-calling-instanceof.md) * [如何从文件里读取字符串](/contents/how-do-i-create-a-java-string-from-the-contents-of-a-file.md) +* [遍历集合时移除元素,怎样避免ConcurrentModificationException异常抛出](/contents/iterating-through-a-collection-avoiding-concurrentmodificationexception-when-reiterating-through-a-collection-avoiding-concurrentmodificationexception-when-re.md) > 网络 @@ -135,7 +136,6 @@ stackoverflow-Java-top-qa - [Can I add jars to maven 2 build classpath without installing them?](http://stackoverflow.com/questions/364114/can-i-add-jars-to-maven-2-build-classpath-without-installing-them) - [Update Eclipse with Android development tools v. 23](http://stackoverflow.com/questions/24437564/update-eclipse-with-android-development-tools-v-23) - [Setting multiple jars in java classpath](http://stackoverflow.com/questions/219585/setting-multiple-jars-in-java-classpath) -- [Iterating through a list, avoiding ConcurrentModificationException when removing in loop](http://stackoverflow.com/questions/223918/iterating-through-a-list-avoiding-concurrentmodificationexception-when-removing) - [What is the Java equivalent for LINQ?](http://stackoverflow.com/questions/1217228/what-is-the-java-equivalent-for-linq) - [Hibernate hbm2ddl.auto possible values and what they do?](http://stackoverflow.com/questions/438146/hibernate-hbm2ddl-auto-possible-values-and-what-they-do) - [How can I pad an integers with zeros on the left?](http://stackoverflow.com/questions/473282/how-can-i-pad-an-integers-with-zeros-on-the-left) From 62805417211533ebb9a27b306853264151f84800 Mon Sep 17 00:00:00 2001 From: AcceptedBoy <972192420@qq.com> Date: 2016年8月18日 10:57:34 +0800 Subject: [PATCH 08/65] add->how-do-you-assert-that-a-certain-exception-is-thrown-in-junit-4-tests.md --- ...in-exception-is-thrown-in-junit-4-tests.md | 63 +++++++++++++++++++ 1 file changed, 63 insertions(+) create mode 100644 contents/how-do-you-assert-that-a-certain-exception-is-thrown-in-junit-4-tests.md diff --git a/contents/how-do-you-assert-that-a-certain-exception-is-thrown-in-junit-4-tests.md b/contents/how-do-you-assert-that-a-certain-exception-is-thrown-in-junit-4-tests.md new file mode 100644 index 0000000..84ca09e --- /dev/null +++ b/contents/how-do-you-assert-that-a-certain-exception-is-thrown-in-junit-4-tests.md @@ -0,0 +1,63 @@ +## JUnit如何断言一个确定的异常抛出 + +#### 问题 + +在JUnit4单元测试中,我要怎样做才能测试出有特定的异常抛出?我能想到的就只有下面的方法: + +```java +@Test +public void testFooThrowsIndexOutOfBoundsException() { + boolean thrown = false; + + try { + foo.doStuff(); + } catch (IndexOutOfBoundsException e) { + thrown = true; + } + + assertTrue(thrown); +} +``` + +#### 回答1 + +在JUnit4后支持下面的写法: +```java +@Test(expected=IndexOutOfBoundsException.class) +public void testIndexOutOfBoundsException() { + ArrayList emptyList = new ArrayList(); + Object o = emptyList.get(0); +} +``` +(译者:在`@Test`注解内提供了`expected`属性,你可以用它来指定一个`Throwble`类型,如果方法调用中抛出了这个异常,那么这条测试用例就相当于通过了) + +#### 回答2 + +如果你使用的是JUnit4.7,你可以使用如下的期望异常规则来验证异常信息: + +```java +public class FooTest { + @Rule + public final ExpectedException exception = ExpectedException.none(); + + @Test + public void doStuffThrowsIndexOutOfBoundsException() { + Foo foo = new Foo(); + + exception.expect(IndexOutOfBoundsException.class); + foo.doStuff(); + } +} +``` + +这种方式比`@Test(expected=IndexOutOfBoundsException.class)`要更好,如果是在调用`foo.doStuff()`方法之前就已经抛出异常的话,测试结果就不是我们想要的了。 +(译者:同时,`ExpectedException`还能够验证异常信息,如`exception.expectMessage("there is an exception!");` + +#### 拓展阅读 + +1. [JUnit:使用ExpectedException进行异常测试](http://www.tuicool.com/articles/ANviIz) +2. [JUnit4 用法详解](http://www.blogjava.net/jnbzwm/archive/2010/12/15/340801.html) + +#### StackOverflow地址: + +[http://stackoverflow.com/questions/156503/how-do-you-assert-that-a-certain-exception-is-thrown-in-junit-4-tests](http://stackoverflow.com/questions/156503/how-do-you-assert-that-a-certain-exception-is-thrown-in-junit-4-tests) \ No newline at end of file From 01ca9246a07be21901ba15ab4ceb7f2bbedb5102 Mon Sep 17 00:00:00 2001 From: AcceptedBoy <972192420@qq.com> Date: 2016年8月18日 11:39:57 +0800 Subject: [PATCH 09/65] update->junit4 && add->create_generic_array --- ...in-exception-is-thrown-in-junit-4-tests.md | 2 +- .../how-to-create-a-generic-array-in-java.md | 76 +++++++++++++++++++ 2 files changed, 77 insertions(+), 1 deletion(-) create mode 100644 contents/how-to-create-a-generic-array-in-java.md diff --git a/contents/how-do-you-assert-that-a-certain-exception-is-thrown-in-junit-4-tests.md b/contents/how-do-you-assert-that-a-certain-exception-is-thrown-in-junit-4-tests.md index 84ca09e..6c5cbdf 100644 --- a/contents/how-do-you-assert-that-a-certain-exception-is-thrown-in-junit-4-tests.md +++ b/contents/how-do-you-assert-that-a-certain-exception-is-thrown-in-junit-4-tests.md @@ -1,4 +1,4 @@ -## JUnit如何断言一个确定的异常抛出 +## JUnit4如何断言确定异常的抛出 #### 问题 diff --git a/contents/how-to-create-a-generic-array-in-java.md b/contents/how-to-create-a-generic-array-in-java.md new file mode 100644 index 0000000..d1d7041 --- /dev/null +++ b/contents/how-to-create-a-generic-array-in-java.md @@ -0,0 +1,76 @@ +## 如何创建泛型java数组 + +#### 问题 + +数组是不能通过泛型创建的,因为我们不能创建不可具体化的类型的数组。如下面的代码: + +```java +public class GenSet { + private E a[]; + + public GenSet() { + a = new E[INITIAL_ARRAY_LENGTH]; //编译期就会报错:不能创建泛型数组 + } +} +``` + +#### 采纳答案 + +* 检查:强类型。`GenSet`明确知道数组中包含的类型是什么(例如通过构造器传入`Class`,当方法中传入类型不是`E`将抛出异常) + +```java +public class GenSet { + + private E[] a; + + public GenSet(Class c, int s) { + // 使用原生的反射方法,在运行时知道其数组对象类型 + @SuppressWarnings("unchecked") + final E[] a = (E[]) Array.newInstance(c, s); + this.a = a; + } + + E get(int i) { + return a[i]; + } + + //...如果传入参数不为E类型,那么强制添加进数组将会抛出异常 + void add(E e) {...} +} +``` + +* 未检查:弱类型。数组内对象不会有任何类型检查,而是作为Object类型传入。 + +在这种情况下,你可以采取如下写法: + +```java +public class GenSet { + + private Object[] a; + + public GenSet(int s) { + a = new Object[s]; + } + + E get(int i) { + @SuppressWarnings("unchecked") + final E e = (E) a[i]; + return e; + } +} +``` + +上述代码在编译期能够通过,但因为泛型擦除的缘故,在程序执行过程中,数组的类型有且仅有`Object`类型存在,这个时候如果我们强制转化为`E`类型的话,在运行时会有`ClassCastException`抛出。所以,要确定好泛型的上界,将上边的代码重写一下: + +```java +public class GenSet { // E has an upper bound of Foo + + private Foo[] a; // E 泛型在运行期会被擦除为Foo类型,所以这里使用Foo[] + + public GenSet(int s) { + a = new Foo[s]; + } + + //... +} +``` \ No newline at end of file From 6aaae682e3f0a46cbbf04ea8f7af2edef0b333b1 Mon Sep 17 00:00:00 2001 From: AcceptedBoy <972192420@qq.com> Date: 2016年8月18日 11:42:02 +0800 Subject: [PATCH 10/65] add_stackoverflow_address->create_generic_array --- contents/how-to-create-a-generic-array-in-java.md | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/contents/how-to-create-a-generic-array-in-java.md b/contents/how-to-create-a-generic-array-in-java.md index d1d7041..086f0d2 100644 --- a/contents/how-to-create-a-generic-array-in-java.md +++ b/contents/how-to-create-a-generic-array-in-java.md @@ -73,4 +73,8 @@ public class GenSet { // E has an upper bound of Foo //... } -``` \ No newline at end of file +``` + +#### StackOverflow地址: + +[http://stackoverflow.com/questions/529085/how-to-create-a-generic-array-in-java](http://stackoverflow.com/questions/529085/how-to-create-a-generic-array-in-java) \ No newline at end of file From 86bf2258cbff6c89ac2f05c904e885fb3e2d8d86 Mon Sep 17 00:00:00 2001 From: AcceptedBoy <972192420@qq.com> Date: 2016年8月18日 18:57:37 +0800 Subject: [PATCH 11/65] add->how-can-i-create-an-executable-jar-with-dependencies-using-maven.md --- ...table-jar-with-dependencies-using-maven.md | 99 +++++++++++++++++++ 1 file changed, 99 insertions(+) create mode 100644 contents/how-can-i-create-an-executable-jar-with-dependencies-using-maven.md diff --git a/contents/how-can-i-create-an-executable-jar-with-dependencies-using-maven.md b/contents/how-can-i-create-an-executable-jar-with-dependencies-using-maven.md new file mode 100644 index 0000000..dab0dcf --- /dev/null +++ b/contents/how-can-i-create-an-executable-jar-with-dependencies-using-maven.md @@ -0,0 +1,99 @@ +## 如何使用maven把项目及其依赖打包为可运行jar包 + +#### 问题 + +我想把java项目打包为可运行的分布式jar包。我该怎样做,才能把项目中maven所依赖的jar包导入到我的项目jar包中? + +#### 回答 + +在`pom.xml`文件中,加入如下的插件: + +```xml + + + + maven-assembly-plugin + + + + + fully.qualified.MainClass + + + + jar-with-dependencies + + + + + +``` + +之后,运行maven命令: + +> mvn clean compile assembly:single + +`clean`,`compile`,`assembly:single`任务将会依次被执行;`compile`任务必须写在`assembly:single`之前,否则打包后的jar包内将不会有你的编译代码。 + +(译注:执行完后,会在你的maven项目的target目录下,生成想要的jar包,而不再需要使用`mvn package`命令进行打包) + +通常情况下,上述maven命令执行后会自动绑定到项目的构建阶段,从而保证了以后在执行`mvn install`命令时的jar包也会被构建。 +(译注:下面是实际上完整的默认的`pom.xml`配置,只不过``可以被省略,若省略则按照下述默认的配置执行) + +```xml + + maven-assembly-plugin + + + + fully.qualified.MainClass + + + + jar-with-dependencies + + + + + make-assembly + package + + single + + + + +``` + +#### 拓展 + +怎样去运行打包后的可运行jar包? + +* 对上述配置中已经指定了`main`函数所在类的jar包,打开命令行窗口,输入命令: + +```java +java -jar jar包的路径/jar包的名字.jar +``` + +例如: + +```Auto +java -jar D:\my_java_project\maven_test.jar +``` + +* 若在pom.xml并没有指定`main`方法所在类,那么该jar的运行应采取如下命令: + +```java +java -cp jar包的路径/jar包的名字.jar main方法所在类的全限定名 +``` + +例如: + +```java +java -cp D:\my_java_project\maven_test.jar com.my.path.MainClass +``` + + +#### StackOverflow地址 + +[http://stackoverflow.com/questions/574594/how-can-i-create-an-executable-jar-with-dependencies-using-maven](http://stackoverflow.com/questions/574594/how-can-i-create-an-executable-jar-with-dependencies-using-maven) \ No newline at end of file From 46310586da02ee4ce8e924b70db6d0d23bb22caa Mon Sep 17 00:00:00 2001 From: AcceptedBoy <972192420@qq.com> Date: 2016年8月18日 19:23:17 +0800 Subject: [PATCH 12/65] add->intellij && update->difference of spring annotation --- ...rmanently-have-line-numbers-in-intellij.md | 29 +++++++++++++++++++ ...onent-repository-service-annotations-in.md | 6 ++-- 2 files changed, 32 insertions(+), 3 deletions(-) create mode 100644 contents/how-can-i-permanently-have-line-numbers-in-intellij.md diff --git a/contents/how-can-i-permanently-have-line-numbers-in-intellij.md b/contents/how-can-i-permanently-have-line-numbers-in-intellij.md new file mode 100644 index 0000000..8f7e071 --- /dev/null +++ b/contents/how-can-i-permanently-have-line-numbers-in-intellij.md @@ -0,0 +1,29 @@ +## 如何让IntelliJ编辑器永久性显示代码行数 + +#### 问题 + +如何让IntelliJ编辑器永久性显示代码行数 + +#### 回答 + +##### IntelliJ 14.0之后的版本 + +打开软件的菜单`File`->`Settings`->`Editor`->`General`->`Appearance`,在右侧的配置`Show line numbers`打勾: +![image1][1] + +##### IntelliJ 8.1.2 - 13.X的版本 + +打开软件的菜单`File`->`Settings`->`Editor`->`Appearance`,在右侧的配置`Show line numbers`打勾: +![images2][2] + +#### 拓展 + +[IntelliJ IDEA 使用教程](http://www.phperz.com/article/15/0923/159068.html) + +#### StackOverflow地址 + +http://stackoverflow.com/questions/13751/how-can-i-permanently-have-line-numbers-in-intellij + + + [1]: http://i.stack.imgur.com/9DL9q.png + [2]: http://i.stack.imgur.com/JVZlJ.jpg \ No newline at end of file diff --git a/contents/whats-the-difference-between-component-repository-service-annotations-in.md b/contents/whats-the-difference-between-component-repository-service-annotations-in.md index be6945a..0dd4196 100644 --- a/contents/whats-the-difference-between-component-repository-service-annotations-in.md +++ b/contents/whats-the-difference-between-component-repository-service-annotations-in.md @@ -2,13 +2,13 @@ #### 问题 -在spring集成的框架中,注解在类上的`@Component`,`@Repository`,`@Service`等注解能否被互换?即这些注解的区别是什么? +在spring集成的框架中,注解在类上的`@Component`,`@Repository`,`@Service`等注解能否被互换?或者说这些注解有什么区别? #### 回答1 引用spring的官方文档中的一段描述: -在Spring2.0之前的版本中,`@Repository`注解可以标记在任何的类上,用来表明该类是用来执行与数据库相关的操作(即dao对象) +在Spring2.0之前的版本中,`@Repository`注解可以标记在任何的类上,用来表明该类是用来执行与数据库相关的操作(即dao对象),并支持自动处理数据库操作产生的异常 在Spring2.5版本中,引入了更多的Spring类注解:`@Component`,`@Service`,`@Controller`。`Component`是一个通用的Spring容器管理的单例bean组件。而`@Repository`, `@Service`, `@Controller`就是针对不同的使用场景所采取的特定功能化的注解组件。 @@ -16,7 +16,7 @@ 最后,如果你不知道要在项目的业务层采用`@Service`还是`@Component`注解。那么,`@Service`是一个更好的选择。 -就如上文所说的,`@Repository`早已被支持了在你的持久层作为一个标记可以去自动处理数据库操作产生的异常(译者注:因为原生的java操作数据库所产生的异常只用了几种,但是产生的原因却有很多种,这样对于数据库操作的报错排查造成了一定的影响;而Spring拓展了原生的持久层异常,针对不同的产生原因有了更多的异常进行描述。所以,在注解了`@Repository`的类上如果数据库操作中抛出了异常,就能对其进行处理,转而抛出的是翻译后的spring专属数据库异常)。 +就如上文所说的,`@Repository`早已被支持了在你的持久层作为一个标记可以去自动处理数据库操作产生的异常(译者注:因为原生的java操作数据库所产生的异常只定义了几种,但是产生数据库异常的原因却有很多种,这样对于数据库操作的报错排查造成了一定的影响;而Spring拓展了原生的持久层异常,针对不同的产生原因有了更多的异常进行描述。所以,在注解了`@Repository`的类上如果数据库操作中抛出了异常,就能对其进行处理,转而抛出的是翻译后的spring专属数据库异常,方便我们对异常进行排查处理)。 | 注解 | 含义 | | ------------- |:-------------:| From 1bac5e9d3b32b0ddecffbc2d958f26099f32bd39 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?=E4=BF=8E=E9=87=91=E5=A5=BD?= Date: 2016年8月22日 18:23:37 +0800 Subject: [PATCH 13/65] android-sdk-installation-doesnt-find-jdk --- ...ndroid-sdk-installation-doesnt-find-jdk.md | 30 +++++++++++++++++++ 1 file changed, 30 insertions(+) create mode 100644 contents/android-sdk-installation-doesnt-find-jdk.md diff --git a/contents/android-sdk-installation-doesnt-find-jdk.md b/contents/android-sdk-installation-doesnt-find-jdk.md new file mode 100644 index 0000000..7b70797 --- /dev/null +++ b/contents/android-sdk-installation-doesnt-find-jdk.md @@ -0,0 +1,30 @@ +##安装Android SDK的时候找不到JDK + +###问题 +我在我的win7 64位的系统上安装Android SDK时,jdk-6u23-windows-x64.exe已经安装上了,但是Android SDK的安装程序却因为找不到已安装的JDK无法继续下去。 +这个问题出现过吗?有没有办法解决呢? + +![](http://ww2.sinaimg.cn/large/0060lm7Tgw1f72ny3m6oaj30ds0a0gmi.jpg) + +###回答1: +当你看到这个提示(找不到jdk)的时候按Back(返回),然后再点Next(下一步)。这个时候,它将会去寻找JDK + +###回答2: +实际安装: + + - 系统:windows 8.1 + - JDK文件: jdk-8u11-windows-x64.exe + - ADT文件:installer_r23.0.2-windows.exe +安装64位JDK,然后尝试第一个回答中的back-next的方法。然后尝试设置JAVA_HOME 根据错误信息的提示,但是,仍旧对我没有用处,然后,尝试如下解决办法: + +按照它说的做,设置JAVA_HOME在你的系统环境变量中,这个路径要使用正斜杠(/)而非反斜杠(\) + +**注意:** +当我把JAVA_HOME设置为C:\Program Files\Java\jdk1.6.0_31的时候还是不行,但是当我设置成C:/Program Files/Java/jdk1.6.0_31的时候就ok了。快把我逼疯了。 + +如果还不行,就把 %JAVA_HOME%加在环境变量Path的头部。 + +下面是我的环境变量的配置: + - JAVA_HOME=C:/Program Files/Java/jdk1.8.0_11 + - JRE_HOME=C:/Program Files/Java/jre8 + - Path=%JAVA_HOME%;C:... From bca7ea7d4d6d405a15a83cdfb4cd2d21051020fd Mon Sep 17 00:00:00 2001 From: giantray <51961070@qq.com> Date: 2016年8月22日 23:33:56 +0800 Subject: [PATCH 14/65] =?UTF-8?q?=E6=9B=B4=E6=96=B0=E5=B7=B2=E7=BF=BB?= =?UTF-8?q?=E8=AF=91=E9=97=AE=E9=A2=98?