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‎汇总.md

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## [1. Java += 操作符实质](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/java-operator.md)
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### Java += 操作符实质
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### 问题
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我之前以为:
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i += j 等同于 i = i + j;
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但假设有:
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```java
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int i = 5;
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long j = 8;
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```
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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
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这是否意味着,i += j,实际是等同于 i= (type of i) (i + j)呢?
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### 回答
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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)
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再照搬下官方文档的说明
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对复合赋值表达式来说,E1 op= E2 (诸如 i += j; i -= j 等等),其实是等同于 E1 = (T)((E1) op (E2)),其中,T是E1这个元素的类型。
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举例来说,如下的代码
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```java
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short x = 3;
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x += 4.6;
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```
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等同于
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```java
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short x = 3;
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x = (short)(x + 4.6);
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```
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stackoverflow链接
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http://stackoverflow.com/questions/8710619/java-operator
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## [2. 将InputStream转换为String](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/read-convert-an-inputstream-to-a-string.md)
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### 将InputStream转换为String
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### 使用Apache库
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不重复造轮子。最靠谱的方法,还是用Apache commons IOUtils
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这样简单几行代码就搞定了
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```java
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StringWriter writer = new StringWriter();
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IOUtils.copy(inputStream, writer, encoding);
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String theString = writer.toString();
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```
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或者
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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
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### 使用原生库
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如果不想引入Apache库,也可以这样做
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```java
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static String convertStreamToString(java.io.InputStream is) {
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java.util.Scanner s = new java.util.Scanner(is).useDelimiter("\\A");
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return s.hasNext() ? s.next() : "";
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}
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```
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stackoverflow讨论地址
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http://stackoverflow.com/questions/309424/read-convert-an-inputstream-to-a-string
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## [3. 将数组转换为List](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/create-arraylist-from-array.md)
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### 将数组转换为List
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### 问题
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假设有数组
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```java
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Element[] array = {new Element(1),new Element(2),new Element(3)};
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```
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如何将其转换为ArrayList`<Element>` arraylist = ???
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### 回答1
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`new ArrayList<Element>(Arrays.asList(array))`
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### 回答2
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Arrays.asList(array)或者Arrays.asList(new Element(1),new Element(2),new Element(3))
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不过,这样做有些坑要注意:
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1. 这样做生成的list,是定长的。也就是说,如果你对它做add或者remove,都会抛UnsupportedOperationException。
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2. 如果修改数组的值,list中的对应值也会改变!
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**Arrays.asList() 返回的是Arrays内部静态类,而不是Java.util.ArrayList的类。这个java.util.Arrays.ArrayList有set(),get(),contains()方法,但是没有任何add() 方法,所以它是固定大小的**
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如果希望避免这两个坑,请改用这个方式
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```java
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Collections.addAll(arraylist, array);
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```
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stackoverflow原址:
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http://stackoverflow.com/questions/157944/how-to-create-arraylist-arraylistt-from-array-t
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## [4. 如何遍历map对象](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/iterate-through-a-hashmap.md)
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## [public,protected,private,不加修饰符。有什么区别呢?](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/in-java-whats-the-difference-between-public-default-protected-and-private.md)
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## [5. 如何测试一个数组是否包含指定的值?](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-can-i-test-if-an-array-contains-a-certain-value.md)
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## [6. 重写(Override)equlas和hashCode方法时应考虑的问题](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/what-issues-should-be-considered-when-overriding-equals-and-hashcode-in-java.md)
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## [7. 从一个多层嵌套循环中直接跳出](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/breaking-out-of-nested-loops-in-java.md)
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## [8. 如何将String转换为Int](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/converting-string-to-int-in-java.md)
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## [9. 如何分割(split)string字符串](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-to-split-a-string-in-java.md)
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## [10. 在java中如何对比(compare)string](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-do-i-compare-strings-in-java.md)
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## [11. `Map<Key,Value>`基于Value值排序](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-to-sort-a-mapkey-value-on-the-values-in-java.md)
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## [12. `HashMap和Hashtable的区别](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/differences-between-hashmap-and-hashtable.md)
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## [13. 如何便捷地将两个数组合到一起](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-to-concatenate-two-arrays-in-java.md)
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## [14. Java 是否支持默认的参数值](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/does-java-support-default-parameter-values.md)
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## [15. Java 产生指定范围的随机数](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/generating-random-integers-in-a-range-with-Java.md)
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## [16. JavaBean 到底是什么](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/what-is-a-javabean-exactly.md)
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## [17. wait()和sleep()的区别](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/difference-between-wait-and-sleep.md)
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## [能否在一个构造器( `constructor` )中调用另一个构造器](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-do-i-call-one-constructor-from-another-in-java.md)
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## [ `finally` 代码块总会被执行么](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/does-finally-always-execute-in-java.md)
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## [如何将String转换为enum](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/convert-a-string-to-an-enum-in-java.md)
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## [在Java中声明数组](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/declare-array-in-java.md)
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## [反射是什么及其用途](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/what-is-reflection-and-why-is-it-useful.md)
