update

README.md

@@ -9,22 +9,22 @@ 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：
+### 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。
 
-####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 +61,6 @@ 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)
@@ -70,6 +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<L,R>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
 
@@ -92,9 +95,17 @@ 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)
+* [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)  
+* [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
 
@@ -107,6 +118,7 @@ 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
 
@@ -118,34 +130,21 @@ 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)
+
+### 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)
-- [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)
-- [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)
 - [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)
-- [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<L,R> 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)
 

contents/Eclipse-set-maximun-line-length-for-auto-formatting.md

@@ -0,0 +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。
+
+stackoverflow原址：http://stackoverflow.com/questions/3697287/eclipse-set-maximum-line-length-for-auto-formatting

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

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
+stackoverflow链接：http://stackoverflow.com/questions/604424/convert-a-string-to-an-enum-in-java

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<String> flavors = new HashSet<String>() {{
+    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
+2009/05/27  16:35             1,602 DemoApp2$1.class
+2009/05/27  16:35             1,976 DemoApp2$10.class
+2009/05/27  16:35             1,919 DemoApp2$11.class
+2009/05/27  16:35             2,404 DemoApp2$12.class
+2009/05/27  16:35             1,197 DemoApp2$13.class
+
+/* snip */
+
+2009/05/27  16:35             1,953 DemoApp2$30.class
+2009/05/27  16:35             1,910 DemoApp2$31.class
+2009/05/27  16:35             2,007 DemoApp2$32.class
+2009/05/27  16:35               926 DemoApp2$33$1$1.class
+2009/05/27  16:35             4,104 DemoApp2$33$1.class
+2009/05/27  16:35             2,849 DemoApp2$33.class
+2009/05/27  16:35               926 DemoApp2$34$1$1.class
+2009/05/27  16:35             4,234 DemoApp2$34$1.class
+2009/05/27  16:35             2,849 DemoApp2$34.class
+
+/* snip */
+
+2009/05/27  16:35               614 DemoApp2$40.class
+2009/05/27  16:35             2,344 DemoApp2$5.class
+2009/05/27  16:35             1,551 DemoApp2$6.class
+2009/05/27  16:35             1,604 DemoApp2$7.class
+2009/05/27  16:35             1,809 DemoApp2$8.class
+2009/05/27  16:35             2,022 DemoApp2$9.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<String> list = new ArrayList<String>() {{
+    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<String> list = new ArrayList<String>() {
+
+    // 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<Integer> intList = [1, 2, 3, 4];
+
+Set<String> strSet = {"Apple", "Banana", "Cactus"};
+
+Map<String, Integer> truthMap = { "answer" : 42 };
+```
+
+但目前还没有这样的语法。
+
+---
+
+<strong>实践</strong>
+
+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<String> l = new ArrayList<String>() {{
+  add("Hello");
+  add("World!");
+}};
+```
+
+* 方法2：初始化`ArrayList`并调用`add`方法
+
+```java
+List<String> l = new ArrayList<String>();
+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<String> l0 = new ArrayList<String>() {{
+      add("Hello");
+      add("World!");
+    }};
+
+    List<String> l1 = new ArrayList<String>() {{
+      add("Hello");
+      add("World!");
+    }};
+
+    /* snip */
+
+    List<String> l999 = new ArrayList<String>() {{
+      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<String> l0 = new ArrayList<String>();
+    l0.add("Hello");
+    l0.add("World!");
+
+    List<String> l1 = new ArrayList<String>();
+    l1.add("Hello");
+    l1.add("World!");
+
+    /* snip */
+
+    List<String> l999 = new ArrayList<String>();
+    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)