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- README.md | 9 ++++----- 1 file changed, 4 insertions(+), 5 deletions(-) diff --git a/README.md b/README.md index 807429a..443a313 100644 --- a/README.md +++ b/README.md @@ -69,6 +69,7 @@ stackoverflow-Java-top-qa * [JSF, Servlet 和 JSP (三种技术)有什么区别](/contents/what-is-the-difference-between-jsf-servlet-and-jsp.md) * [Java内部类和嵌套静态类](/contents/java-inner-class-and-static-nested-class.md) * [@Component, @Repository, @Service的区别](/contents/whats-the-difference-between-component-repository-service-annotations-in.md) +* [如何创建泛型java数组](/contents/how-to-create-a-generic-array-in-java.md) > 编程技巧 @@ -92,7 +93,8 @@ stackoverflow-Java-top-qa * [在调用 instanceof 前需要进行null检查吗](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/is-null-check-needed-before-calling-instanceof.md) * [如何从文件里读取字符串](/contents/how-do-i-create-a-java-string-from-the-contents-of-a-file.md) * [遍历集合时移除元素,怎样避免ConcurrentModificationException异常抛出](/contents/iterating-through-a-collection-avoiding-concurrentmodificationexception-when-reiterating-through-a-collection-avoiding-concurrentmodificationexception-when-re.md) - +* [如何让IntelliJ编辑器永久性显示代码行数](/contents/how-can-i-permanently-have-line-numbers-in-intellij.md) +* [如何使用maven把项目及其依赖打包为可运行jar包](/contents/how-can-i-create-an-executable-jar-with-dependencies-using-maven.md) > 网络 @@ -109,6 +111,7 @@ stackoverflow-Java-top-qa > 测试 * [如何测试 private 方法,变量或者内部类](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/how-to-test-a-class-that-has-private-methods-fields-or-inner-classes.md) +* [JUnit4如何断言确定异常的抛出](/contents/how-do-you-assert-that-a-certain-exception-is-thrown-in-junit-4-tests.md) > Android @@ -117,7 +120,6 @@ stackoverflow-Java-top-qa ### 待翻译问题链接(还剩x问题) - [Proper use cases for Android UserManager.isUserAGoat()?](http://stackoverflow.com/questions/13375357/proper-use-cases-for-android-usermanager-isuseragoat) -- [How can I create an executable jar with dependencies using Maven?](http://stackoverflow.com/questions/574594/how-can-i-create-an-executable-jar-with-dependencies-using-maven) - [Why is executing Java code in comments with certain Unicode characters allowed?](http://stackoverflow.com/questions/30727515/why-is-executing-java-code-in-comments-with-certain-unicode-characters-allowed) - [Convert a String to an enum in Java](http://stackoverflow.com/questions/604424/convert-a-string-to-an-enum-in-java) - [Android SDK installation doesn't find JDK](http://stackoverflow.com/questions/4382178/android-sdk-installation-doesnt-find-jdk) @@ -127,9 +129,7 @@ stackoverflow-Java-top-qa - [How to generate a random alpha-numeric string?](http://stackoverflow.com/questions/41107/how-to-generate-a-random-alpha-numeric-string) - [Comparing Java enum members: == or equals()?](http://stackoverflow.com/questions/1750435/comparing-java-enum-members-or-equals) - [Failed to load the JNI shared Library (JDK)](http://stackoverflow.com/questions/7352493/failed-to-load-the-jni-shared-library-jdk) -- [How do you assert that a certain exception is thrown in JUnit 4 tests?](http://stackoverflow.com/questions/156503/how-do-you-assert-that-a-certain-exception-is-thrown-in-junit-4-tests) - [What exactly is Apache Camel?](http://stackoverflow.com/questions/8845186/what-exactly-is-apache-camel) -- [How can I permanently have line numbers in IntelliJ?](http://stackoverflow.com/questions/13751/how-can-i-permanently-have-line-numbers-in-intellij) - [Access restriction on class due to restriction on required library rt.jar?](http://stackoverflow.com/questions/860187/access-restriction-on-class-due-to-restriction-on-required-library-rt-jar) - [How do I discover memory usage of my application in Android?](http://stackoverflow.com/questions/2298208/how-do-i-discover-memory-usage-of-my-application-in-android) - [Uncatchable ChuckNorrisException](http://stackoverflow.com/questions/13883166/uncatchable-chucknorrisexception) @@ -143,7 +143,6 @@ stackoverflow-Java-top-qa - [Efficiency of Java "Double Brace Initialization"?](http://stackoverflow.com/questions/924285/efficiency-of-java-double-brace-initialization) - [Why does this go into an infinite loop?](http://stackoverflow.com/questions/3831341/why-does-this-go-into-an-infinite-loop) - [decompiling DEX into Java sourcecode](http://stackoverflow.com/questions/1249973/decompiling-dex-into-java-sourcecode) -- [How to create a generic array in Java?](http://stackoverflow.com/questions/529085/how-to-create-a-generic-array-in-java) - [Why does Math.round(0.49999999999999994) return 1](http://stackoverflow.com/questions/9902968/why-does-math-round0-49999999999999994-return-1) - [Eclipse: Set maximum line length for auto formatting?](http://stackoverflow.com/questions/3697287/eclipse-set-maximum-line-length-for-auto-formatting) - [What is the equivalent of the C++ Pair in Java?](http://stackoverflow.com/questions/156275/what-is-the-equivalent-of-the-c-pairl-r-in-java) From f54a51d12bc98629f1f2469f1910608657cd38c9 Mon Sep 17 00:00:00 2001 From: giantray <51961070@qq.com> Date: 2016年8月22日 23:46:12 +0800 Subject: [PATCH 15/65] =?UTF-8?q?=E6=9B=B4=E6=96=B0=E5=B7=B2=E7=BF=BB?= =?UTF-8?q?=E8=AF=91=E9=97=AE=E9=A2=98?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- README.md | 2 +- contents/android-sdk-installation-doesnt-find-jdk.md | 3 +++ 2 files changed, 4 insertions(+), 1 deletion(-) diff --git a/README.md b/README.md index 443a313..e57f95e 100644 --- a/README.md +++ b/README.md @@ -117,12 +117,12 @@ stackoverflow-Java-top-qa * [在Android里面下载文件,并在ProgressDialog显示进度](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/download-a-file-with-android-and-showing-the-progress-in-a-progressdialog.md) * [如何获取Android设备唯一ID](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/is-there-a-unique-android-device-id.md) +* [安装Android SDK的时候找不到JDK](contents/android-sdk-installation-doesnt-find-jdk.md) ### 待翻译问题链接(还剩x问题) - [Proper use cases for Android UserManager.isUserAGoat()?](http://stackoverflow.com/questions/13375357/proper-use-cases-for-android-usermanager-isuseragoat) - [Why is executing Java code in comments with certain Unicode characters allowed?](http://stackoverflow.com/questions/30727515/why-is-executing-java-code-in-comments-with-certain-unicode-characters-allowed) - [Convert a String to an enum in Java](http://stackoverflow.com/questions/604424/convert-a-string-to-an-enum-in-java) -- [Android SDK installation doesn't find JDK](http://stackoverflow.com/questions/4382178/android-sdk-installation-doesnt-find-jdk) - ['Must Override a Superclass Method' Errors after importing a project into Eclipse](http://stackoverflow.com/questions/1678122/must-override-a-superclass-method-errors-after-importing-a-project-into-eclips) - [Fastest way to determine if an integer's square root is an integer](http://stackoverflow.com/questions/295579/fastest-way-to-determine-if-an-integers-square-root-is-an-integer) - [How to fix: Unsupported major.minor version 51.0 error?](http://stackoverflow.com/questions/10382929/how-to-fix-unsupported-major-minor-version-51-0-error) diff --git a/contents/android-sdk-installation-doesnt-find-jdk.md b/contents/android-sdk-installation-doesnt-find-jdk.md index 7b70797..117b823 100644 --- a/contents/android-sdk-installation-doesnt-find-jdk.md +++ b/contents/android-sdk-installation-doesnt-find-jdk.md @@ -28,3 +28,6 @@ - JAVA_HOME=C:/Program Files/Java/jdk1.8.0_11 - JRE_HOME=C:/Program Files/Java/jre8 - Path=%JAVA_HOME%;C:... + +stackoverflow链接: +http://stackoverflow.com/questions/4382178/android-sdk-installation-doesnt-find-jdk \ No newline at end of file From ca5550bd7fc8dc35aeca92c51d9d22a87792e50e Mon Sep 17 00:00:00 2001 From: giantray <51961070@qq.com> Date: 2016年8月24日 09:00:54 +0800 Subject: [PATCH 16/65] =?UTF-8?q?=E6=9B=B4=E6=96=B0=E5=B7=B2=E7=BF=BB?= =?UTF-8?q?=E8=AF=91=E9=97=AE=E9=A2=98?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- README.md | 1 - 1 file changed, 1 deletion(-) diff --git a/README.md b/README.md index e57f95e..624fcdb 100644 --- a/README.md +++ b/README.md @@ -127,7 +127,6 @@ stackoverflow-Java-top-qa - [Fastest way to determine if an integer's square root is an integer](http://stackoverflow.com/questions/295579/fastest-way-to-determine-if-an-integers-square-root-is-an-integer) - [How to fix: Unsupported major.minor version 51.0 error?](http://stackoverflow.com/questions/10382929/how-to-fix-unsupported-major-minor-version-51-0-error) - [How to generate a random alpha-numeric string?](http://stackoverflow.com/questions/41107/how-to-generate-a-random-alpha-numeric-string) -- [Comparing Java enum members: == or equals()?](http://stackoverflow.com/questions/1750435/comparing-java-enum-members-or-equals) - [Failed to load the JNI shared Library (JDK)](http://stackoverflow.com/questions/7352493/failed-to-load-the-jni-shared-library-jdk) - [What exactly is Apache Camel?](http://stackoverflow.com/questions/8845186/what-exactly-is-apache-camel) - [Access restriction on class due to restriction on required library rt.jar?](http://stackoverflow.com/questions/860187/access-restriction-on-class-due-to-restriction-on-required-library-rt-jar) From af7821226f4bab3ccf8f2b5e85ddbb1f327258c2 Mon Sep 17 00:00:00 2001 From: AcceptedBoy <972192420@qq.com> Date: 2016年8月28日 22:57:19 +0800 Subject: [PATCH 17/65] add->efficiency-of-java-double-brace-initialization.md --- ...ncy-of-java-double-brace-initialization.md | 202 ++++++++++++++++++ 1 file changed, 202 insertions(+) create mode 100644 contents/efficiency-of-java-double-brace-initialization.md diff --git a/contents/efficiency-of-java-double-brace-initialization.md b/contents/efficiency-of-java-double-brace-initialization.md new file mode 100644 index 0000000..6066b1c --- /dev/null +++ b/contents/efficiency-of-java-double-brace-initialization.md @@ -0,0 +1,202 @@ +## "Double Brace Initialization"的效率问题 + +#### 问题 + +`Double Brace Initialization`是java的隐藏特性,它有着如下诱人的语法: + +```java +Set flavors = new HashSet() {{ + add("vanilla"); + add("strawberry"); + add("chocolate"); + add("butter pecan"); +}}; +``` + +但是,这个特性听说不是很高效率,是否要限制一次性使用? + +#### 回答 + +当我使用匿名内部类时出现了如下的问题: + +```Auto +2009年05月27日 16:35 1,602 DemoApp21ドル.class +2009年05月27日 16:35 1,976 DemoApp210ドル.class +2009年05月27日 16:35 1,919 DemoApp211ドル.class +2009年05月27日 16:35 2,404 DemoApp212ドル.class +2009年05月27日 16:35 1,197 DemoApp213ドル.class + +/* snip */ + +2009年05月27日 16:35 1,953 DemoApp230ドル.class +2009年05月27日 16:35 1,910 DemoApp231ドル.class +2009年05月27日 16:35 2,007 DemoApp232ドル.class +2009年05月27日 16:35 926 DemoApp233ドル1ドル1ドル.class +2009年05月27日 16:35 4,104 DemoApp233ドル1ドル.class +2009年05月27日 16:35 2,849 DemoApp233ドル.class +2009年05月27日 16:35 926 DemoApp234ドル1ドル1ドル.class +2009年05月27日 16:35 4,234 DemoApp234ドル1ドル.class +2009年05月27日 16:35 2,849 DemoApp234ドル.class + +/* snip */ + +2009年05月27日 16:35 614 DemoApp240ドル.class +2009年05月27日 16:35 2,344 DemoApp25ドル.class +2009年05月27日 16:35 1,551 DemoApp26ドル.class +2009年05月27日 16:35 1,604 DemoApp27ドル.class +2009年05月27日 16:35 1,809 DemoApp28ドル.class +2009年05月27日 16:35 2,022 DemoApp29ドル.class +``` + +这是在我的一个简单应用中所产生的类信息。在这个应用中,使用了大量的匿名内部类,这些类会被单独地编译成`class`文件。 + +`Double Brace Initialization`是一个带有实例初始化块的匿名内部类。这就意味着每一个新的类的产生都会执行一次实例块,这样的目的通常是为了创建一个简单的对象。 + +java虚拟机在使用类之前需要去读取其classes信息,然后执行字节码校验等流程。所以为了保存这些`class`文件,所需要的磁盘空间会增大。 + +这个可以说是`Double Brace Initialization`的开销。所以尽量不要过分使用。 + +--- + +在java的介绍中,`Double Brace Initialization`有着如下的写法: + +```java +List list = new ArrayList() {{ + add("Hello"); + add("World!"); +}}; +``` + +看起来像是java的隐藏特性,其实它只是下面代码的一个重写: + +```java +List list = new ArrayList() { + + // 实例初始化块 + { + add("Hello"); + add("World!"); + } +}; +``` + +所以,它只是在匿名内部类中加上了实例初始化块而已。 + +--- + +Joshua Bloch希望以后的集合代码能够像这样简介: + +```java +List intList = [1, 2, 3, 4]; + +Set strSet = {"Apple", "Banana", "Cactus"}; + +Map truthMap = { "answer" : 42 }; +``` + +但目前还没有这样的语法。 + +--- + +实践 + +做一个简单的试验:创建1000个带着`"Hello"`和`"World!"`元素的`ArrayList` + +* 方法1:Double Brace Initialization + +``` +List l = new ArrayList() {{ + add("Hello"); + add("World!"); +}}; +``` + +* 方法2:初始化`ArrayList`并调用`add`方法 + +```java +List l = new ArrayList(); +l.add("Hello"); +l.add("World!"); +``` + +我修改了java的源文件使之能够为每种上述方法分别创建出1000个实例 + +* 方法1的测试 + +```java +class Test1 { + public static void main(String[] s) { + long st = System.currentTimeMillis(); + + List l0 = new ArrayList() {{ + add("Hello"); + add("World!"); + }}; + + List l1 = new ArrayList() {{ + add("Hello"); + add("World!"); + }}; + + /* snip */ + + List l999 = new ArrayList() {{ + add("Hello"); + add("World!"); + }}; + + System.out.println(System.currentTimeMillis() - st); + } +``` + +* 方法2的测试 + +```java +class Test2 { + public static void main(String[] s) { + long st = System.currentTimeMillis(); + + List l0 = new ArrayList(); + l0.add("Hello"); + l0.add("World!"); + + List l1 = new ArrayList(); + l1.add("Hello"); + l1.add("World!"); + + /* snip */ + + List l999 = new ArrayList(); + l999.add("Hello"); + l999.add("World!"); + + System.out.println(System.currentTimeMillis() - st); + } +} +``` + +然后得出了下面的测试时间: + +```Auto +Test1 Times (ms) Test2 Times (ms) +---------------- ---------------- + 187 0 + 203 0 + 203 0 + 188 0 + 188 0 + 187 0 + 203 0 + 188 0 + 188 0 + 203 0 +``` + +从上面我们可以看到,`Double Brace Initialization`平均时间花费了190ms左右。 +同时,另外一种方法平均只用了0ms。 + +所以,在第一个方法测试程序中,即`Double Brace Initialization`产生了1000个`class`文件。 + +## StackOverflow地址 + +[http://stackoverflow.com/questions/924285/efficiency-of-java-double-brace-initialization](http://stackoverflow.com/questions/924285/efficiency-of-java-double-brace-initialization) \ No newline at end of file From 0ae910dfdc3afd14fb146fffb146c4dca5e8af17 Mon Sep 17 00:00:00 2001 From: bboylin Date: Thu, 1 Sep 2016 01:28:30 +0800 Subject: [PATCH 18/65] finished translating Why does this go into an infinite loop? --- .../why-does-this-go-into-an-infinite-loop.md | 143 ++++++++++++++++++ 1 file changed, 143 insertions(+) create mode 100644 contents/why-does-this-go-into-an-infinite-loop.md diff --git a/contents/why-does-this-go-into-an-infinite-loop.md b/contents/why-does-this-go-into-an-infinite-loop.md new file mode 100644 index 0000000..7b8b6eb --- /dev/null +++ b/contents/why-does-this-go-into-an-infinite-loop.md @@ -0,0 +1,143 @@ +## 这段代码为什么陷入了死循环 + +### 问题 +我写了这样一段代码 + +```java +public class Tests { + public static void main(String[] args) throws Exception { + int x = 0; + while(x<3) { + x = x++; + System.out.println(x); + } + } +} +``` + +我们知道他应该只写x++或者x=x+1就行了,但是x=x++的情况下,x应该先赋值给自己,然后再增加1。为什么X的值一直是0呢? + +--更新 +
