Merge pull request giantray#25 from giantray/yannxia-master

Yannxia master

Author: giantray

README.md

@@ -42,6 +42,7 @@ stackoverflow-Java-top-qa
 * [在Java中声明数组](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/Declare-array-in-Java.md)
 * [反射是什么及其用途](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/What-is-reflection-and-why-is-it-useful.md.md)
 * [为什么不能用string类型进行switch判断](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/Why-can't-I-switch-on-a-String.md)
+* [比较java枚举成员使用equal还是==](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/comparing-java-enum-members-or-equals.md)
 
 > 编程技巧
 

contents/comparing-java-enum-members-or-equals.md

@@ -0,0 +1,83 @@
+## 比较java枚举成员使用equal还是==
+
+### 问题
+我知道Java枚举会被编译成私有构造参数和一堆静态方法的一个类，当去比较两个枚举的时候，总是使用equals()方法，例如：
+```java
+public useEnums(SomeEnum a)
+{
+    if(a.equals(SomeEnum.SOME_ENUM_VALUE))
+    {
+        ...
+    }
+    ...
+}
+```
+除此之外，我也可以使用 == 替代equals() 方法
+```java
+public useEnums2(SomeEnum a)
+{
+    if(a == SomeEnum.SOME_ENUM_VALUE)
+    {
+        ...
+    }
+    ...
+}
+```
+我已经Java编程5年以上了，并且我想我也懂得 == 和 equals() 之间的区别，但是我仍然觉得困扰在这个问题上，哪一个操作符才是我该使用的。
+
+### 答案
+
+二者皆对，如果你看过枚举的源码，你会发现在源码中，equals也仅仅非常简单的 == 。
+我使用 == ，无论如何，这个左值是可以为 null的
+
+
+译者补充 java.lang.Enum 中Equals 代码：
+```java
+public final boolean equals(Object other) {
+    return this==other;
+}
+```
+
+
+### 额外答案
+#### 能使用 == 在枚举判断中？
+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 == 去比较实力，这在语言标准内也是有保证的。
+
+>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.)
+
+#### 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：
+
+##### == 不会抛出 NullPointerException
+```java
+enum Color { BLACK, WHITE };
+
+Color nothing = null;
+if (nothing == Color.BLACK);      // runs fine
+if (nothing.equals(Color.BLACK)); // throws NullPointerException
+```
+##### == 在编译期检测类型兼容性
+```java
+enum Color { BLACK, WHITE };
+enum Chiral { LEFT, RIGHT };
+
+if (Color.BLACK.equals(Chiral.LEFT)); // compiles fine
+if (Color.BLACK == Chiral.LEFT);      // DOESN'T COMPILE!!! Incompatible types!
+```
+
+#### 什么时候使用 == ？
+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.
+
+总而言之，在枚举比较上使用 == ， 因为：
+1. 能正常工作
+2. 更快
+3. 运行时是安全的
+4. 编译期也是安全的