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‎README.md

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* [serialVersionUID 有什么作用？该如何使用？](https://github.com/giantray/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/giantray/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/giantray/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](/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](/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](/contents/when-and-how-should-i-use-a-threadlocal-variable.md)
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* [servlets的运行原理](/contents/how-do-servlets-work-instantiation-shared-variables-and-multithreading.md)
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* [如何计算MD5值](/contents/how-can-i-generate-an-md5-hash.md)
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> 编程技巧
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@@ -101,7 +106,6 @@ stackoverflow-Java-top-qa
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* [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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### 待翻译问题链接(还剩x问题)
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- [Why is subtracting these two times (in 1927) giving a strange result?](http://stackoverflow.com/questions/6841333/why-is-subtracting-these-two-times-in-1927-giving-a-strange-result)
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- [Proper use cases for Android UserManager.isUserAGoat()?](http://stackoverflow.com/questions/13375357/proper-use-cases-for-android-usermanager-isuseragoat)
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- [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)
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- [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)
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- [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)
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- [What exactly is Apache Camel?](http://stackoverflow.com/questions/8845186/what-exactly-is-apache-camel)
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- [How can I permanently have line numbers in IntelliJ?](http://stackoverflow.com/questions/13751/how-can-i-permanently-have-line-numbers-in-intellij)
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- [How do servlets work? Instantiation, shared variables and multithreading](http://stackoverflow.com/questions/3106452/how-do-servlets-work-instantiation-shared-variables-and-multithreading)
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- [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)
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- [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)
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- [How can I generate an MD5 hash?](http://stackoverflow.com/questions/415953/how-can-i-generate-an-md5-hash)
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- [Uncatchable ChuckNorrisException](http://stackoverflow.com/questions/13883166/uncatchable-chucknorrisexception)
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- [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)
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- [Update Eclipse with Android development tools v. 23](http://stackoverflow.com/questions/24437564/update-eclipse-with-android-development-tools-v-23)
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- [Setting multiple jars in java classpath](http://stackoverflow.com/questions/219585/setting-multiple-jars-in-java-classpath)
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- [Iterating through a list, avoiding ConcurrentModificationException when removing in loop](http://stackoverflow.com/questions/223918/iterating-through-a-list-avoiding-concurrentmodificationexception-when-removing)
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- [What is the Java equivalent for LINQ?](http://stackoverflow.com/questions/1217228/what-is-the-java-equivalent-for-linq)
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- [Hibernate hbm2ddl.auto possible values and what they do?](http://stackoverflow.com/questions/438146/hibernate-hbm2ddl-auto-possible-values-and-what-they-do)
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- [How to round a number to n decimal places in Java](http://stackoverflow.com/questions/153724/how-to-round-a-number-to-n-decimal-places-in-java)
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- [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)
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- [Sort ArrayList of custom Objects by property](http://stackoverflow.com/questions/2784514/sort-arraylist-of-custom-objects-by-property)
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- [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)
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- [Efficiency of Java “Double Brace Initialization”?](http://stackoverflow.com/questions/924285/efficiency-of-java-double-brace-initialization)
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- [Why does this go into an infinite loop?](http://stackoverflow.com/questions/3831341/why-does-this-go-into-an-infinite-loop)
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- [When and how should I use a ThreadLocal variable?](http://stackoverflow.com/questions/817856/when-and-how-should-i-use-a-threadlocal-variable)
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- [decompiling DEX into Java sourcecode](http://stackoverflow.com/questions/1249973/decompiling-dex-into-java-sourcecode)
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- [How to create a generic array in Java?](http://stackoverflow.com/questions/529085/how-to-create-a-generic-array-in-java)
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- [Why does Math.round(0.49999999999999994) return 1](http://stackoverflow.com/questions/9902968/why-does-math-round0-49999999999999994-return-1)

‎SUMMARY.md

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\# Summary* [前言](README.md)