下面是字节码 + +```java +public class Tests extends java.lang.Object{ +public Tests(); + Code: + 0: aload_0 + 1: invokespecial #1; //Method java/lang/Object."":()V + 4: return + +public static void main(java.lang.String[]) throws java.lang.Exception; + Code: + 0: iconst_0 + 1: istore_1 + 2: iload_1 + 3: iconst_3 + 4: if_icmpge 22 + 7: iload_1 + 8: iinc 1, 1 + 11: istore_1 + 12: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream; + 15: iload_1 + 16: invokevirtual #3; //Method java/io/PrintStream.println:(I)V + 19: goto 2 + 22: return + +} +``` + + +### 回答 + +一开始我用C#代码来解释,因为C#允许用"ref"关键字来实现int参数的引用传递,所以我决定用我google到的MutableInt类写的标准的java代码来模拟C#中ref关键字作用.我不知道这对我的解答是不是有帮助,我并不是一个专业的java开发者,我知道还有很多更符合语言习惯的方法去解释这个问题。 +也许我写一个函数来模拟x++的作用会解释得更清楚 + +```java +public MutableInt postIncrement(MutableInt x) { + int valueBeforeIncrement = x.intValue(); + x.add(1); + return new MutableInt(valueBeforeIncrement); +} +``` + +对不对?将传递的参数值加1然后返回原值,这就是后增操作符的定义。 +现在让我们看看在你的代码中是怎么执行的。 + +```java +MutableInt x = new MutableInt(); +x = postIncrement(x); +``` + +`postIncrement(x)`函数做了什么?x增加1,对,然后返回x加1之前的值,并且把该返回值赋给x。所以X被赋值的顺序是0,1,0。如果我们重写上面的代码会理解得更清楚。 + +```java +MutableInt x = new MutableInt(); // x is 0. +MutableInt temp = postIncrement(x); // Now x is 1, and temp is 0. +x = temp; // Now x is 0 again. +``` + +你定势地认为,将左边的X换个名称为y,"你能看到x先自增1,然后赋值给了y",这一时也让我有点困惑。bi那个不是x被赋值给y,而是x(自增)之前的值被赋给y.事实上,把左边x变量名改成y结果也没什么不同。 + +```java +MutableInt x = new MutableInt(); // x is 0. +MutableInt y = new MutableInt(); // y is 0. +MutableInt temp = postIncrement(x); // Now x is 1, and temp is 0. +y = temp; // y is still 0. +``` + +可见,x=x++并没有改变x的值,只是让x依次被赋值为x0,x0+1,x0。 + +更新:你可能会怀疑在自增过程中x并没有被赋值为1,下面通过一个demo来解释确实x存在赋值为1的过程。 +demo中调用了x=x++;同时另外开了一个单独的线程不断输出x的值. + +```java +public class Main { + public static volatile int x = 0; + + public static void main(String[] args) { + LoopingThread t = new LoopingThread(); + System.out.println("Starting background thread..."); + t.start(); + + while (true) { + x = x++; + } + } +} + +class LoopingThread extends Thread { + public @Override void run() { + while (true) { + System.out.println(Main.x); + } + } +} +``` + +下面是程序的部分输出,0和1不规则的出现。 + +``` +Starting background thread... +0 +0 +1 +1 +0 +0 +0 +0 +0 +0 +0 +0 +0 +0 +1 +0 +1 +``` + + +stackoverflow链接: +[http://stackoverflow.com/questions/3831341/why-does-this-go-into-an-infinite-loop](http://stackoverflow.com/questions/3831341/why-does-this-go-into-an-infinite-loop) From 3105d5c888362c83c3e5ccbf388b318e5ceb22ed Mon Sep 17 00:00:00 2001 From: sunzhongsheng <1294477941@qq.com> Date: 2016年9月26日 19:39:09 +0800 Subject: [PATCH 19/65] no message --- ...-math-round0-49999999999999994-return-1.md | 96 +++++++++++++++++++ 1 file changed, 96 insertions(+) create mode 100644 contents/why-does-math-round0-49999999999999994-return-1.md diff --git a/contents/why-does-math-round0-49999999999999994-return-1.md b/contents/why-does-math-round0-49999999999999994-return-1.md new file mode 100644 index 0000000..6da947a --- /dev/null +++ b/contents/why-does-math-round0-49999999999999994-return-1.md @@ -0,0 +1,96 @@ +# 为什么数学函数Math.round(0.49999999999999994) 返回 1 + +tags:stackoverflow-java-top-qa + +--- + +###问题 +通过下面的程序你可以看出来,对于任意一个比0.5略小的都是舍去小数向下取整,只有0.5是例外. + +```java +for (int i = 10; i>= 0; i--) { + long l = Double.doubleToLongBits(i + 0.5); + double x; + do { + x = Double.longBitsToDouble(l); + System.out.println(x + " rounded is " + Math.round(x)); + l--; + } while (Math.round(x)> i); +} +``` + +输出为: + +``` +10.5 rounded is 11 +10.499999999999998 rounded is 10 +9.5 rounded is 10 +9.499999999999998 rounded is 9 +8.5 rounded is 9 +8.499999999999998 rounded is 8 +7.5 rounded is 8 +7.499999999999999 rounded is 7 +6.5 rounded is 7 +6.499999999999999 rounded is 6 +5.5 rounded is 6 +5.499999999999999 rounded is 5 +4.5 rounded is 5 +4.499999999999999 rounded is 4 +3.5 rounded is 4 +3.4999999999999996 rounded is 3 +2.5 rounded is 3 +2.4999999999999996 rounded is 2 +1.5 rounded is 2 +1.4999999999999998 rounded is 1 +0.5 rounded is 1 +0.49999999999999994 rounded is 1 +0.4999999999999999 rounded is 0 + +``` +*_译者注:请看输出的最后两行,0.49999999999999994的输出为1,而0.49999999999999999的输出为0* + +我使用的版本是 Java 6 update 31 + +### 回答 +**总结** + +在 Java 6(或者之前的版本),round(x)是用floor(x+0.5)实现的.1 这是一个规范上的bug,恰恰是在这种病理条件下.2Java 7 不再使用这个有问题的实现了. + +**问题** + +0.5+0.49999999999999994 在double的精度下的结果是1 +```java +static void print(double d) { + System.out.printf("%016x\n", Double.doubleToLongBits(d)); +} + +public static void main(String args[]) { + double a = 0.5; + double b = 0.49999999999999994; + + print(a); // 3fe0000000000000 + print(b); // 3fdfffffffffffff + print(a+b); // 3ff0000000000000 + print(1.0); // 3ff0000000000000 +} +``` +这是因为0.49999999999999994的指数比0.5的指数小,所以当它们两个相加时,0.49999999999999994的原数就会发生移位,然后最小精度单位(unit of least precision)/最后置单位(unit of last place)相应的变大了. + +**解决方案** + +自从Java 7以来,OpenJDK(举个栗子)实现如下4: + +```java +public static long round(double a) { + if (a != 0x1.fffffffffffffp-2) // greatest double value less than 0.5 + return (long)floor(a + 0.5d); + else + return 0; +} +``` +1. [http://docs.oracle.com/javase/6/docs/api/java/lang/Math.html#round%28double%29](http://docs.oracle.com/javase/6/docs/api/java/lang/Math.html#round%28double%29) +2. [http://bugs.java.com/bugdatabase/view_bug.do?bug_id=6430675](http://bugs.java.com/bugdatabase/view_bug.do?bug_id=6430675) (credits to @SimonNickerson for finding this) +3. [http://docs.oracle.com/javase/7/docs/api/java/lang/Math.html#round%28double%29](http://docs.oracle.com/javase/7/docs/api/java/lang/Math.html#round%28double%29) +4. [http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7u40-b43/java/lang/Math.java#Math.round%28double%29](http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7u40-b43/java/lang/Math.java#Math.round%28double%29) + +### stackoverflow原文链接:[http://stackoverflow.com/questions/9902968/why-does-math-round0-49999999999999994-return-1](http://stackoverflow.com/questions/9902968/why-does-math-round0-49999999999999994-return-1) \ No newline at end of file From f2be3e883e6ebe26dcb8dc9f97710a39cf231cec Mon Sep 17 00:00:00 2001 From: Lianghui2 <2915057607@qq.com> Date: 2016年9月27日 18:12:03 +0800 Subject: [PATCH 20/65] first edition --- .../how-to-create-a-generic-array2-in-java.md | 77 +++++++++++++++++++ 1 file changed, 77 insertions(+) create mode 100644 contents/how-to-create-a-generic-array2-in-java.md diff --git a/contents/how-to-create-a-generic-array2-in-java.md b/contents/how-to-create-a-generic-array2-in-java.md new file mode 100644 index 0000000..b46d670 --- /dev/null +++ b/contents/how-to-create-a-generic-array2-in-java.md @@ -0,0 +1,77 @@ +如何创建一个java泛型数组 +问题: +由于Java泛型的实现机制,你不能这样写代码: +pulic class GenSet{ + private E a[]; + public GetSet(){ + a=new E[INITIAL_ARRAY_LENGTH]; //error:generic array creation + } +} +在保证类型安全的情况下,我该如何实现创建一个Java泛型数组? +我在一个Java论坛上看到是这样解决的: +import java.lang.reflect.Array; + +class Stack { + public Stack(Class clazz, int capacity) { + array = (T[])Array.newInstance(clazz, capacity); + } + + private final T[] array; +} +但我不懂发生了什么。有人能帮我吗? + +回答: +在回答之前,我得问你一个问题,你的GetSet是"checked"还是"unchecked"? +什么意思呢? +Checked的话,是强类型。GetSet明确地知道包含了什么类型的对象。 +比如,当要传递不是E类型的实参时,它的构造器会被Class引数明确地调用,方法会抛出一个异常。参阅Collections.checkedCollection。 +在这种情况下,你应该这样写: +public class GenSet { + + private E[] a; + + public GenSet(Class c, int s) { + // Use Array native method to create array + // of a type only known at run time + @SuppressWarnings("unchecked") + final E[] a = (E[]) Array.newInstance(c, s); + this.a = a; + } + + E get(int i) { + return a[i]; + } +} +Unchecked的话:弱类型。实际上要传递任何对象的实参时 +是没有类型检查的。 +在这种情况下,你应当这样写: +public class GenSet { + + private Object[] a; + + public GenSet(int s) { + a = new Object[s]; + } + + E get(int i) { + @SuppressWarnings("unchecked") + final E e = (E) a[i]; + return e; + } +} +注意数组里的元素类型应当是可擦除的形参。 +public class GenSet { // E has an upper bound of Foo + + private Foo[] a; // E erases to Foo, so use Foo[] + + public GenSet(int s) { + a = new Foo[s]; + } + + ... +} +所有的这些结果来自Java一个有名,存心,不足的泛型:它通过使用 +erasure实现,所以"泛型"类在运行时创建是不知道它的实参类型的, +所以不能提供类型安全,除非某些明确的机制(比如类型检查)已经实现了。 +StackOverflow地址: +http://stackoverflow.com/questions/529085/how-to-create-a-generic-array-in-java \ No newline at end of file From 12c00b74efb1dab2315f3d09f8ae0ed1fe4034fb Mon Sep 17 00:00:00 2001 From: Lianghui2 <2915057607@qq.com> Date: 2016年9月27日 23:59:39 +0800 Subject: [PATCH 21/65] unfinished --- contents/what-is-the-equivalent-of-the-c++pair-in-java.md | 6 ++++++ 1 file changed, 6 insertions(+) create mode 100644 contents/what-is-the-equivalent-of-the-c++pair-in-java.md diff --git a/contents/what-is-the-equivalent-of-the-c++pair-in-java.md b/contents/what-is-the-equivalent-of-the-c++pair-in-java.md new file mode 100644 index 0000000..250f2ef --- /dev/null +++ b/contents/what-is-the-equivalent-of-the-c++pair-in-java.md @@ -0,0 +1,6 @@ +Java里什么是与C++的Pair相等的? +问题: +Java里没有Pair是不是一个好理由?那什么会和C++这个结构相等呢?似乎1.6版本提供了一些类似的(比如AbstractMap.SimpleEntry),但这看起来很费解。 + +回答: + From 9c52fc0deee0fbdc985e87a5c7169d81f5968504 Mon Sep 17 00:00:00 2001 From: Lianghui2 <2915057607@qq.com> Date: 2016年9月28日 08:25:26 +0800 Subject: [PATCH 22/65] first-commit --- contents/what-is-the-equivalent-of-the-c++pair-in-java.md | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/contents/what-is-the-equivalent-of-the-c++pair-in-java.md b/contents/what-is-the-equivalent-of-the-c++pair-in-java.md index 250f2ef..377cd9e 100644 --- a/contents/what-is-the-equivalent-of-the-c++pair-in-java.md +++ b/contents/what-is-the-equivalent-of-the-c++pair-in-java.md @@ -3,4 +3,10 @@ Java里什么是与C++的Pair相等的? Java里没有Pair是不是一个好理由?那什么会和C++这个结构相等呢?似乎1.6版本提供了一些类似的(比如AbstractMap.SimpleEntry),但这看起来很费解。 回答: +在comp.lang.java.help的一个论坛上,Hunter Gratzner对于java的Pair结构给出了一些论点。主要的论点是Pair类不能传达出两个值的关系的语义(你怎么能知道第一和第二代表什么?) +一个最好的校验是写一个非常简单的类,像Mike建议的,每一个应用上的pair类,Map.Entry都能很好的取代。 +总的来说,我建议是最好是一个Position(x,y)类,一个Range(begin,end)和一个Entry(key,value)类,而不是一个不能告诉我应该怎么做的广泛Pair(first,second)类。 + +stackoverflow的地址: +http://stackoverflow.com/questions/156275/what-is-the-equivalent-of-the-c-pairl-r-in-java From 3688c5e159282e29b826a1f148652d1c3b9708b1 Mon Sep 17 00:00:00 2001 From: Lianghui2 <2915057607@qq.com> Date: 2016年9月29日 19:46:20 +0800 Subject: [PATCH 23/65] =?UTF-8?q?=E7=AC=AC=E4=B8=89=E7=AF=87?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- .../Eclipse-set-maximun-line-length-for-auto-formatting.md | 4 ++++ 1 file changed, 4 insertions(+) create mode 100644 contents/Eclipse-set-maximun-line-length-for-auto-formatting.md diff --git a/contents/Eclipse-set-maximun-line-length-for-auto-formatting.md b/contents/Eclipse-set-maximun-line-length-for-auto-formatting.md new file mode 100644 index 0000000..b1f8579 --- /dev/null +++ b/contents/Eclipse-set-maximun-line-length-for-auto-formatting.md @@ -0,0 +1,4 @@ +为自动代码调整设置最大的行数? +问题:我正在学习Java。如果我在Eclipse Helios里使用ctrl+shift+f的组合键,它会自动调整我的代码。一定程度下,它会改变行数。我想增加行数的最大值。应该怎么做? + +回答,在偏好设置里,分别点击Java->Code Style->Fomatter->edit,在菜单栏Line Wrapping下会有行的宽度选择(Maximun line width).你将需要编辑你的代码轮廓。 \ No newline at end of file From 8d0b2d8ef1a2e8528d4a8f1fd1dd42f94720cab2 Mon Sep 17 00:00:00 2001 From: Lianghui2 <2915057607@qq.com> Date: 2016年9月30日 21:07:24 +0800 Subject: [PATCH 24/65] =?UTF-8?q?=E7=AC=AC=E5=9B=9B=E7=AF=87?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- .../failed-to-load-the-JNI-shared-library(JDK).md | 15 +++++++++++++++ 1 file changed, 15 insertions(+) create mode 100644 contents/failed-to-load-the-JNI-shared-library(JDK).md diff --git a/contents/failed-to-load-the-JNI-shared-library(JDK).md b/contents/failed-to-load-the-JNI-shared-library(JDK).md new file mode 100644 index 0000000..6ac55e2 --- /dev/null +++ b/contents/failed-to-load-the-JNI-shared-library(JDK).md @@ -0,0 +1,15 @@ +加载JNI共享库失败(JDK) +问题:当我试图打开Eclipse时,会弹出一个提示写着: +Failed to load the JNI shared library "C:/JDK/bin/client/jvm.dll"`. +然后,Eclipse会强制关闭。 +我做了以下几点: +·我检查那个路径有没有存在什么,真的有存在。 +·我的Eclipse和Java SE Development Kit都是64位的。我检查我的系统,它能处理64位。 +·我也在Google和Stack Overflow搜索解决方法,我找到唯一的方法是下载一个32位版本的JDK和Eclipse。 +下载32位版本是我没办法下的办法。但还有其他的解决方法吗? + +回答: +你需要一个三件套: +·64位的操作系统 +·64位的Java +·64位的Eclipse From d4cacdbca11876fa81af0002e910dee540672870 Mon Sep 17 00:00:00 2001 From: Lianghui2 <2915057607@qq.com> Date: 2016年9月30日 21:09:24 +0800 Subject: [PATCH 25/65] Update failed-to-load-the-JNI-shared-library(JDK).md --- contents/failed-to-load-the-JNI-shared-library(JDK).md | 1 + 1 file changed, 1 insertion(+) diff --git a/contents/failed-to-load-the-JNI-shared-library(JDK).md b/contents/failed-to-load-the-JNI-shared-library(JDK).md index 6ac55e2..bd489d9 100644 --- a/contents/failed-to-load-the-JNI-shared-library(JDK).md +++ b/contents/failed-to-load-the-JNI-shared-library(JDK).md @@ -13,3 +13,4 @@ Failed to load the JNI shared library "C:/JDK/bin/client/jvm.dll"`. ·64位的操作系统 ·64位的Java ·64位的Eclipse + From fa4a9731131c104e5673b86e355c0fef33570cdc Mon Sep 17 00:00:00 2001 From: Lianghui2 <2915057607@qq.com> Date: 2016年9月30日 21:10:35 +0800 Subject: [PATCH 26/65] Update failed-to-load-the-JNI-shared-library(JDK).md --- contents/failed-to-load-the-JNI-shared-library(JDK).md | 1 + 1 file changed, 1 insertion(+) diff --git a/contents/failed-to-load-the-JNI-shared-library(JDK).md b/contents/failed-to-load-the-JNI-shared-library(JDK).md index bd489d9..b60b2cd 100644 --- a/contents/failed-to-load-the-JNI-shared-library(JDK).md +++ b/contents/failed-to-load-the-JNI-shared-library(JDK).md @@ -14,3 +14,4 @@ Failed to load the JNI shared library "C:/JDK/bin/client/jvm.dll"`. ·64位的Java ·64位的Eclipse + From 5e57dac278aa777df983177aee4ab5a9ba446d81 Mon Sep 17 00:00:00 2001 From: Lianghui2 <2915057607@qq.com> Date: 2016年9月30日 21:47:50 +0800 Subject: [PATCH 27/65] =?UTF-8?q?=E4=B8=BA=E4=BB=80=E4=B9=88=E4=B8=8D?= =?UTF-8?q?=E8=A1=8C?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- "345円244円207円345円277円230円.md" | 0 1 file changed, 0 insertions(+), 0 deletions(-) create mode 100644 "345円244円207円345円277円230円.md" diff --git "a/345円244円207円345円277円230円.md" "b/345円244円207円345円277円230円.md" new file mode 100644 index 0000000..e69de29 From d72929b775f3dccf76ad3e48e8539e6d8c322fb1 Mon Sep 17 00:00:00 2001 From: Lianghui2 <2915057607@qq.com> Date: 2016年9月30日 21:55:47 +0800 Subject: [PATCH 28/65] =?UTF-8?q?Update=20=E5=A4=87=E5=BF=98.md?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- "345円244円207円345円277円230円.md" | 1 + 1 file changed, 1 insertion(+) diff --git "a/345円244円207円345円277円230円.md" "b/345円244円207円345円277円230円.md" index e69de29..c2e3070 100644 --- "a/345円244円207円345円277円230円.md" +++ "b/345円244円207円345円277円230円.md" @@ -0,0 +1 @@ +就是不行了 From cc14bc87d3125c4f1c8950286caa4232073188e7 Mon Sep 17 00:00:00 2001 From: Lianghui2 <2915057607@qq.com> Date: Sun, 2 Oct 2016 21:46:05 +0800 Subject: [PATCH 29/65] =?UTF-8?q?=E7=AC=AC=E4=BA=94=E7=AF=87?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- contents/lookup-enum-by-string-value (2).md | 13 +++++++++++++ 1 file changed, 13 insertions(+) create mode 100644 contents/lookup-enum-by-string-value (2).md diff --git a/contents/lookup-enum-by-string-value (2).md b/contents/lookup-enum-by-string-value (2).md new file mode 100644 index 0000000..34db1b2 --- /dev/null +++ b/contents/lookup-enum-by-string-value (2).md @@ -0,0 +1,13 @@ +如何将枚举转换成数组 +问题: +假设我有一个枚举类是这样的: +public enum Blah { + A, B, C, D +} +我想要将枚举类的值转化成一个数组,比如"A"怎么可能是Blah.A.怎么有可能做到这点?我需要Enum.valueOf()这个方法吗?如果是这样,我应该怎么用他? + +回答: +是的,Blah.valuOf("A")将会给你Blah.A。 +valueOf()和values()这些静态方法是在编译时创建的,而且不会出现在源代码里。不过他们确实有出现在Javadoc,比如Dialog.ModalityType有这两个方法。 +stackoverflow链接: +http://stackoverflow.com/questions/604424/lookup-enum-by-string-value From deb7717cc691f4f45218888328c6260fb514a3fc Mon Sep 17 00:00:00 2001 From: Lianghui2 <2915057607@qq.com> Date: Sun, 2 Oct 2016 22:05:25 +0800 Subject: [PATCH 30/65] =?UTF-8?q?=E7=AC=AC=E4=BA=94=E7=AF=87?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- contents/lookup-enum-by-string-value2.md | 13 +++++++++++++ 1 file changed, 13 insertions(+) create mode 100644 contents/lookup-enum-by-string-value2.md diff --git a/contents/lookup-enum-by-string-value2.md b/contents/lookup-enum-by-string-value2.md new file mode 100644 index 0000000..34db1b2 --- /dev/null +++ b/contents/lookup-enum-by-string-value2.md @@ -0,0 +1,13 @@ +如何将枚举转换成数组 +问题: +假设我有一个枚举类是这样的: +public enum Blah { + A, B, C, D +} +我想要将枚举类的值转化成一个数组,比如"A"怎么可能是Blah.A.