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Commit 9e3a9cc

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Merge pull request #1 from giantray/master
update
2 parents ca5550b + db68803 commit 9e3a9cc

28 files changed

+1138
-183
lines changed

‎README.md‎

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Original file line numberDiff line numberDiff line change
@@ -9,22 +9,22 @@ stackoverflow-Java-top-qa
99
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
1010

1111
-------------
12-
### 如何参与翻译(欢迎加入翻译组QQ群485011036)
13-
####如何参与:
12+
### 如何参与翻译
13+
####如何参与:
1414
- 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。
1515
- 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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####一些基本的约定:
17+
####一些基本的约定:
1818
- 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
1919
- 在每篇翻译文档内容的最后,要附上stackoverflow的原文链接
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####每个人可以做(但不限于):
21+
####每个人可以做(但不限于):
2222
- 找未翻译的问题进行翻译
2323
- 优化已翻译的问题
2424
- 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)
2525
- 输出gitbook版本(现在直接在github上查看,体验不好)
2626

27-
####文档优化反馈:
27+
####文档优化反馈:
2828
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!
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@@ -61,7 +61,6 @@ stackoverflow-Java-top-qa
6161
* [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)
6262
* [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)
6363
* [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)
64-
* [Java 中如何将 String 转换为 enum](/contents/lookup-enum-by-string-value.md)
6564
* [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)
6665
* [servlets的运行原理](/contents/how-do-servlets-work-instantiation-shared-variables-and-multithreading.md)
6766
* [如何计算MD5值](/contents/how-can-i-generate-an-md5-hash.md)
@@ -70,6 +69,10 @@ stackoverflow-Java-top-qa
7069
* [Java内部类和嵌套静态类](/contents/java-inner-class-and-static-nested-class.md)
7170
* [@Component, @Repository, @Service的区别](/contents/whats-the-difference-between-component-repository-service-annotations-in.md)
7271
* [如何创建泛型java数组](/contents/how-to-create-a-generic-array-in-java.md)
72+
* [如何在整数左填充0](/contents/how-can-i-pad-an-integers-with-zeros-on-the-left.md)
73+
* [Java里什么是与C++的Pair<L,R>相等的?](/contents/what-is-the-equivalent-of-the-c++pair-in-java.md)
74+
* [为什么数学函数Math.round(0.49999999999999994) 返回 1](/contents/why-does-math-round0-49999999999999994-return-1.md)
75+
* [这段代码为什么陷入了死循环](/contents/why-does-this-go-into-an-infinite-loop.md)
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> 编程技巧
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@@ -92,9 +95,17 @@ stackoverflow-Java-top-qa
9295
* [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)
9396
* [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)
9497
* [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)
95-
* [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)
98+
* [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)
9699
* [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)
97100
* [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)
101+
* [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)
102+
* [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)
103+
* [Apache Camel是什么](/contents/what-exactly-is-apache-camel.md)
104+
* [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)
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* [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)
106+
* [加载JNI共享库失败(JDK)](/contents/failed-to-load-the-JNI-shared-library(JDK).md)
107+
* [如何对Java class文件进行反编译](/contents/how-do-i-decompile-java-class-files.md)
108+
* [如何在classpath中设置多个jar包](/contents/setting-multiple-jars-in-java-classpath.md)
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> 网络
100111
@@ -107,6 +118,7 @@ stackoverflow-Java-top-qa
107118
* [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)
108119
* [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)
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* [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)
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* ["Double Brace Initialization"的效率问题](/contents/efficiency-of-java-double-brace-initialization.md)
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> 测试
112124