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

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## 比较java枚举成员使用equal还是==
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### 问题
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我知道Java枚举会被编译成一个包含私有构造参数和一堆静态方法的类，当去比较两个枚举的时候，总是使用equals()方法，例如：
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```java
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public useEnums(SomeEnum a)
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{
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if(a.equals(SomeEnum.SOME_ENUM_VALUE))
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{
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...
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}
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...
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}
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```
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除此之外，我也可以使用 == 替代equals() 方法
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```java
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public useEnums2(SomeEnum a)
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{
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if(a == SomeEnum.SOME_ENUM_VALUE)
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{
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...
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}
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...
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}
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```
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我有5年以上的java编程经验，并且我想我也懂得 == 和 equals() 之间的区别，但是我仍然觉得很困惑，哪一个操作符才是我该使用的。
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### 答案
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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 == 。
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我使用 == ，因为无论如何，这个左值是可以为 null的
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译者补充 java.lang.Enum 中Equals 代码：
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```java
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public final boolean equals(Object other) {
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return this==other;
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}
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```
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### 额外答案
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#### 能在枚举中使用 == 进行判断？
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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。
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>JLS 8.9 Enums
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>An enum type has no instances other than those defined by its enum constants.
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>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.
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>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.)
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#### 什么时候 == 和 equals 不一样？
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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:
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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：
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##### == 不会抛出 NullPointerException
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```java
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enum Color { BLACK, WHITE };
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Color nothing = null;
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if (nothing == Color.BLACK); // runs fine
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if (nothing.equals(Color.BLACK)); // throws NullPointerException
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```
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##### == 在编译期检测类型兼容性
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```java
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enum Color { BLACK, WHITE };
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enum Chiral { LEFT, RIGHT };
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if (Color.BLACK.equals(Chiral.LEFT)); // compiles fine
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if (Color.BLACK == Chiral.LEFT); // DOESN'T COMPILE!!! Incompatible types!
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```
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#### 什么时候使用 == ？
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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.
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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。
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> Item 1: Consider static factory methods instead of constructors
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[...] 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.
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总而言之，在枚举比较上使用 == ， 因为：
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1. 能正常工作
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2. 更快
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3. 运行时是安全的
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4. 编译期也是安全的
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stackoverlfow链接：http://stackoverflow.com/questions/1750435/comparing-java-enum-members-or-equals
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# 比较java中枚举成员是用“==”还是equals()
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## 问题
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java枚举被编译成带有私有构造器和一堆public的静态成员的类。当比较枚举中的两个成员时，经常使用.equals()方法，例如
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public useEnums(SomeEnum a)
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{
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if(a.equals(SomeEnum.SOME_ENUM_VALUE))
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{
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...
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}
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...
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}
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然而，偶然间使用“==”代替equals方法
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public useEnums2(SomeEnum a)
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{
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if(a == SomeEnum.SOME_ENUM_VALUE)
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{
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...
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}
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...
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}
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应该使用哪个呢？
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## 解答
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### 回答1
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技术上来说，都是对的，如果你看了equals的源码，它简单地遵从“==” ，然而我一般使用“==” 因为它对于空指针，比较安全
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### 回答2
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#### 能在枚举时使用用“==”吗？
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可以，枚举值有小型实例控制，允许你用“==”去比较实例，在文档中有说明：
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JLS 8.9 枚举
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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。
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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。
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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。）
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这足够强力地支持Josh的建议，如果你坚持使用单例模式，最好的方法是用枚举类型强化单例属性（见Effective Java第二版中的第三条：用私有构造器或者枚举类型强化Singleton属性，或者单例模式的线程安全
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>http://stackoverflow.com/questions/2912281/thread-safety-in-singleton/ ）
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#### “==”和equals的区别
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通常情况下，==并不是可以替换equals，然而在枚举中是可以的。它们之间有两个重要的不同：
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“==”从不会抛出空指针异常
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enum Color { BLACK, WHITE };
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Color nothing = null;
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if (nothing == Color.BLACK); // 正常运行
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if (nothing.equals(Color.BLACK)); // 抛出空指
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针异常
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在编译期，"=="会检查其类型的兼容性
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enum Color { BLACK, WHITE };
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enum Chiral { LEFT, RIGHT };
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if (Color.BLACK.equals(Chiral.LEFT)); // 编译正常
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if (Color.BLACK == Chiral.LEFT); // 无法编译，类型不兼容
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#### 在适用时，“==”可以被使用吗
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Bloch（effective java的作者）明确指出不可变类可以控制它们实例保证客户端“==”是可用的。枚举就被明确地证明了
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考虑静态工厂方法代替构造器
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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
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总之，在枚举中使用“==”优势：
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- 能运行
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- 更快
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- 在运行期更安全
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- 在编译期更安全
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备注：强有力的反击了那些认为foo.equals(bar)比foo==bar更有可读性的人们。
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原文地址：
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> http://stackoverflow.com/questions/1750435/comparing-java-enum-members-or-equals

‎contents/how-can-i-generate-an-md5-hash.md

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##如何计算MD5值
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###问题
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Java中有没有方法可以计算一个String的MD5值？
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###回答
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你可以用 ```MessageDigest``` 的MD5实例来计算String的MD5值。
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使用 ```MessageDigest``` 和 String 时，一定要显式声明你的数据编码类型。如果你使用无参的 ```String.getBytes()``` , 它会以当前平台的默认编码来转换数据。不同平台的默认编码可能是不同的，这可能会导致你的数据不一致。
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``` java
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import java.security.*;
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..
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byte[] bytesOfMessage = yourString.getBytes("UTF-8");
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MessageDigest md = MessageDigest.getInstance("MD5");
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byte[] thedigest = md.digest(bytesOfMessage);
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```
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如果你的要计算的数据量很大，你可以循环使用 ```.update(byte[])``` 方法来加载数据。加载完毕后用 ```.digest()``` 方法来得到计算出的MD5值。
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stackoverflow链接
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http://stackoverflow.com/questions/415953/how-can-i-generate-an-md5-hash

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