怎么有可能做到这点?我需要Enum.valueOf()这个方法吗?如果是这样,我应该怎么用他? + +回答: +是的,Blah.valuOf("A")将会给你Blah.A。 +valueOf()和values()这些静态方法是在编译时创建的,而且不会出现在源代码里。不过他们确实有出现在Javadoc,比如Dialog.ModalityType有这两个方法。 +stackoverflow链接: +http://stackoverflow.com/questions/604424/lookup-enum-by-string-value From ca3ff7f1da208fde6433569629491247774272e3 Mon Sep 17 00:00:00 2001 From: Lianghui2 <2915057607@qq.com> Date: 2016年10月15日 23:30:40 +0800 Subject: [PATCH 31/65] woqu --- contents/lookup-enum-by-string-value (2).md | 13 ------------- 1 file changed, 13 deletions(-) delete mode 100644 contents/lookup-enum-by-string-value (2).md diff --git a/contents/lookup-enum-by-string-value (2).md b/contents/lookup-enum-by-string-value (2).md deleted file mode 100644 index 34db1b2..0000000 --- a/contents/lookup-enum-by-string-value (2).md +++ /dev/null @@ -1,13 +0,0 @@ -如何将枚举转换成数组 -问题: -假设我有一个枚举类是这样的: -public enum Blah { - A, B, C, D -} -我想要将枚举类的值转化成一个数组,比如"A"怎么可能是Blah.A.怎么有可能做到这点?我需要Enum.valueOf()这个方法吗?如果是这样,我应该怎么用他? - -回答: -是的,Blah.valuOf("A")将会给你Blah.A。 -valueOf()和values()这些静态方法是在编译时创建的,而且不会出现在源代码里。不过他们确实有出现在Javadoc,比如Dialog.ModalityType有这两个方法。 -stackoverflow链接: -http://stackoverflow.com/questions/604424/lookup-enum-by-string-value From b58ab999f79c28252274938b1e32b75d385e71e3 Mon Sep 17 00:00:00 2001 From: Akuma Date: 2016年10月20日 14:57:53 +0800 Subject: [PATCH 32/65] fix a typo --- ...examples-of-gof-design-patterns-in-javas-core-libraries.md | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/contents/examples-of-gof-design-patterns-in-javas-core-libraries.md b/contents/examples-of-gof-design-patterns-in-javas-core-libraries.md index 6ea5bdf..97ce66b 100644 --- a/contents/examples-of-gof-design-patterns-in-javas-core-libraries.md +++ b/contents/examples-of-gof-design-patterns-in-javas-core-libraries.md @@ -54,7 +54,7 @@ ### [装饰模式](http://en.wikipedia.org/wiki/Decorator_pattern) -- [java.io.InputStream](http://docs.oracle.com/javase/6/docs/api/java/io/InputStream.html),[OutputStream](http://docs.oracle.com/javase/6/docs/api/java/io/OutputStream.html),[Reader](http://docs.oracle.com/javase/6/docs/api/java/io/Reader.html) 和 [Writer](http://docs.oracle.com/javase/6/docs/api/java/io/Writer.html) 的所有资料都有一个使用 InputStream,OutputStream,Reader,Writer 的构造器 +- [java.io.InputStream](http://docs.oracle.com/javase/6/docs/api/java/io/InputStream.html), [OutputStream](http://docs.oracle.com/javase/6/docs/api/java/io/OutputStream.html), [Reader](http://docs.oracle.com/javase/6/docs/api/java/io/Reader.html) 和 [Writer](http://docs.oracle.com/javase/6/docs/api/java/io/Writer.html) 的所有子类都有一个使用 InputStream, OutputStream, Reader, Writer 的构造器 - [java.util.Collections](http://docs.oracle.com/javase/6/docs/api/java/util/Collections.html) 中的 [checkedXXX()](http://docs.oracle.com/javase/6/docs/api/java/util/Collections.html#checkedCollection%28java.util.Collection,%20java.lang.Class%29), [synchronizedXXX()](http://docs.oracle.com/javase/6/docs/api/java/util/Collections.html#synchronizedCollection%28java.util.Collection%29) 和 [unmodifiableXXX()](http://docs.oracle.com/javase/6/docs/api/java/util/Collections.html#unmodifiableCollection%28java.util.Collection%29) 方法 - [javax.servlet.http.HttpServletRequestWrapper](http://docs.oracle.com/javaee/6/api/javax/servlet/http/HttpServletRequestWrapper.html) 和 [HttpServletResponseWrapper](http://docs.oracle.com/javaee/6/api/javax/servlet/http/HttpServletResponseWrapper.html) @@ -148,4 +148,4 @@ - stackoverflow原址: -http://stackoverflow.com/questions/1673841/examples-of-gof-design-patterns-in-javas-core-libraries \ No newline at end of file +http://stackoverflow.com/questions/1673841/examples-of-gof-design-patterns-in-javas-core-libraries From 941d6da8fbf444eeb22aeb1f286da4a6ca9125ba Mon Sep 17 00:00:00 2001 From: jinzhencheng Date: 2016年10月30日 14:07:20 +0800 Subject: [PATCH 33/65] =?UTF-8?q?=E6=B7=BB=E5=8A=A0=E4=BA=86<<=e5=bd=93?= =?UTF-8?q?=E5=AF=BC=E5=85=A5=E9=A1=B9=E7=9B=AE=E5=88=B0eclipse=E6=97=B6?= =?UTF-8?q?=E6=8A=A5=E9=94=99=E3=80=8B=E7=9A=84=E7=BF=BB=E8=AF=91?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- ...s-after-importing-a-project-into-eclips.md | 35 +++++++++++++++++++ 1 file changed, 35 insertions(+) create mode 100644 contents/must-override-a-superclass-method-errors-after-importing-a-project-into-eclips.md diff --git a/contents/must-override-a-superclass-method-errors-after-importing-a-project-into-eclips.md b/contents/must-override-a-superclass-method-errors-after-importing-a-project-into-eclips.md new file mode 100644 index 0000000..723403f --- /dev/null +++ b/contents/must-override-a-superclass-method-errors-after-importing-a-project-into-eclips.md @@ -0,0 +1,35 @@ + +##问题 +大多数情况下,当我重新导入项目到eclipse的时候,我重写的方法都不能被正确格式化,导致这样的错误: +> The method must override a superclass method. + +需要说明的是这是一个Android项目,不知道什么原因,方法的参数被篡改了,因此,我不得不手动的把他们改回来。 +例如: +```java +list.setOnCreateContextMenuListener(new OnCreateContextMenuListener() { + //这儿的参数名是正确的 + public void onCreateContextMenu(ContextMenu menu, View v, + ContextMenuInfo menuInfo) { + } +}); +``` +初始化的时候被篡改成了这样: +```java +list.setOnCreateContextMenuListener(new OnCreateContextMenuListener() { + //这儿的参数被篡改成了这样 + public void onCreateContextMenu(ContextMenu arg1, View arg2, + ContextMenuInfo arg3) { + } +}); +``` +奇怪的是,如果我移除我的代码并使用eclipse自动创建方法的话,它还是会是相同的参数(被篡改的)。因此,我真不知道那儿的问题,它本应该自动格式化代码的。 +要是手动的去修改被篡改的参数名,那是一个非常痛苦的过程。要是有人能解释为什么会出现这样的情况以及怎样去解决它,我感激不尽。 +是不是因为我格式化的这个方法,是另一个方法里面的参数而导致的这样的问题呢? + +##回答 +Eclipse的默认执行环境是java 1.5况且你使用了类的声明接口方法(在java 1.6中能使用@Ovrride注释,但是在java 1.5中一个方法只能重写父类的方法) + +打开你的项目,然后找到preference并且设置java的编译版本为1.6,同时也确保你的eclipse是使用JRE 1.6 来执行你的程序的。 + +Stack Overflow原地址:http://stackoverflow.com/questions/1678122/must-override-a-superclass-method-errors-after-importing-a-project-into-eclips + From 62d40b18bba335a745a7ffa6ab99d8445dc5039c Mon Sep 17 00:00:00 2001 From: jinzhencheng Date: 2016年10月31日 17:31:43 +0800 Subject: [PATCH 34/65] update 'Sort ArrayList of custom Objects by property' --- ...arraylist-of-custom-objects-by-property.md | 75 +++++++++++++++++++ 1 file changed, 75 insertions(+) create mode 100644 contents/sort-arraylist-of-custom-objects-by-property.md diff --git a/contents/sort-arraylist-of-custom-objects-by-property.md b/contents/sort-arraylist-of-custom-objects-by-property.md new file mode 100644 index 0000000..5c279a4 --- /dev/null +++ b/contents/sort-arraylist-of-custom-objects-by-property.md @@ -0,0 +1,75 @@ + + +##通过对象属性对常规对象的ArrayList进行排序 + +###问题 +我读过使用Comparator对常规类的ArrayList进行排序的示例,但是它们大多数使用comparedTo(),据我了解这是一个对字符串进行操作的方法。 +我想要对一个由常规对象构成的ArrayList,通过它的属性(一个Date对象,getStartDate())对ArrayList进行排序。通常情况下我这样比较它们: +```java +item1.getStartDate().before(item2.getStartDate()) +``` +所以我能不能像下面一样: +```java +public class CustomComparator { + public boolean compare(Object object1, Object object2) { + return object1.getStartDate().before(object2.getStartDate()); + } +} + +public class RandomName { + ... + Collections.sort(Database.arrayList, new CustomComparator); + ... +} +``` + +###回答 + 以前Date声明了Comparable,它有一个像处理字符串操作那样的compareTo方法。因此你可以这样: +```java +public class CustomComparator implements Comparator { + @Override + public int compare(MyObject o1, MyObject o2) { + return o1.getStartDate().compareTo(o2.getStartDate()); + } +} +``` +这儿的compare()方法必须返回int,所以不能像你预期那样直接返回boolean. +你的代码看起来应该是这样: +```java +Collections.sort(Database.arrayList, new CustomComparator()); +``` +如果你不需要重复使用comparetor的话,一种简单的方法是,把它写成一个内部类的样子: +```java +Collections.sort(Database.arrayList, new Comparator() { + @Override + public int compare(MyObject o1, MyObject o2) { + return o1.getStartDate().compareTo(o2.getStartDate()); + } +}); +``` +自java8 开始,你可以使用lambda表达式一种简洁的方式来写Comparator() +```java +Collections.sort(Database.arrayList, + (o1, o2) -> o1.getStartDate().compareTo(o2.getStartDate())); +``` +并且List有一个sort(Comparator)方法,所以你可以进一步简化它 +```java + +Database.arrayList.sort((o1, o2) -> o1.getStartDate().compareTo(o2.getStartDate())); +``` +这是一种司空见惯的方法,使用Comparable为一个类创建一个Comprator +```java +Database.arrayList.sort(Comparator.comparing(MyObject::getStartDate)); +All of these are equivalent forms. +``` + +stackoverflow原地址:http://stackoverflow.com/questions/2784514/sort-arraylist-of-custom-objects-by-property + + + + + + + + + From 0e1260d652a9ecbf2b055794a5429de05eaad863 Mon Sep 17 00:00:00 2001 From: lcs1795192 <1845251476@qq.com> Date: 2017年1月17日 14:48:36 +0800 Subject: [PATCH 35/65] =?UTF-8?q?=E6=B7=BB=E5=8A=A0=E7=AC=AC=E4=B8=80?= =?UTF-8?q?=E4=B8=AA=E7=BF=BB=E8=AF=91?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- ...ses-for-android-usermanager-isuseragoat.md | 38 +++++++++++++++++++ 1 file changed, 38 insertions(+) create mode 100644 contents/proper-use-cases-for-android-usermanager-isuseragoat.md diff --git a/contents/proper-use-cases-for-android-usermanager-isuseragoat.md b/contents/proper-use-cases-for-android-usermanager-isuseragoat.md new file mode 100644 index 0000000..06301bd --- /dev/null +++ b/contents/proper-use-cases-for-android-usermanager-isuseragoat.md @@ -0,0 +1,38 @@ +##安卓中"UserManger.isUserAGoat()"的合适案例? + +###问题描述: +我正在看在安卓4.2里介绍的新API。当看到"UserManger"类时,我遇到了如下的"method": +public boolean isUserAGoat() +Used to determine whether the user making this call is subject to teleportations. + +Returns whether the user making this call is a goat. +这个应该怎样使用和什么时候使用? + +###回答: +根据他们的资料,在API21前,这个"method"用来返回"false" +/** + * Used to determine whether the user making this call is subject to + * teleportations. + * @return whether the user making this call is a goat + */ +public boolean isUserAGoat() { + return false; +} +看起来这个"method"对我们开发者没有真正的用途。一些人之前认为它可能是个复活节彩蛋。 +再次编辑: +在API21它改为判断是否存在有包"com.coffeestainstudios.goatsimulator"的已安装app。 +/** + * Used to determine whether the user making this call is subject to + * teleportations. + * + *

As of {@link android.os.Build.VERSION_CODES#LOLLIPOP}, this method can + * now automatically identify goats using advanced goat recognition technology.

+ * + * @return Returns true if the user making this call is a goat. + */ +public boolean isUserAGoat() { + return mContext.getPackageManager() + .isPackageAvailable("com.coffeestainstudios.goatsimulator"); +} + +[stackoverflow链接:Proper use cases for Android UserManager.isUserAGoat()](http://stackoverflow.com/questions/13375357/proper-use-cases-for-android-usermanager-isuseragoat) From 3fc6af09e6bad5bfa6d1f75a65f8f1852612562b Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?=E7=8E=8B=E5=B0=8F=E8=BF=87?= <2638334066@qq.com> Date: Sun, 5 Mar 2017 23:17:13 +0800 Subject: [PATCH 36/65] =?UTF-8?q?=E6=9B=B4=E6=96=B0=E4=BA=86convert-a-stri?= =?UTF-8?q?ng-to-an-enum-in-java=EF=BC=8C=E5=A2=9E=E5=8A=A0=E4=BA=86?= =?UTF-8?q?=E4=B8=80=E7=A7=8D=E5=B8=B8=E7=94=A8=E5=9B=9E=E7=AD=94?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 新增加的其他回答,在编程中更加常用,建议补充上。 --- .../convert-a-string-to-an-enum-in-java.md | 34 ++++++++++++++++++- 1 file changed, 33 insertions(+), 1 deletion(-) diff --git a/contents/convert-a-string-to-an-enum-in-java.md b/contents/convert-a-string-to-an-enum-in-java.md index 5e11958..d9474b9 100644 --- a/contents/convert-a-string-to-an-enum-in-java.md +++ b/contents/convert-a-string-to-an-enum-in-java.md @@ -19,6 +19,38 @@ Enum.valueOf()是否能实现以上目的,如果是,那我如何使用? ### 其他答案 +当文本和枚举值不同时,可以采用这种方式: +```java +public enum Blah { + A("text1"), + B("text2"), + C("text3"), + D("text4"); + + private String text; + + Blah(String text) { + this.text = text; + } + + public String getText() { + return this.text; + } + + public static Blah fromString(String text) { + for (Blah b : Blah.values()) { + if (b.text.equalsIgnoreCase(text)) { + return b; + } + } + return null; + } +} +``` +fromString方法中,throw new IllegalArgumentException("No constant with text " + text + " found") 会比直接返回null更优秀. + +### 其他答案 + 我有一个挺赞的工具方法: ```java /** @@ -47,4 +79,4 @@ public static MyEnum fromString(String name) { } ``` -stackoverflow链接:http://stackoverflow.com/questions/604424/convert-a-string-to-an-enum-in-java \ No newline at end of file +stackoverflow链接:http://stackoverflow.com/questions/604424/convert-a-string-to-an-enum-in-java From 7aa258eba9165190942f7066c4f9745a11ec68ef Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?=E7=8E=8B=E5=B0=8F=E8=BF=87?= <2638334066@qq.com> Date: Sun, 5 Mar 2017 23:25:36 +0800 Subject: [PATCH 37/65] =?UTF-8?q?Convert=20a=20String=20to=20an=20enum=20i?= =?UTF-8?q?n=20Java=20=E9=97=AE=E9=A2=98=E5=9C=A8=E4=B8=80=E5=B9=B4?= =?UTF-8?q?=E5=89=8D=E5=B7=B2=E8=A2=AB=E5=9B=9E=E7=AD=94?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 该问题已被回答,建议更新Read.md - [Convert a String to an enum in Java](http://stackoverflow.com/questions/604424/convert-a-string-to-an-enum-in-java) --- README.md | 1 - 1 file changed, 1 deletion(-) diff --git a/README.md b/README.md index 624fcdb..dbf9fd3 100644 --- a/README.md +++ b/README.md @@ -122,7 +122,6 @@ stackoverflow-Java-top-qa ### 待翻译问题链接(还剩x问题) - [Proper use cases for Android UserManager.isUserAGoat()?](http://stackoverflow.com/questions/13375357/proper-use-cases-for-android-usermanager-isuseragoat) - [Why is executing Java code in comments with certain Unicode characters allowed?](http://stackoverflow.com/questions/30727515/why-is-executing-java-code-in-comments-with-certain-unicode-characters-allowed) -- [Convert a String to an enum in Java](http://stackoverflow.com/questions/604424/convert-a-string-to-an-enum-in-java) - ['Must Override a Superclass Method' Errors after importing a project into Eclipse](http://stackoverflow.com/questions/1678122/must-override-a-superclass-method-errors-after-importing-a-project-into-eclips) - [Fastest way to determine if an integer's square root is an integer](http://stackoverflow.com/questions/295579/fastest-way-to-determine-if-an-integers-square-root-is-an-integer) - [How to fix: Unsupported major.minor version 51.0 error?](http://stackoverflow.com/questions/10382929/how-to-fix-unsupported-major-minor-version-51-0-error) From 5e42d2487b8e35815568c9888c433145a3789153 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?=E7=8E=8B=E5=B0=8F=E8=BF=87?= <2638334066@qq.com> Date: Mon, 6 Mar 2017 01:05:22 +0800 Subject: [PATCH 38/65] =?UTF-8?q?=E5=88=9B=E5=BB=BA=E4=BA=86apache-camel?= =?UTF-8?q?=E6=98=AF=E4=BB=80=E4=B9=88=E7=9A=84=E9=97=AE=E9=A2=98=E5=9B=9E?= =?UTF-8?q?=E7=AD=94?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 1.创建了what-exactly-is-apache-camel.md 2.翻译问题和答案 3.增加了术语解释 4.对问题的通俗理解 --- contents/what-exactly-is-apache-camel.md | 36 ++++++++++++++++++++++++ 1 file changed, 36 insertions(+) create mode 100644 contents/what-exactly-is-apache-camel.md diff --git a/contents/what-exactly-is-apache-camel.md b/contents/what-exactly-is-apache-camel.md new file mode 100644 index 0000000..b2b3931 --- /dev/null +++ b/contents/what-exactly-is-apache-camel.md @@ -0,0 +1,36 @@ +### 问题描述 + +我不明白Camel到底是干什么的. +希望你能在101字以内介绍一下Camel: + 它到底是什么? + 如何在java中使用它? + 它是和服务器相关的么? + 它是一个独立的程序? +请解释一下Camel是什么. + +### 答案 +如果你有5-10分钟时间,我建议你读一下Jonathan Anstey关于Apache Camel的文章.