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* [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)
119131
* [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)
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* [安装Android SDK的时候找不到JDK](contents/android-sdk-installation-doesnt-find-jdk.md)
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* [安卓中"UserManger.isUserAGoat()"的合适案例](contents/proper-use-cases-for-android-usermanager-isuseragoat.md)
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### 待翻译问题链接(还剩x问题)
123-
-[Proper use cases for Android UserManager.isUserAGoat()?](http://stackoverflow.com/questions/13375357/proper-use-cases-for-android-usermanager-isuseragoat)
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### 待翻译问题链接(还剩 13 问题)
124137
- [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)
125-
- [Convert a String to an enum in Java](http://stackoverflow.com/questions/604424/convert-a-string-to-an-enum-in-java)
126-
- ['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)
127138
- [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)
128139
- [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)
129-
- [How to generate a random alpha-numeric string?](http://stackoverflow.com/questions/41107/how-to-generate-a-random-alpha-numeric-string)
130-
- [Failed to load the JNI shared Library (JDK)](http://stackoverflow.com/questions/7352493/failed-to-load-the-jni-shared-library-jdk)
131-
- [What exactly is Apache Camel?](http://stackoverflow.com/questions/8845186/what-exactly-is-apache-camel)
132140
- [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)
133141
- [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)
134142
- [Uncatchable ChuckNorrisException](http://stackoverflow.com/questions/13883166/uncatchable-chucknorrisexception)
135143
- [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)
136144
- [Update Eclipse with Android development tools v. 23](http://stackoverflow.com/questions/24437564/update-eclipse-with-android-development-tools-v-23)
137-
- [Setting multiple jars in java classpath](http://stackoverflow.com/questions/219585/setting-multiple-jars-in-java-classpath)
138145
- [What is the Java equivalent for LINQ?](http://stackoverflow.com/questions/1217228/what-is-the-java-equivalent-for-linq)
139146
- [Hibernate hbm2ddl.auto possible values and what they do?](http://stackoverflow.com/questions/438146/hibernate-hbm2ddl-auto-possible-values-and-what-they-do)
140-
- [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)
141-
- [Sort ArrayList of custom Objects by property](http://stackoverflow.com/questions/2784514/sort-arraylist-of-custom-objects-by-property)
142-
- [Efficiency of Java "Double Brace Initialization"?](http://stackoverflow.com/questions/924285/efficiency-of-java-double-brace-initialization)
143-
- [Why does this go into an infinite loop?](http://stackoverflow.com/questions/3831341/why-does-this-go-into-an-infinite-loop)
144147
- [decompiling DEX into Java sourcecode](http://stackoverflow.com/questions/1249973/decompiling-dex-into-java-sourcecode)
145-
- [Why does Math.round(0.49999999999999994) return 1](http://stackoverflow.com/questions/9902968/why-does-math-round0-49999999999999994-return-1)
146-
- [Eclipse: Set maximum line length for auto formatting?](http://stackoverflow.com/questions/3697287/eclipse-set-maximum-line-length-for-auto-formatting)
147-
- [What is the equivalent of the C++ Pair<L,R> in Java?](http://stackoverflow.com/questions/156275/what-is-the-equivalent-of-the-c-pairl-r-in-java)
148-
- [How do I "decompile" Java class files?](http://stackoverflow.com/questions/272535/how-do-i-decompile-java-class-files)
149148
- [Useful Eclipse Java Code Templates [closed]](http://stackoverflow.com/questions/1028858/useful-eclipse-java-code-templates)
150149
- [How to call SOAP web service in Android](http://stackoverflow.com/questions/297586/how-to-call-soap-web-service-in-android)
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为自动代码调整设置最大的行数?
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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?
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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。
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stackoverflow原址:http://stackoverflow.com/questions/3697287/eclipse-set-maximum-line-length-for-auto-formatting