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## [为什么不能用string类型进行switch判断](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/why-cant-i-switch-on-a-string.md)
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## [比较java枚举成员使用equal还是==](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/comparing-java-enum-members-or-equals.md)
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## [用java怎么创建一个文件并向该文件写文本内容](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-to-create-a-file-and-write-to-a-file-in-java.md)
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## [serialVersionUID 有什么作用?该如何使用?](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/what-is-a-serialversionuid-and-why-should-i-use-it.md)
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## [为什么Java的```Vector```类被认为是过时的或者废弃的](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/why-is-java-vector-class-considered-obsolete-or-deprecated.md)
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## [Java的foreach循环是如何工作的](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-does-the-java-for-each-loop-work.md)
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## [为什么相减这两个时间(1927年)会得到奇怪的结果](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/why-is-subtracting-these-two-times-in-1927-giving-a-strange-result.md)
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## [Java 中如何将 String 转换为 enum](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/lookup-enum-by-string-value.md)
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## [该什么时候使用 ThreadLocal变量,它是如何工作的](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/when-and-how-should-i-use-a-threadlocal-variable.md)
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## [servlets的运行原理](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-do-servlets-work-instantiation-shared-variables-and-multithreading.md)
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## [如何计算MD5值](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-can-i-generate-an-md5-hash.md)
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## [Java中软引用和弱引用的区别](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/what-is-the-difference-between-a-soft-reference-and-a-weak-reference-in-java.md)
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## [JSF, Servlet 和 JSP (三种技术)有什么区别](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/what-is-the-difference-between-jsf-servlet-and-jsp.md)
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## [Java内部类和嵌套静态类](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/java-inner-class-and-static-nested-class.md)
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## [@Component, @Repository, @Service的区别](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/whats-the-difference-between-component-repository-service-annotations-in.md)
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## [如何创建泛型java数组](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-to-create-a-generic-array-in-java.md)
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# 编程技巧
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## [去掉烦人的"!=null"(判空语句](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/avoiding-null-statements-in-java.md)
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## [获取完整的堆栈信息](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/get-current-stack-trace-in-java.md)
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## [如何用一行代码初始化一个ArrayList](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/initialization-of-an-arraylist-in-one-line.md)
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## [初始化静态map](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-can-i-initialize-a-static-map.md)
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## [给3个布尔变量,当其中有2个或者2个以上为true才返回true](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/check-if-at-least-two-out-of-three-booleans-are-true.md)
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## [输出 Java 数组最简单的方式](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/whats-the-simplest-way-to-print-a-java-array.md)
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## [为什么以下用随机生成的文字会得出 "hello world"?](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/why-does-this-code-using-random-strings-print-hello-world.md)
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## [什么在java中存放密码更倾向于char[]而不是String](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/why-is-char-preferred-over-string-for-passwords-in-java.md)
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## [如何避免在JSP文件中使用Java代码](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-to-avoid-java-code-in-jsp-files.md)
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* [Java 源码里的设计模式](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/examples-of-gof-design-patterns-in-javas-core-libraries.md)
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* [如何产生一个随机的字母数字串作为 session 的唯一标识符](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-to-generate-a-random-alpha-numeric-string.md)
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* [如何创建单例](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/what-is-an-efficient-way-to-implement-a-singleton-in-java.md)
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* [实现Runnable接口 VS. 继承Thread类](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/implements-runnable-vs-extends-thread.md)
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* [我应该用哪一个@NotNull注解](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/which-notnull-java-annotation-should-i-use.md)
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* [怎样将堆栈追踪信息转换为字符串](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-can-i-convert-a-stack-trace-to-a-string.md)
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* [如何处理 java.lang.outOfMemoryError PermGen space error](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/dealing-with-java-lang-outofmemoryerror-permgen-space-error.md)
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* [如何在整数左填充0](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-can-i-pad-an-integers-with-zeros-on-the-left.md)
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* [在调用 instanceof 前需要进行null检查吗](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/is-null-check-needed-before-calling-instanceof.md)
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* [如何从文件里读取字符串](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-do-i-create-a-java-string-from-the-contents-of-a-file.md)
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* [遍历集合时移除元素,怎样避免ConcurrentModificationException异常抛出](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/iterating-through-a-collection-avoiding-concurrentmodificationexception-when-reiterating-through-a-collection-avoiding-concurrentmodificationexception-when-re.md)
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* [如何让IntelliJ编辑器永久性显示代码行数](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-can-i-permanently-have-line-numbers-in-intellij.md)
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* [如何使用maven把项目及其依赖打包为可运行jar包](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-can-i-create-an-executable-jar-with-dependencies-using-maven.md)
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> 网络
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* [如何使用java.net.URLConnection接收及发送HTTP请求](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/using-java-net-urlconnection-to-fire-and-handle-http-requests.md)
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> 性能
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* [LinkedList、ArrayList各自的使用场景,如何确认应该用哪一个呢?](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/when-to-use-linkedlist-over-arraylist.md)
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* [StringBuilder和StringBuffer有哪些区别呢](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/stringbuilder-and-stringbuffer.md)
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* [为什么处理排序的数组要比非排序的快](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/why-is-processing-a-sorted-array-faster-than-an-unsorted-array.md)
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* [如何使用Java创建一个内存泄漏的程序](https://github.com/linpeiyou/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/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/why-is-printing-b-dramatically-slower-than-printing.md)
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> 测试
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* [如何测试 private 方法,变量或者内部类](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-to-test-a-class-that-has-private-methods-fields-or-inner-classes.md)
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* [JUnit4如何断言确定异常的抛出](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/how-do-you-assert-that-a-certain-exception-is-thrown-in-junit-4-tests.md)
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> Android
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* [在Android里面下载文件,并在ProgressDialog显示进度](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/download-a-file-with-android-and-showing-the-progress-in-a-progressdialog.md)
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* [如何获取Android设备唯一ID](https://github.com/linpeiyou/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](https://github.com/linpeiyou/stackoverflow-java-top-qa/blob/master/contents/android-sdk-installation-doesnt-find-jdk.md)

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