这是一篇非常棒的文章,简要的介绍了一些Apache Camel的概念,以及用java实现了一个 +实例.Jonathan Anstey是这样描述的: +Apache Camel是一个开源的Java框架,其整合的目的主要是为了使开发人员更容易、方便的开发程序.它提供了如下内容: + +(1)所有被广泛使用的`企业集成模式`的具体实现(`EIPs`) + +(2)连接不同的数据传输和API + +(3)容易使用`领域特定语言(DSL)`建立EIPs和高效的数据传输 + +### 术语解析 + +`EIPs`:企业集成模式的简称,使用消息传递进行企业应用集成,比如消息中间件,将不同程序之间连接在一起. + +`DSL`:DSL编程又称为声明式编程,DSL是在模型之上建立的一种更加灵活的对模型化的理解和使用方式,通俗点说你只需要告诉程序你想要什么,不必每一步都指挥它如何 +执行,SQL语句就是其中的代表. + +### 通俗点讲 + +_Camel:将数据从一方获得,该数据可以是消息、文件流、JSON的多种形式的数据,然后处理,再发送,整合了多种数据获取、处理、发送方式,方便开发者使用_ + + +stackoverflow链接 +http://stackoverflow.com/questions/8845186/what-exactly-is-apache-camel +_译者:[王小过](https://github.com/whp1473)_ From 9ceec4f12a4e0f96fbbb8b8f57179adacf5df6fd Mon Sep 17 00:00:00 2001 From: RWBUPT <1291839723@qq.com> Date: Thu, 9 Mar 2017 20:40:07 +0800 Subject: [PATCH 39/65] Create why-does-math.round-(0.49999999999999994)-return-1.md --- ...th.round-(0.49999999999999994)-return-1.md | 92 +++++++++++++++++++ 1 file changed, 92 insertions(+) create mode 100644 contents/why-does-math.round-(0.49999999999999994)-return-1.md diff --git a/contents/why-does-math.round-(0.49999999999999994)-return-1.md b/contents/why-does-math.round-(0.49999999999999994)-return-1.md new file mode 100644 index 0000000..fb7bf35 --- /dev/null +++ b/contents/why-does-math.round-(0.49999999999999994)-return-1.md @@ -0,0 +1,92 @@ +##为什么Math.round(0.49999999999999994)返回1? + +###问题描述: + +在下面的程序中你可以看到每个稍微比.5小的值都被向下舍入了,除了`0.5`那个。 + +``` +for (int i = 10; i>= 0; i--) { + long l = Double.doubleToLongBits(i + 0.5); + double x; + do { + x = Double.longBitsToDouble(l); + System.out.println(x + " rounded is " + Math.round(x)); + l--; + } while (Math.round(x)> i); +} +``` +输出 +``` +10.5 rounded is 11 +10.499999999999998 rounded is 10 +9.5 rounded is 10 +9.499999999999998 rounded is 9 +8.5 rounded is 9 +8.499999999999998 rounded is 8 +7.5 rounded is 8 +7.499999999999999 rounded is 7 +6.5 rounded is 7 +6.499999999999999 rounded is 6 +5.5 rounded is 6 +5.499999999999999 rounded is 5 +4.5 rounded is 5 +4.499999999999999 rounded is 4 +3.5 rounded is 4 +3.4999999999999996 rounded is 3 +2.5 rounded is 3 +2.4999999999999996 rounded is 2 +1.5 rounded is 2 +1.4999999999999998 rounded is 1 +0.5 rounded is 1 +0.49999999999999994 rounded is 1 +0.4999999999999999 rounded is 0 +``` +我使用 Java 6 update 31。 + +###回答: + +**总结** + +在Java 6中(而且很可能更早),`round x`以`floor(x+0.5)`的方式执行[1]。这是一个规则上的bug,它恰好会导致这样一种不合道理的情况[2]。Java 7不再授权这种分离的执行方式[3]。 + +**问题** + +0.5+0.49999999999999994在双精度数中正好表示1: +``` +static void print(double d) { + System.out.printf("%016x\n", Double.doubleToLongBits(d)); +} + +public static void main(String args[]) { + double a = 0.5; + double b = 0.49999999999999994; + + print(a); // 3fe0000000000000 + print(b); // 3fdfffffffffffff + print(a+b); // 3ff0000000000000 + print(1.0); // 3ff0000000000000 +} +``` +这是因为0.49999999999999994有一个比0.5小的指数,所以它们相加时,它的尾数被移位了,ULP(ULP表示现有数值最后一位代表的最小数值单位,小于ULP的数值将无法累加到现有数值)变的更大。 + +**解决方法** + +从Java 7开始,OpenJDK(如下例)这样执行它(round函数)[4]。 +``` +public static long round(double a) { + if (a != 0x1.fffffffffffffp-2) // greatest double value less than 0.5 + return (long)floor(a + 0.5d); + else + return 0; +} +``` +[1] http://docs.oracle.com/javase/6/docs/api/java/lang/Math.html#round%28double%29 + +[2] http://bugs.java.com/bugdatabase/view_bug.do?bug_id=6430675 (credits to @SimonNickerson for finding this) + +[3] http://docs.oracle.com/javase/7/docs/api/java/lang/Math.html#round%28double%29 + +[4] http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7u40-b43/java/lang/Math.java#Math.round%28double%29 + + +[stackoverflow链接:Why does Math.round(0.49999999999999994) return 1?](http://stackoverflow.com/questions/9902968/why-does-math-round0-49999999999999994-return-1) From a58d15e95d537f66caaa5c29a252b7e901f96e9e Mon Sep 17 00:00:00 2001 From: mysterin Date: 2017年4月10日 16:05:34 +0800 Subject: [PATCH 40/65] =?UTF-8?q?=E4=BF=AE=E6=94=B9stackoverflow=E7=9A=84?= =?UTF-8?q?=E9=93=BE=E6=8E=A5?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 原来的链接是指向到so回答的第二页,打开过去并不是原答案所在,需要返回第一页才能看到原答案。 --- contents/avoiding-null-statements-in-java.md | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/contents/avoiding-null-statements-in-java.md b/contents/avoiding-null-statements-in-java.md index aab0bc7..c2a1348 100644 --- a/contents/avoiding-null-statements-in-java.md +++ b/contents/avoiding-null-statements-in-java.md @@ -99,4 +99,4 @@ ParserFactory.getParser().findAction(someInput).doSomething(); - 如果你想返回null,请停下来想一想,这个地方是否更应该抛出一个异常 stackoverflow链接: -http://stackoverflow.com/questions/271526/avoiding-null-statements-in-java?page=2&tab=votes#tab-top +http://stackoverflow.com/questions/271526/avoiding-null-statements?page=1&tab=votes#tab-top From 77a1c0f90ca1f58dec253735f3503b6c1b836490 Mon Sep 17 00:00:00 2001 From: tuxiantian <791978859@qq.com> Date: Thu, 8 Jun 2017 16:31:37 +0800 Subject: [PATCH 41/65] Update iterate-through-a-hashmap.md --- contents/iterate-through-a-hashmap.md | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/contents/iterate-through-a-hashmap.md b/contents/iterate-through-a-hashmap.md index 91807ac..24ac1ec 100644 --- a/contents/iterate-through-a-hashmap.md +++ b/contents/iterate-through-a-hashmap.md @@ -1,6 +1,6 @@ # HashMap遍历 # -在Java中有多种遍历HashMAp的方法。让我们回顾一下最常见的方法和它们各自的优缺点。由于所有的Map都实现了Map接口,所以接下来方法适用于所有Map(如:HaspMap,TreeMap,LinkedMap,HashTable,etc) +在Java中有多种遍历HashMap的方法。让我们回顾一下最常见的方法和它们各自的优缺点。由于所有的Map都实现了Map接口,所以接下来方法适用于所有Map(如:HaspMap,TreeMap,LinkedMap,HashTable,etc) ## 方法#1 使用For-Each迭代entries ## @@ -74,4 +74,4 @@ 如果你只需要使用key或者value使用方法#2,如果你坚持使用java的老版本(java 5 以前的版本)或者打算在迭代的时候移除entries,使用方法#3。其他情况请使用#1方法。避免使用#4方法。 stackoverflow链接: -http://stackoverflow.com/questions/1066589/iterate-through-a-hashmap \ No newline at end of file +http://stackoverflow.com/questions/1066589/iterate-through-a-hashmap From 124eadd509f55fa975b1a0f30ec9fb62edbe9ee7 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?=E6=9D=8E=E5=A5=95=E6=85=A7?= Date: 2017年9月26日 00:31:18 +0800 Subject: [PATCH 42/65] =?UTF-8?q?look-enum-by-string-value.md=20=E6=8E=92?= =?UTF-8?q?=E7=89=88=E5=92=8C=E6=9B=B4=E6=96=B0?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- contents/lookup-enum-by-string-value.md | 31 ++++++++++++------------- 1 file changed, 15 insertions(+), 16 deletions(-) diff --git a/contents/lookup-enum-by-string-value.md b/contents/lookup-enum-by-string-value.md index 160aafa..f1cb45a 100644 --- a/contents/lookup-enum-by-string-value.md +++ b/contents/lookup-enum-by-string-value.md @@ -1,18 +1,13 @@ -Java 中如何将 String 转换为 enum -======= +# Java 中如何将 String 转换为 enum -###问题 - -###我有一个 enum 类 - -``` java +### 问题 + enum 类 +```java public enum Blah { A, B, C, D } ``` -我想要找到一个 `String` 对应的 enum 值。例如, `"A"` 将是 `Blah.A`.如何做到? - -我需要使用 `Enum.valueOf()` 方法吗? 如果是该如何使用? +如何根据枚举类型的值(比如 "A" ) 得到 `Blah.A`? --- @@ -20,7 +15,8 @@ public enum Blah { 是的, `Blah.valueOf("A")` 将会给你 `Blah.A`. -静态方法 `valueof()` 和 `values()` 在编译时期被插入,并不存在于源码中。但是在Javadoc中;例如,[`Dialog.ModalityType`](http://docs.oracle.com/javase/7/docs/api/java/awt/Dialog.ModalityType.html "Dialog.ModalityType")中显示了这两个方法。 +静态方法 `valueof()` 和 `values()` 在编译时期被插入,并不存在于源码中。 +但是在Javadoc中会显示;例如,[`Dialog.ModalityType`](http://docs.oracle.com/javase/7/docs/api/java/awt/Dialog.ModalityType.html "Dialog.ModalityType")中显示了这两个方法。 ### A2 @@ -80,7 +76,7 @@ public static > T getEnumFromString(Class c, String string) return null; } ``` -之后,在我的enum类中通常如此使用来减少打字: +之后,在我的enum类中通常如此使用来减少代码量: ``` java public static MyEnum fromString(String name) { return getEnumFromString(MyEnum.class, name); @@ -91,8 +87,8 @@ public static MyEnum fromString(String name) { _评论区对于答主的异常处理一片指责 -译者注_ -###A4 -如果你不想编写自己的工具类,可以使用 Google的 `guava` 库: +### A4 +如果你不想编写自己的工具类,可以使用 Google的 [Google guava](https://github.com/google/guava) 库: ``` java Enums.getIfPresent(Blah.class, "A") ``` @@ -100,8 +96,11 @@ Enums.getIfPresent(Blah.class, "A") _完整方法签名 `Optional getIfPresent(Class enumClass, String value)` , `Optional` 对象可以优雅的解决null值问题 -译者注_ +> 注意: 返回的是 `Google Optional` 而不是 `Java Optional` + --- _其他的答案都大同小异,感兴趣的可以看原帖_ stackoverflow链接 -http://stackoverflow.com/questions/604424/lookup-enum-by-string-value -_译者:[MagicWolf](https://github.com/DaiDongLiang)_ +[Lookup enum by string value +](https://stackoverflow.com/questions/604424/lookup-enum-by-string-value) +_译者:[MagicWolf](https://github.com/DaiDongLiang)_ \ No newline at end of file From c24cd350cd9c5eecbf97ed07c8280aacd4be1743 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?=E6=9D=8E=E5=A5=95=E6=85=A7?= Date: 2017年9月26日 00:39:24 +0800 Subject: [PATCH 43/65] update README --- README.md | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-) diff --git a/README.md b/README.md index 624fcdb..6da8904 100644 --- a/README.md +++ b/README.md @@ -10,21 +10,22 @@ stackoverflow-Java-top-qa ------------- ### 如何参与翻译(欢迎加入翻译组QQ群485011036) -####如何参与: +#### 如何参与: - 请从下文"待翻译问题链接"中寻找你感兴趣的问答进行翻译。翻译好的问答,放到contents目录下,无需更新readme.md文档。之后提一个PR,我负责合并PR并更新到readme中。 - 另外,为了避免多人重复新翻译一个问题,你可以提issue,说明你计划翻译的问题及时间点,我可以先更新到下面的"未翻译问题"中,说明已有人领了这个问题。当然,也不一定要提issue,一般情况下,只要及时提pr,我及时审核,出现"撞车"的概率并不高。 -####一些基本的约定: +#### 一些基本的约定: - 文档的文件名,和stackoverflowhich-notnull-java-annotation-should-i-usew上的url保持一致。例如,http://stackoverflow.com/questions/8710619/java-operator 的文件名, 就是java-operator.md - 在每篇翻译文档内容的最后,要附上stackoverflow的原文链接 -####每个人可以做(但不限于): +#### 每个人可以做(但不限于): - 找未翻译的问题进行翻译 - 优化已翻译的问题 - 输出问答精简汇总版(把所有的问答集中到一个md文件,然后尽量精简,让别人可以在一天内把这100个问题的精髓都看完) - 输出gitbook版本(现在直接在github上查看,体验不好) -####文档优化反馈: +#### 文档优化反馈: + 请大家多多反馈,优化已翻译好的文章:可以到[吐槽区](https://github.com/giantray/stackoverflow-java-top-qa/issues/66)吐槽,也可以在已翻译文章基础上进行优化,提新的PR。文章质量的提升,需要大家一起努力! @@ -61,7 +62,6 @@ stackoverflow-Java-top-qa * [为什么Java的```Vector```类被认为是过时的或者废弃的](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/why-is-java-vector-class-considered-obsolete-or-deprecated.md) * [Java的foreach循环是如何工作的](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/how-does-the-java-for-each-loop-work.md) * [为什么相减这两个时间(1927年)会得到奇怪的结果](/contents/why-is-subtracting-these-two-times-in-1927-giving-a-strange-result.md) -* [Java 中如何将 String 转换为 enum](/contents/lookup-enum-by-string-value.md) * [该什么时候使用 ThreadLocal变量,它是如何工作的](/contents/when-and-how-should-i-use-a-threadlocal-variable.md) * [servlets的运行原理](/contents/how-do-servlets-work-instantiation-shared-variables-and-multithreading.md) * [如何计算MD5值](/contents/how-can-i-generate-an-md5-hash.md) @@ -122,7 +122,6 @@ stackoverflow-Java-top-qa ### 待翻译问题链接(还剩x问题) - [Proper use cases for Android UserManager.isUserAGoat()?](http://stackoverflow.com/questions/13375357/proper-use-cases-for-android-usermanager-isuseragoat) - [Why is executing Java code in comments with certain Unicode characters allowed?](http://stackoverflow.com/questions/30727515/why-is-executing-java-code-in-comments-with-certain-unicode-characters-allowed) -- [Convert a String to an enum in Java](http://stackoverflow.com/questions/604424/convert-a-string-to-an-enum-in-java) - ['Must Override a Superclass Method' Errors after importing a project into Eclipse](http://stackoverflow.com/questions/1678122/must-override-a-superclass-method-errors-after-importing-a-project-into-eclips) - [Fastest way to determine if an integer's square root is an integer](http://stackoverflow.com/questions/295579/fastest-way-to-determine-if-an-integers-square-root-is-an-integer) - [How to fix: Unsupported major.minor version 51.0 error?](http://stackoverflow.com/questions/10382929/how-to-fix-unsupported-major-minor-version-51-0-error) From d13b3b8f66b8b608f825a6a308c2f108cf889bf5 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?=E7=A7=8B=E5=90=8D=E5=B1=B1=E8=BD=A6=E7=A5=9E?= Date: 2017年10月16日 21:41:10 -0500 Subject: [PATCH 44/65] =?UTF-8?q?=E4=BF=AE=E5=A4=8D=E6=96=87=E6=A1=A3?= =?UTF-8?q?=E6=98=BE=E7=A4=BA=E9=94=99=E8=AF=AF?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit markdown标准语法应该有个空格 --- contents/java-operator.md | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/contents/java-operator.md b/contents/java-operator.md index cee3e03..ab967cc 100644 --- a/contents/java-operator.md +++ b/contents/java-operator.md @@ -1,6 +1,6 @@ -##Java += 操作符实质 +## Java += 操作符实质 -###问题 +### 问题 我之前以为: i += j 等同于 i = i + j; 但假设有: @@ -11,7 +11,7 @@ long j = 8; 这时 i = i + j 不能编译,但 i += j 却可以编译。这说明两者还是有差别的 这是否意味着,i += j,实际是等同于 i= (type of i) (i + j)呢? -###回答 +### 回答 这个问题,其实官方文档中已经解答了。 请看这里 [§15.26.2 Compound Assignment Operators](http://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.26.2) From 8ddef1aca6269e818ebd50a4cc5b71d970efcae9 Mon Sep 17 00:00:00 2001 From: LI Daobing Date: Tue, 3 Jul 2018 13:21:25 +0800 Subject: [PATCH 45/65] Update how-to-sort-a-mapkey-value-on-the-values-in-java.md --- .../how-to-sort-a-mapkey-value-on-the-values-in-java.md | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/contents/how-to-sort-a-mapkey-value-on-the-values-in-java.md b/contents/how-to-sort-a-mapkey-value-on-the-values-in-java.md index b9fecfb..31b36ec 100644 --- a/contents/how-to-sort-a-mapkey-value-on-the-values-in-java.md +++ b/contents/how-to-sort-a-mapkey-value-on-the-values-in-java.md @@ -1,6 +1,6 @@ -##`Map`基于Value值排序 +## `Map`基于Value值排序 -###方法1: +### 方法1: 使用TreeMap,可以参考下面的代码 ```java public class Testing { @@ -43,7 +43,7 @@ class ValueComparator implements Comparator { ``` 译注:如果不自己写Comparator,treemap默认是用key来排序 -###方法2: +### 方法2: 先通过linkedlist排好序,再放到LinkedHashMap中 ```java @@ -74,4 +74,4 @@ public class MapUtil 译注:这两种方法,我简单测试了下,如果map的size在十万级别以上,两者的耗时都是几百毫秒,第二个方法会快一些。否则,第一个方法快一些。因此,如果你处理的map,都是几十万级别以下的大小,两种方式随意使用,看个人喜欢了。 stackoverflow链接: -http://stackoverflow.com/questions/109383/how-to-sort-a-mapkey-value-on-the-values-in-java \ No newline at end of file +http://stackoverflow.com/questions/109383/how-to-sort-a-mapkey-value-on-the-values-in-java From 79c54d97926ac510a3213b14735ad3cada20f94b Mon Sep 17 00:00:00 2001 From: LI Daobing Date: Tue, 3 Jul 2018 13:23:05 +0800 Subject: [PATCH 46/65] Update README.md --- README.md | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/README.md b/README.md index 624fcdb..3bdf099 100644 --- a/README.md +++ b/README.md @@ -10,21 +10,21 @@ stackoverflow-Java-top-qa ------------- ### 如何参与翻译(欢迎加入翻译组QQ群485011036) -####如何参与: +#### 如何参与: - 请从下文"待翻译问题链接"中寻找你感兴趣的问答进行翻译。翻译好的问答,放到contents目录下,无需更新readme.md文档。之后提一个PR,我负责合并PR并更新到readme中。 - 另外,为了避免多人重复新翻译一个问题,你可以提issue,说明你计划翻译的问题及时间点,我可以先更新到下面的"未翻译问题"中,说明已有人领了这个问题。当然,也不一定要提issue,一般情况下,只要及时提pr,我及时审核,出现"撞车"的概率并不高。 -####一些基本的约定: +#### 一些基本的约定: - 文档的文件名,和stackoverflowhich-notnull-java-annotation-should-i-usew上的url保持一致。例如,http://stackoverflow.com/questions/8710619/java-operator 的文件名, 就是java-operator.md - 在每篇翻译文档内容的最后,要附上stackoverflow的原文链接 -####每个人可以做(但不限于): +#### 每个人可以做(但不限于): - 找未翻译的问题进行翻译 - 优化已翻译的问题 - 输出问答精简汇总版(把所有的问答集中到一个md文件,然后尽量精简,让别人可以在一天内把这100个问题的精髓都看完) - 输出gitbook版本(现在直接在github上查看,体验不好) -####文档优化反馈: +#### 文档优化反馈: 请大家多多反馈,优化已翻译好的文章:可以到[吐槽区](https://github.com/giantray/stackoverflow-java-top-qa/issues/66)吐槽,也可以在已翻译文章基础上进行优化,提新的PR。