‎contents/avoiding-null-statements-in-java.md‎

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- 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
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stackoverflow链接:
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http://stackoverflow.com/questions/271526/avoiding-null-statements-in-java?page=2&tab=votes#tab-top
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http://stackoverflow.com/questions/271526/avoiding-null-statements?page=1&tab=votes#tab-top

‎contents/convert-a-string-to-an-enum-in-java.md‎

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### 其他答案
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当文本和枚举值不同时,可以采用这种方式:
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```java
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public enum Blah {
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A("text1"),
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B("text2"),
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C("text3"),
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D("text4");
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private String text;
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Blah(String text) {
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this.text = text;
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}
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public String getText() {
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return this.text;
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}
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public static Blah fromString(String text) {
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for (Blah b : Blah.values()) {
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if (b.text.equalsIgnoreCase(text)) {
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return b;
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}
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}
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return null;
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}
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}
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```
50+
fromString方法中,throw new IllegalArgumentException("No constant with text " + text + " found") 会比直接返回null更优秀.
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### 其他答案
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2254
我有一个挺赞的工具方法:
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```java
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/**
@@ -47,4 +79,4 @@ public static MyEnum fromString(String name) {
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}
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```
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stackoverflow链接:http://stackoverflow.com/questions/604424/convert-a-string-to-an-enum-in-java
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stackoverflow链接:http://stackoverflow.com/questions/604424/convert-a-string-to-an-enum-in-java
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## "Double Brace Initialization"的效率问题
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#### 问题
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`Double Brace Initialization`是java的隐藏特性,它有着如下诱人的语法:
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```java
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Set<String> flavors = new HashSet<String>() {{
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add("vanilla");
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add("strawberry");
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add("chocolate");
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add("butter pecan");
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}};
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```
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但是,这个特性听说不是很高效率,是否要限制一次性使用?
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#### 回答
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当我使用匿名内部类时出现了如下的问题:
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```Auto
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2009年05月27日 16:35 1,602 DemoApp21ドル.class
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2009年05月27日 16:35 1,976 DemoApp210ドル.class
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2009年05月27日 16:35 1,919 DemoApp211ドル.class
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2009年05月27日 16:35 2,404 DemoApp212ドル.class
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2009年05月27日 16:35 1,197 DemoApp213ドル.class
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/* snip */
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2009年05月27日 16:35 1,953 DemoApp230ドル.class
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2009年05月27日 16:35 1,910 DemoApp231ドル.class
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2009年05月27日 16:35 2,007 DemoApp232ドル.class
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2009年05月27日 16:35 926 DemoApp233ドル1ドル1ドル.class
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2009年05月27日 16:35 4,104 DemoApp233ドル1ドル.class
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2009年05月27日 16:35 2,849 DemoApp233ドル.class
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2009年05月27日 16:35 926 DemoApp234ドル1ドル1ドル.class
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2009年05月27日 16:35 4,234 DemoApp234ドル1ドル.class
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2009年05月27日 16:35 2,849 DemoApp234ドル.class
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/* snip */
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2009年05月27日 16:35 614 DemoApp240ドル.class
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2009年05月27日 16:35 2,344 DemoApp25ドル.class
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2009年05月27日 16:35 1,551 DemoApp26ドル.class
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2009年05月27日 16:35 1,604 DemoApp27ドル.class
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2009年05月27日 16:35 1,809 DemoApp28ドル.class
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2009年05月27日 16:35 2,022 DemoApp29ドル.class
49+
```
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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。
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`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。
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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。
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这个可以说是`Double Brace Initialization`的开销。所以尽量不要过分使用。
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---
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在java的介绍中,`Double Brace Initialization`有着如下的写法:
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```java
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List<String> list = new ArrayList<String>() {{
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add("Hello");
66+
add("World!");
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}};
68+
```
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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:
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```java
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List<String> list = new ArrayList<String>() {
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// 实例初始化块
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{
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add("Hello");
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add("World!");
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}
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};
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```
82+
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所以,它只是在匿名内部类中加上了实例初始化块而已。
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---
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Joshua Bloch希望以后的集合代码能够像这样简介:
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```java
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List<Integer> intList = [1, 2, 3, 4];
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Set<String> strSet = {"Apple", "Banana", "Cactus"};
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Map<String, Integer> truthMap = { "answer" : 42 };
95+
```
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但目前还没有这样的语法。
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---
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<strong>实践</strong>
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做一个简单的试验:创建1000个带着`"Hello"`和`"World!"`元素的`ArrayList`
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* 方法1:Double Brace Initialization
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```
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List<String> l = new ArrayList<String>() {{
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add("Hello");
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add("World!");
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}};
112+
```
113+
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* 方法2:初始化`ArrayList`并调用`add`方法
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```java
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List<String> l = new ArrayList<String>();
118+
l.add("Hello");
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l.add("World!");
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```
121+
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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
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* 方法1的测试
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```java
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class Test1 {
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public static void main(String[] s) {
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long st = System.currentTimeMillis();
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List<String> l0 = new ArrayList<String>() {{
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add("Hello");
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add("World!");
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}};
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List<String> l1 = new ArrayList<String>() {{
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add("Hello");
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add("World!");
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}};
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/* snip */
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List<String> l999 = new ArrayList<String>() {{
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add("Hello");
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add("World!");
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}};
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System.out.println(System.currentTimeMillis() - st);
149+
}
150+
```
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* 方法2的测试
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```java
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class Test2 {
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public static void main(String[] s) {
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long st = System.currentTimeMillis();
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List<String> l0 = new ArrayList<String>();
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l0.add("Hello");
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l0.add("World!");
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List<String> l1 = new ArrayList<String>();
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l1.add("Hello");
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l1.add("World!");
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/* snip */
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List<String> l999 = new ArrayList<String>();
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l999.add("Hello");
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l999.add("World!");
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System.out.println(System.currentTimeMillis() - st);
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}
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}
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```
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然后得出了下面的测试时间:
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```Auto
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Test1 Times (ms) Test2 Times (ms)
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---------------- ----------------
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187 0
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203 0
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203 0
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188 0
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188 0
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187 0
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203 0
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188 0
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188 0
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203 0
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```
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从上面我们可以看到,`Double Brace Initialization`平均时间花费了190ms左右。
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同时,另外一种方法平均只用了0ms。
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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。
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## StackOverflow地址
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[http://stackoverflow.com/questions/924285/efficiency-of-java-double-brace-initialization](http://stackoverflow.com/questions/924285/efficiency-of-java-double-brace-initialization)

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