文章质量的提升,需要大家一起努力! From 74b36bb1486b39baaf094dd5768fbc170d9e5b24 Mon Sep 17 00:00:00 2001 From: razyer <623065552@qq.com> Date: 2018年8月15日 21:03:09 +0800 Subject: [PATCH 47/65] =?UTF-8?q?=E8=B0=83=E6=95=B4markdown=E6=A0=BC?= =?UTF-8?q?=E5=BC=8F=EF=BC=8C=E6=8F=90=E9=AB=98=E6=98=BE=E7=A4=BA=E5=92=8C?= =?UTF-8?q?=E9=98=85=E8=AF=BB=E6=95=88=E6=9E=9C?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- ...ation-shared-variables-and-multithreading.md | 17 +++++++++-------- 1 file changed, 9 insertions(+), 8 deletions(-) diff --git a/contents/how-do-servlets-work-instantiation-shared-variables-and-multithreading.md b/contents/how-do-servlets-work-instantiation-shared-variables-and-multithreading.md index 148ab53..4539697 100644 --- a/contents/how-do-servlets-work-instantiation-shared-variables-and-multithreading.md +++ b/contents/how-do-servlets-work-instantiation-shared-variables-and-multithreading.md @@ -1,13 +1,14 @@ -##How do servlets work? Instantiation, shared variables and multithreading -###问题: +## How do servlets work? Instantiation, shared variables and multithreading +### 问题: 假设,我有一个web服务器可以支持无数的servlets,对于通过这些servlets的信息,我正在获取这些servlets的上下文环境,并设置session变量。 现在,如果有两个或者更多的user用户发送请求到这个服务器,session变量会发生什么变化?session对于所有的user是公共的还是不同的user拥有不同的session。如果用户彼此之间的session是不同的,那么服务器怎么区分辨别不同的用户呢? 另外一些相似的问题,如果有N个用户访问一个具体的servlets,那么这个servlets是只在第一个用户第一次访问的时候实例化,还是为每一个用户各自实例化呢? -###答案: -####ServletContext + +### 答案: +#### ServletContext 当servletcontainer(像tomcat)启动的时候,它会部署和加载所有的webapplications,当一个webapplication加载完成后,servletcontainer就会创建一个ServletContext,并且保存在服务器的内存中。这个webapp的web.xml会被解析,web.xml中的每个```, and ```或者通过注解```@WebServlet, @WebFilter and @WebListener```,都会被创建一次并且也保存在服务器的内存中。对于所有filter,```init()```方法会被直接触发,当servletcontainer关闭的时候,它会unload所有的webapplications,触发所有实例化的servlets和filters的```destroy()```方法,最后,servletcontext和所有的servlets,filter和listener实例都会被销毁。 -####HttpServletRequest and HttpServletResponse +#### HttpServletRequest and HttpServletResponse servletcontainer 是附属于webserver的,而这个webserver会持续监听一个目标端口的```HTTP request```请求,这个端口在开发中经常会被设置成8080,而在生产环境会被设置成80。当一个客户端(比如用户的浏览器)发送一个HTTP request,servletcontainer就会创建新的HttpServletRequest对象和HttpServletResponse对象。。。。 在有filter的情况下,```doFilter()```方法会被触发。当代码调用```chain.doFilter(request, response)```时候,请求会经过下一个过滤器filter,如果没有了过滤器,会到达servlet。在servlets的情况下,```service()```触发,然后根据```request.getMethod()```确定执行doGet()还是```doPost()```,如果当前servlet找不到请求的方法,返回405error。 @@ -22,14 +23,14 @@ request对象提供了HTTP请求所有的信息,比如request headers和reques 只要没超过``````设定的值,httpSession对象会一直存在,``````大小可以在web.xml中设定,默认是30分钟。所以如果连续30分钟之内客户端不再访问webapp,servletcontainer就会销毁对应的session。接下来的request请求即使cookies依旧存在,但是却不再有对应的session了。servletcontainer 会创建新的session。 另外一方面,session cookie在浏览器端有默认的生命时长,就是只要浏览器一直在运行,所以当浏览器关闭,浏览器端的cookie会被销毁。 -####最后 +#### 总结 - 只要webapp存在,ServletContext 一定会存在。并且ServletContext 是被所有session和request共享的。 - 只要客户端用同一个浏览器和webapp交互并且该session没有在服务端超时,HttpSession 就会一直存在。并且在同一个会话中所有请求都是共享的。 - 只有当完整的response响应到达,HttpServletRequest 和 HttpServletResponse才不再存活,并且不被共享。 - 只要webapp存在,servlet、filter和listener就会存在。他们被所有请求和会话共享。 - 只要问题中的对象存在,任何设置在ServletContext, HttpServletRequest 和 HttpSession中的属性就会存在。 -####线程安全 +#### 线程安全 就是说,你主要关注的是线程安全性。你应该了解到,servlets和filter是被所有请求共享的。这正是Java的美妙之处,它的多线程和不同的线程可以充分利用同样的实例instance,否则对于每一个request请求都要重复创建和调用init()和destroy()开销太大。 但是你也应该注意到,你不应该把任何请求或会话作用域的数据作为一个servlet或过滤器的实例变量。这样会被其他会话的请求共享,并且那是线程不安全的!下面的例子阐明的这点: @@ -49,4 +50,4 @@ public class ExampleServlet extends HttpServlet { stackoverflow链接: -http://stackoverflow.com/questions/3106452/how-do-servlets-work-instantiation-shared-variables-and-multithreading \ No newline at end of file +http://stackoverflow.com/questions/3106452/how-do-servlets-work-instantiation-shared-variables-and-multithreading From 6e490df68e6dc374767a8a4e5cdfdaed8537e8c9 Mon Sep 17 00:00:00 2001 From: jayknoxqu Date: 2018年10月17日 22:13:00 +0800 Subject: [PATCH 48/65] Update how-can-i-initialize-a-static-map.md MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 修正文中错误代码! --- contents/how-can-i-initialize-a-static-map.md | 26 +++++++++---------- 1 file changed, 13 insertions(+), 13 deletions(-) diff --git a/contents/how-can-i-initialize-a-static-map.md b/contents/how-can-i-initialize-a-static-map.md index 71b83a5..a85057f 100644 --- a/contents/how-can-i-initialize-a-static-map.md +++ b/contents/how-can-i-initialize-a-static-map.md @@ -52,23 +52,23 @@ 我喜欢用Guava(是 Collection 框架的增强)的方法初始化一个静态的,不可改变的map - static fianl Map myMap = ImmutablMap.of( - 1,"one", - 2, "two" - ) + static final Map MY_MAP = ImmutableMap.of( + 1, "one", + 2, "two" + ); + · 当map的 entry个数超过5个时,你就不能使用`ImmutableMap.of`。可以试试`ImmutableMap.bulider()` - static fianl Map myMap = ImmutableMap.builder() - { - .put(1, "one") - .put(2, "two") - - .put(15, "fifteen") - .build(); - } + static final Map MY_MAP = ImmutableMap.builder() + .put(1, "one") + .put(2, "two") + // ... + .put(15, "fifteen") + .build(); + # 原文链接 # -http://stackoverflow.com/questions/507602/how-can-i-initialize-a-static-map \ No newline at end of file +http://stackoverflow.com/questions/507602/how-can-i-initialize-a-static-map From 5eee13fde9ecc97c227a4c3f060165ac10c87f94 Mon Sep 17 00:00:00 2001 From: indicolite Date: 2018年10月26日 14:44:29 +0800 Subject: [PATCH 49/65] =?UTF-8?q?=E6=A0=BC=E5=BC=8F=E4=BF=AE=E6=94=B9?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- README.md | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/README.md b/README.md index 624fcdb..3bdf099 100644 --- a/README.md +++ b/README.md @@ -10,21 +10,21 @@ stackoverflow-Java-top-qa ------------- ### 如何参与翻译(欢迎加入翻译组QQ群485011036) -####如何参与: +#### 如何参与: - 请从下文"待翻译问题链接"中寻找你感兴趣的问答进行翻译。翻译好的问答,放到contents目录下,无需更新readme.md文档。之后提一个PR,我负责合并PR并更新到readme中。 - 另外,为了避免多人重复新翻译一个问题,你可以提issue,说明你计划翻译的问题及时间点,我可以先更新到下面的"未翻译问题"中,说明已有人领了这个问题。当然,也不一定要提issue,一般情况下,只要及时提pr,我及时审核,出现"撞车"的概率并不高。 -####一些基本的约定: +#### 一些基本的约定: - 文档的文件名,和stackoverflowhich-notnull-java-annotation-should-i-usew上的url保持一致。例如,http://stackoverflow.com/questions/8710619/java-operator 的文件名, 就是java-operator.md - 在每篇翻译文档内容的最后,要附上stackoverflow的原文链接 -####每个人可以做(但不限于): +#### 每个人可以做(但不限于): - 找未翻译的问题进行翻译 - 优化已翻译的问题 - 输出问答精简汇总版(把所有的问答集中到一个md文件,然后尽量精简,让别人可以在一天内把这100个问题的精髓都看完) - 输出gitbook版本(现在直接在github上查看,体验不好) -####文档优化反馈: +#### 文档优化反馈: 请大家多多反馈,优化已翻译好的文章:可以到[吐槽区](https://github.com/giantray/stackoverflow-java-top-qa/issues/66)吐槽,也可以在已翻译文章基础上进行优化,提新的PR。文章质量的提升,需要大家一起努力! From b7d20aad25e82a1dbbfc479e643a03540007cf37 Mon Sep 17 00:00:00 2001 From: 245455758 <30688227+245455758@users.noreply.github.com> Date: 2018年10月28日 21:26:45 +0800 Subject: [PATCH 50/65] Create how-to-get-an-enum-value-from-a-string-value-in-java.md MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 创建文件 --- contents/how-to-get-an-enum-value-from-a-string-value-in-java.md | 1 + 1 file changed, 1 insertion(+) create mode 100644 contents/how-to-get-an-enum-value-from-a-string-value-in-java.md diff --git a/contents/how-to-get-an-enum-value-from-a-string-value-in-java.md b/contents/how-to-get-an-enum-value-from-a-string-value-in-java.md new file mode 100644 index 0000000..8b13789 --- /dev/null +++ b/contents/how-to-get-an-enum-value-from-a-string-value-in-java.md @@ -0,0 +1 @@ + From cf30c73f9e87a9398cbc850adee21a10e8d96520 Mon Sep 17 00:00:00 2001 From: 245455758 <30688227+245455758@users.noreply.github.com> Date: 2018年10月28日 22:32:14 +0800 Subject: [PATCH 51/65] =?UTF-8?q?=E5=88=9D=E6=AD=A5=E5=AE=8C=E6=88=90?= =?UTF-8?q?=E4=BA=86=E7=BF=BB=E8=AF=91?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- ...-enum-value-from-a-string-value-in-java.md | 24 +++++++++++++++++++ 1 file changed, 24 insertions(+) diff --git a/contents/how-to-get-an-enum-value-from-a-string-value-in-java.md b/contents/how-to-get-an-enum-value-from-a-string-value-in-java.md index 8b13789..66d634b 100644 --- a/contents/how-to-get-an-enum-value-from-a-string-value-in-java.md +++ b/contents/how-to-get-an-enum-value-from-a-string-value-in-java.md @@ -1 +1,25 @@ +## 标题 +### 问题: + 假如说我有一个如下的枚举类 + +```java + public enum Blah { + A, B, C, D + } +``` +而我想要找出具有指定名称的枚举类型对应的的字符串类型的枚举常量,打个比方``"A"``对应的值为``Blah.A``。 +此时我应该怎么做? +我是不是应该使用``Enum.valueOf()``这个方法?如果是的话,我又该怎么使用它? + +### 回答: +是的,``Blah.valueOf("A")``将会给你你想要的``Blah.A`` +不过需要注意的是,你输入的名字必须是绝对匹配的,像``Blah.valueOf("a")``和``Blah.valueOf("A ")``都会抛出``IllegalArgumentException`` +注:第一个表达式``a`` 与``A``不匹配 +第二个表达式``A ``后面多了一个空格 + +静态方法``valueOf()``和``values()``在编译时创建并且不会出现在编译后的源码里。但尽管如此,这两个方法还是确实出现在了Java文档里,[文档连接](https://docs.oracle.com/javase/7/docs/api/java/awt/Dialog.ModalityType.html) + +stackoverflow讨论地址 [https://stackoverflow.com/questions/604424/how-to-get-an-enum-value-from-a-string-value-in-java](https://stackoverflow.com/questions/604424/how-to-get-an-enum-value-from-a-string-value-in-java) + + From f01f9808a7d13e4e194be4c317b724d96aca7fa3 Mon Sep 17 00:00:00 2001 From: 245455758 <30688227+245455758@users.noreply.github.com> Date: 2018年10月28日 22:39:53 +0800 Subject: [PATCH 52/65] Update how-to-get-an-enum-value-from-a-string-value-in-java.md MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 完善页面 --- .../how-to-get-an-enum-value-from-a-string-value-in-java.md | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/contents/how-to-get-an-enum-value-from-a-string-value-in-java.md b/contents/how-to-get-an-enum-value-from-a-string-value-in-java.md index 66d634b..b8a0a12 100644 --- a/contents/how-to-get-an-enum-value-from-a-string-value-in-java.md +++ b/contents/how-to-get-an-enum-value-from-a-string-value-in-java.md @@ -1,7 +1,7 @@ -## 标题 +## 怎样得到指定名称的枚举类型对应的的字符串类型的枚举常量 ### 问题: - 假如说我有一个如下的枚举类 +假如说我有一个如下的枚举类 ```java public enum Blah { @@ -13,7 +13,7 @@ 我是不是应该使用``Enum.valueOf()``这个方法?如果是的话,我又该怎么使用它? ### 回答: -是的,``Blah.valueOf("A")``将会给你你想要的``Blah.A`` +是的,``Blah.valueOf("A")``将会给你你想要的``Blah.A`` 不过需要注意的是,你输入的名字必须是绝对匹配的,像``Blah.valueOf("a")``和``Blah.valueOf("A ")``都会抛出``IllegalArgumentException`` 注:第一个表达式``a`` 与``A``不匹配 第二个表达式``A ``后面多了一个空格 From ca2b24df27edcc6d236dda52acdbb6b77ff9022c Mon Sep 17 00:00:00 2001 From: 245455758 <30688227+245455758@users.noreply.github.com> Date: 2018年10月29日 11:28:52 +0800 Subject: [PATCH 53/65] =?UTF-8?q?=E5=88=9B=E5=BB=BA=E6=96=87=E4=BB=B6?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- contents/why-does-this-go-into-an-infinite-loop | 1 + 1 file changed, 1 insertion(+) create mode 100644 contents/why-does-this-go-into-an-infinite-loop diff --git a/contents/why-does-this-go-into-an-infinite-loop b/contents/why-does-this-go-into-an-infinite-loop new file mode 100644 index 0000000..8b13789 --- /dev/null +++ b/contents/why-does-this-go-into-an-infinite-loop @@ -0,0 +1 @@ + From 7500914ef82d80e07c0c6a9c07212e67e3bee87b Mon Sep 17 00:00:00 2001 From: 245455758 <30688227+245455758@users.noreply.github.com> Date: 2018年10月29日 11:31:45 +0800 Subject: [PATCH 54/65] =?UTF-8?q?=E5=AE=8C=E6=88=90=E6=A0=87=E9=A2=98?= =?UTF-8?q?=E7=A1=AE=E5=AE=9A?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- contents/why-does-this-go-into-an-infinite-loop | 1 + 1 file changed, 1 insertion(+) diff --git a/contents/why-does-this-go-into-an-infinite-loop b/contents/why-does-this-go-into-an-infinite-loop index 8b13789..33787a9 100644 --- a/contents/why-does-this-go-into-an-infinite-loop +++ b/contents/why-does-this-go-into-an-infinite-loop @@ -1 +1,2 @@ +## 为什么它会进入死循环 From 6850f0cc36a8a7b34afbfd73856a2b09e8c9e3a2 Mon Sep 17 00:00:00 2001 From: withlimin <42736147+withlimin@users.noreply.github.com> Date: 2019年4月11日 11:46:08 +0800 Subject: [PATCH 55/65] Create how-do-i-decompile-java-class-files.md https://stackoverflow.com/questions/272535/how-do-i-decompile-java-class-files --- .../how-do-i-decompile-java-class-files.md | 65 +++++++++++++++++++ 1 file changed, 65 insertions(+) create mode 100644 contents/how-do-i-decompile-java-class-files.md diff --git a/contents/how-do-i-decompile-java-class-files.md b/contents/how-do-i-decompile-java-class-files.md new file mode 100644 index 0000000..8a28fc2 --- /dev/null +++ b/contents/how-do-i-decompile-java-class-files.md @@ -0,0 +1,65 @@ +## 如何对Java class文件进行反编译 ## + +可以用什么程序来编译class文件 会得到java代码还是JVM编译的代码? +在这个网站上性能讨论的问题上经常看到进行反编译文件来看编译器如何优化一些东西 + +"反编译"的艺术也可以被认为是逆向工程。虽然有时在逆向工程时你并不总是能够访问二进制文件。 + + +没人提到 bytecodeviewer.com吗,它可以反编译为java源码和字节码(对于java源码是基于JAD) + +**www.javadecompilers.com** + + 它是最流行的Java编译器,用c++编写,速度很快 + 比较过时,且不提供支持,不支持Java5及以后的版本 +这个网站也列出了其他的工具。 + +正如Salvador Valencia在评论(2017年9月)中所说 javadecompiler提供了一个SaaS,您可以将.cl​​ass文件上传到云端,并返回反编译代码。 +(原答案:2008年10月) +定义J2SE 5.0(Java SE 5)主要功能的JSR 176的最终版本已于2004年9月30日发布。 +由Pavel Kouznetsov先生编写的着名Java反编译器JAD支持的最新Java版本是JDK 1.3。 + + +Java Decompiler(一个快速Java反编译器)具有: +显式支持反编译和分析Java 5+".class"文件。 +一个很好的GUI: +![](https://i.imgur.com/iXIWcl8.png) + +它适用于从JDK 1.1.8到JDK 1.7.0以及其他(Jikes,JRockit等)的编译器。 +它具有在线实时演示版本,实际上功能齐全!你可以在页面上删除一个jar文件,看看反编译的源代码而不安装任何东西。 + + +比如: +Procyon:开源(Apache 2) +Krakatau:开源(GPLv3) +CFR:开源(MIT) +JAD +DJ Java Decompiler +Mocha +还有很多。 + +这些产生Java代码,可以让你看到JVM字节码。 +要查看Java源代码,请检查一些反编译器。去搜索jad。 +如果要查看字节码,只需使用JDK附带的javap即可。 + + + +我试了几个,Procyon对我来是最好的。它正在积极开发中,并支持最新版Java的许多功能。 + +以下是我试过的其他的: +CFR +还可以,但是经常反编译失败。我会密切注意这个。还积极开发支持最新的Java功能。 +Krakatau +采用不同的方法,它尝试输出等效的Java代码,而不是尝试重建原始源,这有可能使混淆代码更好。根据我的测试,它与Procyon大致相当,但仍然很高兴有不同的东西。我确实必须使用-skip命令行标志,因此它不会停止错误。积极开发,有趣的是它是用Python编写的。 +JD-GUI +工作,但Procyon的输出要好得多。这是一个将Procyon输出与原始和JD-GUI进行比较的页面。 JD-GUI也可以作为Eclipse插件使用,它根本不适用于我。似乎不是开源的,发展似乎是零星的。 +JAD +工作,但只支持Java 1.4及更低版本。也可以作为Eclipse插件使用。不再在开发中。 + + +我使用JAD Decompiler。 +有一个Eclipse插件,jadeclipse。这挺好用的。 + + +JD-GUI真的很棒。你可以打开一个jar文件并浏览代码,就好像正在使用IDE一样。好东西。 + From a2fe2a2463a868b961c42b45ec937b3e65b4dbcf Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?=E6=9E=81=E9=80=9F=E8=9C=97=E7=89=9B?= <31986081+jisu-woniu@users.noreply.github.com> Date: 2019年4月20日 18:49:48 -0500 Subject: [PATCH 56/65] =?UTF-8?q?=E4=BB=A3=E7=A0=81=E6=A0=BC=E5=BC=8F?= =?UTF-8?q?=E4=BF=AE=E5=A4=8D?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- contents/read-convert-an-inputstream-to-a-string.md | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/contents/read-convert-an-inputstream-to-a-string.md b/contents/read-convert-an-inputstream-to-a-string.md index 55aefec..8807e6f 100644 --- a/contents/read-convert-an-inputstream-to-a-string.md +++ b/contents/read-convert-an-inputstream-to-a-string.md @@ -9,8 +9,9 @@ IOUtils.copy(inputStream, writer, encoding); String theString = writer.toString(); ``` 或者 +```java String theString = IOUtils.toString(inputStream, encoding)//这个方法其实封装了上面的方法,减少了一个参数 - +``` ###使用原生库 如果不想引入Apache库,也可以这样做 ```java From 451cea2dfee4e5a0f0e41fae0a85b94756c2be72 Mon Sep 17 00:00:00 2001 From: zsgwsjj <749940957@qq.com> Date: 2019年5月14日 10:37:52 +0800 Subject: [PATCH 57/65] Update java-operator.md --- contents/java-operator.md | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/contents/java-operator.md b/contents/java-operator.md index cee3e03..ab967cc 100644 --- a/contents/java-operator.md +++ b/contents/java-operator.md @@ -1,6 +1,6 @@ -##Java += 操作符实质 +## Java += 操作符实质 -###问题 +### 问题 我之前以为: i += j 等同于 i = i + j; 但假设有: @@ -11,7 +11,7 @@ long j = 8; 这时 i = i + j 不能编译,但 i += j 却可以编译。这说明两者还是有差别的 这是否意味着,i += j,实际是等同于 i= (type of i) (i + j)呢? -###回答 +### 回答 这个问题,其实官方文档中已经解答了。 请看这里 [§15.26.2 Compound Assignment Operators](http://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.26.2) From 84c06679902c840e9c596dfb78e8c7e0e698c04a Mon Sep 17 00:00:00 2001 From: Jarbitz Date: 2019年5月21日 17:29:36 +0800 Subject: [PATCH 58/65] Update how-to-sort-a-mapkey-value-on-the-values-in-java.md title # mark --- .../how-to-sort-a-mapkey-value-on-the-values-in-java.md | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/contents/how-to-sort-a-mapkey-value-on-the-values-in-java.md b/contents/how-to-sort-a-mapkey-value-on-the-values-in-java.md index b9fecfb..31b36ec 100644 --- a/contents/how-to-sort-a-mapkey-value-on-the-values-in-java.md +++ b/contents/how-to-sort-a-mapkey-value-on-the-values-in-java.md @@ -1,6 +1,6 @@ -##`Map`基于Value值排序 +## `Map`基于Value值排序 -###方法1: +### 方法1: 使用TreeMap,可以参考下面的代码 ```java public class Testing { @@ -43,7 +43,7 @@ class ValueComparator implements Comparator { ``` 译注:如果不自己写Comparator,treemap默认是用key来排序 -###方法2: +### 方法2: 先通过linkedlist排好序,再放到LinkedHashMap中 ```java @@ -74,4 +74,4 @@ public class MapUtil 译注:这两种方法,我简单测试了下,如果map的size在十万级别以上,两者的耗时都是几百毫秒,第二个方法会快一些。否则,第一个方法快一些。因此,如果你处理的map,都是几十万级别以下的大小,两种方式随意使用,看个人喜欢了。 stackoverflow链接: -http://stackoverflow.com/questions/109383/how-to-sort-a-mapkey-value-on-the-values-in-java \ No newline at end of file +http://stackoverflow.com/questions/109383/how-to-sort-a-mapkey-value-on-the-values-in-java From b6ef88397d1cd2bb6317fa5b74f551feb14e271f Mon Sep 17 00:00:00 2001 From: seuwh2018 Date: 2019年5月27日 15:57:52 +0800 Subject: [PATCH 59/65] translation the question Why does this go into an infinite loop? --- contents/what-the-equivalent-of-x=x++.md | 106 +++++++++++++++++++++++ 1 file changed, 106 insertions(+) create mode 100644 contents/what-the-equivalent-of-x=x++.md diff --git a/contents/what-the-equivalent-of-x=x++.md b/contents/what-the-equivalent-of-x=x++.md new file mode 100644 index 0000000..1857b40 --- /dev/null +++ b/contents/what-the-equivalent-of-x=x++.md @@ -0,0 +1,106 @@ +## x=x++最终x等于多少? +### 问题描述: +``` +public class Tests { + public static void main(String[] args) throws Exception { + int x = 0; + while(x<3) { + x = x++; + System.out.println(x); + } + } +} +``` +如上,程序会进入死循环,我们知道通常对于x自增是通过x++或者x=x+1,那么为什么x=x++不是对x自增再赋值给x? + +### 回答: +明确x=x++处理流程,相当于: +```java +int temp=x; +x++; +x=temp; +``` +#### 解释: +考虑如下类: +```java +class test { + public static void main(String[] args) { + int i=0; + i=i++; + } +} +``` +通过运行反汇编: +```bash +$ javap -c test +``` +可以得到: +```java +Compiled from "test.java" +class test extends java.lang.Object{ +test(); + Code: + 0: aload_0 + 1: invokespecial #1; //Method java/lang/Object."":()V + 4: return + +public static void main(java.lang.String[]); + Code: + 0: iconst_0 + 1: istore_1 + 2: iload_1 + 3: iinc 1, 1 + 6: istore_1 + 7: return +} +``` +以下来解释其中的助记符: +* iconst_0: 常量0被push入栈。 + +* istore_1: 栈顶元素被pop出,然后存储在索引为1的局部变量也就是x内。 + +* iload_1 : 索引为1的局部变量(即x)的值被push入栈,此时栈顶值为0。 + +* iinc 1, 1 :索引为1的量增加为1,也就是x变为1。 + +* istore_1 : 栈顶值被pop出,存入索引为1的局部变量中,因此x再次变为0。 + +以上,可以知道x=x++由于入栈出栈,x的值并未变化。 + +#### 对比:x=++x +```java +class test { + public static void main(String[] args) { + int i=0; + i=++i; + } +} +``` +运行 +```bash +$ javac test.java +$ javap -c test +``` +得到: +```java +Compiled from "test.java" +class test { + test(); + Code: + 0: aload_0 + 1: invokespecial #1 // Method java/lang/Object."":()V + 4: return + + public static void main(java.lang.String[]); + Code: + 0: iconst_0 + 1: istore_1 + 2: iinc 1, 1 + 5: iload_1 + 6: istore_1 + 7: return +} +``` +可以看出是先对x进行加一,再将x的值push入栈,随后再将栈顶值赋给x。 + + From 739fe6dee9f407c0639e665a473f95a28f0ab8c4 Mon Sep 17 00:00:00 2001 From: lizeyang Date: 2019年9月14日 15:25:02 +0800 Subject: [PATCH 60/65] Merge branch 'smj' of https://github.com/AutumnLight/stackoverflow-java-top-qa into AutumnLight-smj # Conflicts: # contents/how-can-i-generate-an-md5-hash.md # contents/why-is-printing-b-dramatically-slower-than-printing.md --- ...setting-multiple-jars-in-java-classpath.md | 38 ++++ ...ing-b-dramatically-slower-than-printing.md | 165 ++++++------------ 2 files changed, 94 insertions(+), 109 deletions(-) create mode 100644 contents/setting-multiple-jars-in-java-classpath.md diff --git a/contents/setting-multiple-jars-in-java-classpath.md b/contents/setting-multiple-jars-in-java-classpath.md new file mode 100644 index 0000000..6549e70 --- /dev/null +++ b/contents/setting-multiple-jars-in-java-classpath.md @@ -0,0 +1,38 @@ +##如何在classpath中设置多个jar包? + +###问题 +是否有一个方法可以在classpath选项中包含一个文件夹(目录)下的所有jar包? +我尝试运行`java -classpath lib/*.jar:. my.package.Program`,但是无法找到相应的类文件,可是这些类文件确实存在于命令中的jar包中。我是否需要在classpath中分别指定所有的jar包? + +###回答 +在使用Java6或者以上版本时,classpath选项可以支持通配符(wildcards)。使用方法如下: +* 使用直接引用(`"`) +* 使用 `*` 而不是 `*.jar` + +**Windows平台** + +`java -cp "Test.jar;lib/*" my.package.MainClass` + +**Unix平台** + +`java -cp "Test.jar:lib/*" my.package.MainClass` + +Unix平台与Windows平台基本一样,除了使用冒号 `:` 替代分号 `;` 之外。如果你不能使用通配符,也可以使用`bash`完成上述功能,命令如下(其中lib是一个包含所有jar包的目录): +`java -cp $(echo lib/*.jar | tr ' ' ':')` + +注意事项:classpath选项与-jar选项并不能兼容。可以查看:[Execute jar file with multiple classpath libraries from command prompt](http://stackoverflow.com/questions/13018100/execute-jar-file-with-multiple-classpath-libraries-from-command-prompt) + +**对于通配符的理解** +来自[Classpath](http://java.sun.com/javase/6/docs/technotes/tools/windows/classpath.html)文档: + +类路径可以包含一个基本文件名通配符`*`,其等价于指定一个特定目录下的所有以.jar或.JAR为后缀的文件的列表。例如,一个类路径的条目为`foo/*`,其指定了foo目录下的所有jar文件。一个仅仅包含`*`的classpath条目(entry)指定了当前目录下的所有jar包。 + +一个包含了`*`的classpath条目不能匹配特定目录下的class文件。为了既能匹配foo目录下的类文件,也能匹配jar包,可以使用`foo;foo/*`或`foo/*;foo`。对于前者而言,类文件和资源选择的顺序先是foo目录下的类文件和资源,之后才是jar包;而后者则正好相反。 + +通配符并不能递归地搜索子目录下的jar包。例如,`foo/*`只找`foo`目录下的jar包,而不会寻找`foo/bar`,`foo/baz`等目录下的jar包。 + +一个目录中的jar包枚举顺序并不固定,这不仅和平台有关,甚至可能会在同一个机器上因为时间不同而表现不同。一个结构良好(well-constructed)的应用不应该依赖于某个特定的顺序。如果特定顺序是不可避免的时候,就需要在classpath中显示枚举所有的jar包了。 + +在类加载进程中,通配符的扩展在早期完成,优先于程序main函数的调用,而不是在其后。每个包含通配符的类路径都被替换为所在目录下所有jar包的序列。例如,如果目录`foo`包含`a.jar`,`b.jar`以及`c.jar`,因此类路径`foo/*`被扩展为`foo/a.jar;foo/b.jar;foo/c.jar`,并且以上字符串被作为系统属性`java.class.path`的值。 + +环境变量`CLASSPATH`与命令行选项-classpath或者-cp并没有什么不同。也就是说,通配符既可以应用于命令行`-classpath/-cp`选项中,也可以应用于环境变量`CLASSPATH`中。 \ No newline at end of file diff --git a/contents/why-is-printing-b-dramatically-slower-than-printing.md b/contents/why-is-printing-b-dramatically-slower-than-printing.md index 970dbde..4887753 100644 --- a/contents/why-is-printing-b-dramatically-slower-than-printing.md +++ b/contents/why-is-printing-b-dramatically-slower-than-printing.md @@ -1,114 +1,61 @@ -# 为什么打印"B"会明显的比打印"#"慢 - -## 问题 - -我生成了两个`1000`x`1000`的矩阵: - -第一个矩阵:`O`和`#`。 -第二个矩阵:`O`和`B`。 - -使用如下的代码,生成第一个矩阵需要8.52秒: - - Random r = new Random(); - for (int i = 0; i < 1000; i++) { - for (int j = 0; j < 1000; j++) { - if(r.nextInt(4) == 0) { - System.out.print("O"); - } else { - System.out.print("#"); - } - } - - System.out.println(""); - } - - -而使用这段代码,生成第二个矩阵花费了259.152秒: - - Random r = new Random(); - for (int i = 0; i < 1000; i++) { - for (int j = 0; j < 1000; j++) { - if(r.nextInt(4) == 0) { - System.out.print("O"); - } else { - System.out.print("B"); //only line changed - } +##为什么打印B要比打印#慢很多? + +###问题 +我生成了两个大小为 1000 * 1000 的矩阵 +第一个矩阵:O和# +第二个矩阵:O和B +使用以下代码,第一个矩阵仅用时8.25s就完成了: +```java +Random r = new Random(); +for (int i = 0; i < 1000; i++) { + for (int j = 0; j < 1000; j++) { + if(r.nextInt(4) == 0) { + System.out.print("O"); + } else { + System.out.print("#"); } - - System.out.println(""); } -如此大的运行时间差异的背后究竟是什么原因呢? - ---- - -正如评论中所建议的,只打印`System.out.print("#");`用时7.8871秒,而`System.out.print("B");`则给出`still printing...`。 - -另外有人指出这段代码对他们来说是正常的, 我使用了[Ideone.com](http://ideone.com),这两段代码的执行速度是相同的。 - -测试条件: - - - 我在Netbeans 7.2中运行测试,由控制台显示输出 - - 我使用了`System.nanoTime()`来计算时间 - -## 解答一 - -*纯粹的推测*是因为你使用的终端尝试使用[单词换行][1]而不是字符换行,并且它认为`B`是一个单词而`#`却不是。所以当它到达行尾并且寻找一个换行的地方的时候,如果是`#`就可以马上换行;而如果是`B`,它则需要花更长时间搜索,因为可能会有更多的内容才能换行(在某些终端会非常费时,比如说它会先输出退格再输出空格去覆盖被换行的那部分字符)。 - -但这都只是纯粹的推测。 - - - [1]: http://en.wikipedia.org/wiki/Word_wrap - - -##解答二 - -我用Eclipse和Netbeans 8.0.2做了测试,他们的Java版本都是1.8;我用了`System.nanoTime()`来计时。 - -##Eclipse: - -我得到了**用时相同的结果** - 大约**1.564秒**。 - -##Netbeans: - -* 使用"#": **1.536秒** -* 使用"B": **44.164秒** - -所以看起来像是Netbeans输出到控制台的性能问题。 - -在做了更多研究以后我发现问题所在是Netbeans [换行][1] 的最大缓存(这并不限于`System.out.println`命令),参见以下代码: - - for (int i = 0; i < 1000; i++) { - long t1 = System.nanoTime(); - System.out.print("BBB......BBB"); \\<-contain 1000 "B" - long t2 = System.nanoTime(); - System.out.println(t2-t1); - System.out.println(""); + System.out.println(""); + } +```` +而使用相同的代码时,第二个矩阵却执行了259.152s +````java +Random r = new Random(); +for (int i = 0; i < 1000; i++) { + for (int j = 0; j < 1000; j++) { + if(r.nextInt(4) == 0) { + System.out.print("O"); + } else { + System.out.print("B"); + } } -每一个循环所花费的时间都不到1毫秒,除了 **每第五个循环**会花掉大约225毫秒。像这样(单位是毫秒): - - BBB...31744 - BBB...31744 - BBB...31744 - BBB...31744 - BBB...226365807 - BBB...31744 - BBB...31744 - BBB...31744 - BBB...31744 - BBB...226365807 - . - . - . - -以此类推。 - -##总结: - -1. 使用Eclipse打印"B"完全没有问题 -1. Netbeans有换行的问题但是可以被解决(因为在Eclipse并没有这个问题)(而不用在B后面添加空格("B "))。 - - [1]: http://en.wikipedia.org/wiki/Line_wrap_and_word_wrap - -stackoverflow原址:http://stackoverflow.com/questions/21947452/why-is-printing-b-dramatically-slower-than-printing + System.out.println(""); + } +```` +为什么两者的执行时间会有如此巨大的差距? +- - - +评论中一些人建议仅执行`System.out.print("#");`以及`System.out.print("B");`前者用时7.8871s,而后者仍然在打印(即花费时间过多--译者注) +另外有些人指出在他们那里工作正常(即两者花费时间差不错--译者注),我在[Ideone.com](http://ideone.com/)环境中运行两段代码,执行时间也差不多。 + +测试条件: +- Netbeans 7.2,结果输出在IDE的控制台 +- 使用`System.nanoTime()`度量时间 + +###回答 +纯推测:你正在使用的终端试图进行["自动换行"(word-wrapping)](http://en.wikipedia.org/wiki/Word_wrap),而不是"字符换行"(character-wrapping),并且将'B'视为一个单词字符(word character),而'#'视为一个非单词字符(non-word character)。因此当输出到达行尾时,控制台搜索一个位置用来换行,当遇到'#'时可以立即执行换行;然而遇到'B'时,控制台必须继续搜索,并且可能有更多的字符需要换行(这个操作在一些控制台上可能花销很大,例如,输出退格,然后输出空白字符来覆盖那些需要被换行的字符)。 +但是,这仅仅是理论推测。 + +**译者注:** +对于"word-wrapping"和"character-wrapping",我的理解是,它们的区别在于换行时是否在一个单词内部分割,例如在 charac-ter 中的-处换行,"word-wrapping"会将character全部移到下一行,而"character-wrapping"则将ter移到下一行,而charac依旧在原来的位置。 +**word-wrapping** +``` +******* +character +``` +**character-wrapping** +``` +*******charac +ter +``` \ No newline at end of file From 13a99f77dfb412b12456a7a0e6a18eb5fa040661 Mon Sep 17 00:00:00 2001 From: lizeyang Date: 2019年9月14日 15:30:59 +0800 Subject: [PATCH 61/65] =?UTF-8?q?=E4=BF=AE=E5=A4=8D=E9=93=BE=E6=8E=A5?= =?UTF-8?q?=E9=94=99=E8=AF=AF=E3=80=82close=20#97?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- README.md | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/README.md b/README.md index a21ea81..4580d52 100644 --- a/README.md +++ b/README.md @@ -91,7 +91,7 @@ stackoverflow-Java-top-qa * [如何在整数左填充0](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/how-can-i-pad-an-integers-with-zeros-on-the-left.md) * [在调用 instanceof 前需要进行null检查吗](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/is-null-check-needed-before-calling-instanceof.md) * [如何从文件里读取字符串](/contents/how-do-i-create-a-java-string-from-the-contents-of-a-file.md) -* [遍历集合时移除元素,怎样避免ConcurrentModificationException异常抛出](/contents/iterating-through-a-collection-avoiding-concurrentmodificationexception-when-reiterating-through-a-collection-avoiding-concurrentmodificationexception-when-re.md) +* [遍历集合时移除元素,怎样避免ConcurrentModificationException异常抛出](/contents/iterating-through-a-collection-avoiding-concurrentmodificationexception-when-re.md) * [如何让IntelliJ编辑器永久性显示代码行数](/contents/how-can-i-permanently-have-line-numbers-in-intellij.md) * [如何使用maven把项目及其依赖打包为可运行jar包](/contents/how-can-i-create-an-executable-jar-with-dependencies-using-maven.md) From c35cced4863cc655b7969534ad4138867a9fb6ef Mon Sep 17 00:00:00 2001 From: lizeyang Date: 2019年9月14日 15:58:48 +0800 Subject: [PATCH 62/65] =?UTF-8?q?=E6=9B=B4=E6=96=B0readme?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- README.md | 22 ++++++++++++---------- 1 file changed, 12 insertions(+), 10 deletions(-) diff --git a/README.md b/README.md index 4580d52..6d711f0 100644 --- a/README.md +++ b/README.md @@ -69,6 +69,10 @@ stackoverflow-Java-top-qa * [Java内部类和嵌套静态类](/contents/java-inner-class-and-static-nested-class.md) * [@Component, @Repository, @Service的区别](/contents/whats-the-difference-between-component-repository-service-annotations-in.md) * [如何创建泛型java数组](/contents/how-to-create-a-generic-array-in-java.md) +* [如何在整数左填充0](/contents/how-can-i-pad-an-integers-with-zeros-on-the-left.md) +* [Java里什么是与C++的Pair相等的?](/contents/what-is-the-equivalent-of-the-c++pair-in-java.md) +* [为什么数学函数Math.round(0.49999999999999994) 返回 1](/contents/why-does-math-round0-49999999999999994-return-1.md) +* [这段代码为什么陷入了死循环](/contents/why-does-this-go-into-an-infinite-loop.md) > 编程技巧 @@ -94,6 +98,11 @@ stackoverflow-Java-top-qa * [遍历集合时移除元素,怎样避免ConcurrentModificationException异常抛出](/contents/iterating-through-a-collection-avoiding-concurrentmodificationexception-when-re.md) * [如何让IntelliJ编辑器永久性显示代码行数](/contents/how-can-i-permanently-have-line-numbers-in-intellij.md) * [如何使用maven把项目及其依赖打包为可运行jar包](/contents/how-can-i-create-an-executable-jar-with-dependencies-using-maven.md) +* [重新导入项目到eclipse时遇到'Must Override a Superclass Method'报错](/contents/must-override-a-superclass-method-errors-after-importing-a-project-into-eclips.md) +* [如何产生一个随机的字母数字串作为 session 的唯一标识符](/contents/how-to-generate-a-random-alpha-numeric-string.md) +* [Apache Camel是什么](/contents/what-exactly-is-apache-camel.md) +* [通过对象属性对常规对象的ArrayList进行排序](/contents/sort-arraylist-of-custom-objects-by-property.md) + > 网络 @@ -106,6 +115,7 @@ stackoverflow-Java-top-qa * [为什么处理排序的数组要比非排序的快](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/why-is-processing-a-sorted-array-faster-than-an-unsorted-array.md) * [如何使用Java创建一个内存泄漏的程序](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/creating-a-memory-leak-with-java.md) * [为什么打印"B"会明显的比打印"#"慢](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/why-is-printing-b-dramatically-slower-than-printing.md) +* ["Double Brace Initialization"的效率问题](/contents/efficiency-of-java-double-brace-initialization.md) > 测试 @@ -117,16 +127,14 @@ stackoverflow-Java-top-qa * [在Android里面下载文件,并在ProgressDialog显示进度](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/download-a-file-with-android-and-showing-the-progress-in-a-progressdialog.md) * [如何获取Android设备唯一ID](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/is-there-a-unique-android-device-id.md) * [安装Android SDK的时候找不到JDK](contents/android-sdk-installation-doesnt-find-jdk.md) +* [安卓中"UserManger.isUserAGoat()"的合适案例](contents/proper-use-cases-for-android-usermanager-isuseragoat.md) + ### 待翻译问题链接(还剩x问题) -- [Proper use cases for Android UserManager.isUserAGoat()?](http://stackoverflow.com/questions/13375357/proper-use-cases-for-android-usermanager-isuseragoat) - [Why is executing Java code in comments with certain Unicode characters allowed?](http://stackoverflow.com/questions/30727515/why-is-executing-java-code-in-comments-with-certain-unicode-characters-allowed) -- ['Must Override a Superclass Method' Errors after importing a project into Eclipse](http://stackoverflow.com/questions/1678122/must-override-a-superclass-method-errors-after-importing-a-project-into-eclips) - [Fastest way to determine if an integer's square root is an integer](http://stackoverflow.com/questions/295579/fastest-way-to-determine-if-an-integers-square-root-is-an-integer) - [How to fix: Unsupported major.minor version 51.0 error?](http://stackoverflow.com/questions/10382929/how-to-fix-unsupported-major-minor-version-51-0-error) -- [How to generate a random alpha-numeric string?](http://stackoverflow.com/questions/41107/how-to-generate-a-random-alpha-numeric-string) - [Failed to load the JNI shared Library (JDK)](http://stackoverflow.com/questions/7352493/failed-to-load-the-jni-shared-library-jdk) -- [What exactly is Apache Camel?](http://stackoverflow.com/questions/8845186/what-exactly-is-apache-camel) - [Access restriction on class due to restriction on required library rt.jar?](http://stackoverflow.com/questions/860187/access-restriction-on-class-due-to-restriction-on-required-library-rt-jar) - [How do I discover memory usage of my application in Android?](http://stackoverflow.com/questions/2298208/how-do-i-discover-memory-usage-of-my-application-in-android) - [Uncatchable ChuckNorrisException](http://stackoverflow.com/questions/13883166/uncatchable-chucknorrisexception) @@ -135,14 +143,8 @@ stackoverflow-Java-top-qa - [Setting multiple jars in java classpath](http://stackoverflow.com/questions/219585/setting-multiple-jars-in-java-classpath) - [What is the Java equivalent for LINQ?](http://stackoverflow.com/questions/1217228/what-is-the-java-equivalent-for-linq) - [Hibernate hbm2ddl.auto possible values and what they do?](http://stackoverflow.com/questions/438146/hibernate-hbm2ddl-auto-possible-values-and-what-they-do) -- [How can I pad an integers with zeros on the left?](http://stackoverflow.com/questions/473282/how-can-i-pad-an-integers-with-zeros-on-the-left) -- [Sort ArrayList of custom Objects by property](http://stackoverflow.com/questions/2784514/sort-arraylist-of-custom-objects-by-property) -- [Efficiency of Java "Double Brace Initialization"?](http://stackoverflow.com/questions/924285/efficiency-of-java-double-brace-initialization) -- [Why does this go into an infinite loop?](http://stackoverflow.com/questions/3831341/why-does-this-go-into-an-infinite-loop) - [decompiling DEX into Java sourcecode](http://stackoverflow.com/questions/1249973/decompiling-dex-into-java-sourcecode) -- [Why does Math.round(0.49999999999999994) return 1](http://stackoverflow.com/questions/9902968/why-does-math-round0-49999999999999994-return-1) - [Eclipse: Set maximum line length for auto formatting?](http://stackoverflow.com/questions/3697287/eclipse-set-maximum-line-length-for-auto-formatting) -- [What is the equivalent of the C++ Pair in Java?](http://stackoverflow.com/questions/156275/what-is-the-equivalent-of-the-c-pairl-r-in-java) - [How do I "decompile" Java class files?](http://stackoverflow.com/questions/272535/how-do-i-decompile-java-class-files) - [Useful Eclipse Java Code Templates [closed]](http://stackoverflow.com/questions/1028858/useful-eclipse-java-code-templates) - [How to call SOAP web service in Android](http://stackoverflow.com/questions/297586/how-to-call-soap-web-service-in-android) From 926e85d9bcd7e627f2f5eee63b508c98bd2f4b65 Mon Sep 17 00:00:00 2001 From: lizeyang Date: 2019年9月14日 16:25:03 +0800 Subject: [PATCH 63/65] =?UTF-8?q?=E9=83=A8=E5=88=86=E7=BF=BB=E8=AF=91?= =?UTF-8?q?=E6=B2=A1=E6=9C=89=E5=8A=A0stackoverlow=E5=8E=9F=E6=96=87?= =?UTF-8?q?=E5=9C=B0=E5=9D=80?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- README.md | 9 +- ...maximun-line-length-for-auto-formatting.md | 4 +- ...led-to-load-the-JNI-shared-library(JDK).md | 2 + .../how-do-i-decompile-java-class-files.md | 2 + ...setting-multiple-jars-in-java-classpath.md | 4 +- contents/what-the-equivalent-of-x=x++.md | 106 ------------------ .../why-does-this-go-into-an-infinite-loop | 2 - "345円244円207円345円277円230円.md" | 1 - 8 files changed, 14 insertions(+), 116 deletions(-) delete mode 100644 contents/what-the-equivalent-of-x=x++.md delete mode 100644 contents/why-does-this-go-into-an-infinite-loop delete mode 100644 "345円244円207円345円277円230円.md" diff --git a/README.md b/README.md index 6d711f0..447ce35 100644 --- a/README.md +++ b/README.md @@ -102,7 +102,10 @@ stackoverflow-Java-top-qa * [如何产生一个随机的字母数字串作为 session 的唯一标识符](/contents/how-to-generate-a-random-alpha-numeric-string.md) * [Apache Camel是什么](/contents/what-exactly-is-apache-camel.md) * [通过对象属性对常规对象的ArrayList进行排序](/contents/sort-arraylist-of-custom-objects-by-property.md) - +* [为Eclipse自动代码格式化设置行的最大长度?](/contents/Eclipse-set-maximun-line-length-for-auto-formatting.md) +* [加载JNI共享库失败(JDK)](/contents/failed-to-load-the-JNI-shared-library(JDK).md) +* [如何对Java class文件进行反编译](/contents/how-do-i-decompile-java-class-files.md) +* [如何在classpath中设置多个jar包](/contents/setting-multiple-jars-in-java-classpath.md) > 网络 @@ -134,18 +137,14 @@ stackoverflow-Java-top-qa - [Why is executing Java code in comments with certain Unicode characters allowed?](http://stackoverflow.com/questions/30727515/why-is-executing-java-code-in-comments-with-certain-unicode-characters-allowed) - [Fastest way to determine if an integer's square root is an integer](http://stackoverflow.com/questions/295579/fastest-way-to-determine-if-an-integers-square-root-is-an-integer) - [How to fix: Unsupported major.minor version 51.0 error?](http://stackoverflow.com/questions/10382929/how-to-fix-unsupported-major-minor-version-51-0-error) -- [Failed to load the JNI shared Library (JDK)](http://stackoverflow.com/questions/7352493/failed-to-load-the-jni-shared-library-jdk) - [Access restriction on class due to restriction on required library rt.jar?](http://stackoverflow.com/questions/860187/access-restriction-on-class-due-to-restriction-on-required-library-rt-jar) - [How do I discover memory usage of my application in Android?](http://stackoverflow.com/questions/2298208/how-do-i-discover-memory-usage-of-my-application-in-android) - [Uncatchable ChuckNorrisException](http://stackoverflow.com/questions/13883166/uncatchable-chucknorrisexception) - [Can I add jars to maven 2 build classpath without installing them?](http://stackoverflow.com/questions/364114/can-i-add-jars-to-maven-2-build-classpath-without-installing-them) - [Update Eclipse with Android development tools v. 23](http://stackoverflow.com/questions/24437564/update-eclipse-with-android-development-tools-v-23) -- [Setting multiple jars in java classpath](http://stackoverflow.com/questions/219585/setting-multiple-jars-in-java-classpath) - [What is the Java equivalent for LINQ?](http://stackoverflow.com/questions/1217228/what-is-the-java-equivalent-for-linq) - [Hibernate hbm2ddl.auto possible values and what they do?](http://stackoverflow.com/questions/438146/hibernate-hbm2ddl-auto-possible-values-and-what-they-do) - [decompiling DEX into Java sourcecode](http://stackoverflow.com/questions/1249973/decompiling-dex-into-java-sourcecode) -- [Eclipse: Set maximum line length for auto formatting?](http://stackoverflow.com/questions/3697287/eclipse-set-maximum-line-length-for-auto-formatting) -- [How do I "decompile" Java class files?](http://stackoverflow.com/questions/272535/how-do-i-decompile-java-class-files) - [Useful Eclipse Java Code Templates [closed]](http://stackoverflow.com/questions/1028858/useful-eclipse-java-code-templates) - [How to call SOAP web service in Android](http://stackoverflow.com/questions/297586/how-to-call-soap-web-service-in-android) diff --git a/contents/Eclipse-set-maximun-line-length-for-auto-formatting.md b/contents/Eclipse-set-maximun-line-length-for-auto-formatting.md index b1f8579..d00fbae 100644 --- a/contents/Eclipse-set-maximun-line-length-for-auto-formatting.md +++ b/contents/Eclipse-set-maximun-line-length-for-auto-formatting.md @@ -1,4 +1,6 @@ 为自动代码调整设置最大的行数? 问题:我正在学习Java。如果我在Eclipse Helios里使用ctrl+shift+f的组合键,它会自动调整我的代码。一定程度下,它会改变行数。我想增加行数的最大值。应该怎么做? -回答,在偏好设置里,分别点击Java->Code Style->Fomatter->edit,在菜单栏Line Wrapping下会有行的宽度选择(Maximun line width).你将需要编辑你的代码轮廓。 \ No newline at end of file +回答,在偏好设置里,分别点击Java->Code Style->Fomatter->edit,在菜单栏Line Wrapping下会有行的宽度选择(Maximun line width).你将需要编辑你的代码轮廓。 + +stackoverflow原址:http://stackoverflow.com/questions/3697287/eclipse-set-maximum-line-length-for-auto-formatting \ No newline at end of file diff --git a/contents/failed-to-load-the-JNI-shared-library(JDK).md b/contents/failed-to-load-the-JNI-shared-library(JDK).md index b60b2cd..8fffc87 100644 --- a/contents/failed-to-load-the-JNI-shared-library(JDK).md +++ b/contents/failed-to-load-the-JNI-shared-library(JDK).md @@ -14,4 +14,6 @@ Failed to load the JNI shared library "C:/JDK/bin/client/jvm.dll"`. ·64位的Java ·64位的Eclipse +- stackoverflow原址: +http://stackoverflow.com/questions/7352493/failed-to-load-the-jni-shared-library-jdk diff --git a/contents/how-do-i-decompile-java-class-files.md b/contents/how-do-i-decompile-java-class-files.md index 8a28fc2..f050bdd 100644 --- a/contents/how-do-i-decompile-java-class-files.md +++ b/contents/how-do-i-decompile-java-class-files.md @@ -63,3 +63,5 @@ JAD JD-GUI真的很棒。你可以打开一个jar文件并浏览代码,就好像正在使用IDE一样。好东西。 +### 原文链接 +http://stackoverflow.com/questions/272535/how-do-i-decompile-java-class-files \ No newline at end of file diff --git a/contents/setting-multiple-jars-in-java-classpath.md b/contents/setting-multiple-jars-in-java-classpath.md index 6549e70..e87a69b 100644 --- a/contents/setting-multiple-jars-in-java-classpath.md +++ b/contents/setting-multiple-jars-in-java-classpath.md @@ -35,4 +35,6 @@ Unix平台与Windows平台基本一样,除了使用冒号 `:` 替代分号 `;` 在类加载进程中,通配符的扩展在早期完成,优先于程序main函数的调用,而不是在其后。每个包含通配符的类路径都被替换为所在目录下所有jar包的序列。例如,如果目录`foo`包含`a.jar`,`b.jar`以及`c.jar`,因此类路径`foo/*`被扩展为`foo/a.jar;foo/b.jar;foo/c.jar`,并且以上字符串被作为系统属性`java.class.path`的值。 -环境变量`CLASSPATH`与命令行选项-classpath或者-cp并没有什么不同。也就是说,通配符既可以应用于命令行`-classpath/-cp`选项中,也可以应用于环境变量`CLASSPATH`中。 \ No newline at end of file +环境变量`CLASSPATH`与命令行选项-classpath或者-cp并没有什么不同。也就是说,通配符既可以应用于命令行`-classpath/-cp`选项中,也可以应用于环境变量`CLASSPATH`中。 + +stackoverflow原地址:http://stackoverflow.com/questions/219585/setting-multiple-jars-in-java-classpath \ No newline at end of file diff --git a/contents/what-the-equivalent-of-x=x++.md b/contents/what-the-equivalent-of-x=x++.md deleted file mode 100644 index 1857b40..0000000 --- a/contents/what-the-equivalent-of-x=x++.md +++ /dev/null @@ -1,106 +0,0 @@ -## x=x++最终x等于多少? -### 问题描述: -``` -public class Tests { - public static void main(String[] args) throws Exception { - int x = 0; - while(x<3) { - x = x++; - System.out.println(x); - } - } -} -``` -如上,程序会进入死循环,我们知道通常对于x自增是通过x++或者x=x+1,那么为什么x=x++不是对x自增再赋值给x? - -### 回答: -明确x=x++处理流程,相当于: -```java -int temp=x; -x++; -x=temp; -``` -#### 解释: -考虑如下类: -```java -class test { - public static void main(String[] args) { - int i=0; - i=i++; - } -} -``` -通过运行反汇编: -```bash -$ javap -c test -``` -可以得到: -```java -Compiled from "test.java" -class test extends java.lang.Object{ -test(); - Code: - 0: aload_0 - 1: invokespecial #1; //Method java/lang/Object."":()V - 4: return - -public static void main(java.lang.String[]); - Code: - 0: iconst_0 - 1: istore_1 - 2: iload_1 - 3: iinc 1, 1 - 6: istore_1 - 7: return -} -``` -以下来解释其中的助记符: -* iconst_0: 常量0被push入栈。 - -* istore_1: 栈顶元素被pop出,然后存储在索引为1的局部变量也就是x内。 - -* iload_1 : 索引为1的局部变量(即x)的值被push入栈,此时栈顶值为0。 - -* iinc 1, 1 :索引为1的量增加为1,也就是x变为1。 - -* istore_1 : 栈顶值被pop出,存入索引为1的局部变量中,因此x再次变为0。 - -以上,可以知道x=x++由于入栈出栈,x的值并未变化。 - -#### 对比:x=++x -```java -class test { - public static void main(String[] args) { - int i=0; - i=++i; - } -} -``` -运行 -```bash -$ javac test.java -$ javap -c test -``` -得到: -```java -Compiled from "test.java" -class test { - test(); - Code: - 0: aload_0 - 1: invokespecial #1 // Method java/lang/Object."":()V - 4: return - - public static void main(java.lang.String[]); - Code: - 0: iconst_0 - 1: istore_1 - 2: iinc 1, 1 - 5: iload_1 - 6: istore_1 - 7: return -} -``` -可以看出是先对x进行加一,再将x的值push入栈,随后再将栈顶值赋给x。 - - diff --git a/contents/why-does-this-go-into-an-infinite-loop b/contents/why-does-this-go-into-an-infinite-loop deleted file mode 100644 index 33787a9..0000000 --- a/contents/why-does-this-go-into-an-infinite-loop +++ /dev/null @@ -1,2 +0,0 @@ -## 为什么它会进入死循环 - diff --git "a/345円244円207円345円277円230円.md" "b/345円244円207円345円277円230円.md" deleted file mode 100644 index c2e3070..0000000 --- "a/345円244円207円345円277円230円.md" +++ /dev/null @@ -1 +0,0 @@ -就是不行了 From 35dfc6f8b8f3669e30205eb291af3e148406bd2f Mon Sep 17 00:00:00 2001 From: lizeyang Date: 2019年9月14日 16:26:02 +0800 Subject: [PATCH 64/65] =?UTF-8?q?=E6=9B=B4=E6=96=B0readme?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- README.md | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/README.md b/README.md index 447ce35..10f931c 100644 --- a/README.md +++ b/README.md @@ -133,7 +133,7 @@ stackoverflow-Java-top-qa * [安卓中"UserManger.isUserAGoat()"的合适案例](contents/proper-use-cases-for-android-usermanager-isuseragoat.md) -### 待翻译问题链接(还剩x问题) +### 待翻译问题链接(还剩 13 问题) - [Why is executing Java code in comments with certain Unicode characters allowed?](http://stackoverflow.com/questions/30727515/why-is-executing-java-code-in-comments-with-certain-unicode-characters-allowed) - [Fastest way to determine if an integer's square root is an integer](http://stackoverflow.com/questions/295579/fastest-way-to-determine-if-an-integers-square-root-is-an-integer) - [How to fix: Unsupported major.minor version 51.0 error?](http://stackoverflow.com/questions/10382929/how-to-fix-unsupported-major-minor-version-51-0-error) From db68803d271ce43b7fdf4e42f6680154c91f3d9a Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?=E6=9D=8E=E6=B3=BD=E6=89=AC?= <51961070@qq.com> Date: 2019年9月17日 08:31:00 +0800 Subject: [PATCH 65/65] Update README.md --- README.md | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/README.md b/README.md index 10f931c..e6c4632 100644 --- a/README.md +++ b/README.md @@ -9,7 +9,7 @@ stackoverflow-Java-top-qa 对于参与翻译的人,这也是很好的一个学习、理解过程,欢迎大家一起来翻译 ------------- -### 如何参与翻译(欢迎加入翻译组QQ群485011036) +### 如何参与翻译 #### 如何参与: - 请从下文"待翻译问题链接"中寻找你感兴趣的问答进行翻译。翻译好的问答,放到contents目录下,无需更新readme.md文档。之后提一个PR,我负责合并PR并更新到readme中。 - 另外,为了避免多人重复新翻译一个问题,你可以提issue,说明你计划翻译的问题及时间点,我可以先更新到下面的"未翻译问题"中,说明已有人领了这个问题。当然,也不一定要提issue,一般情况下,只要及时提pr,我及时审核,出现"撞车"的概率并不高。

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