==和equals的简单比较-白红宇

==和equals的简单比较

阅读量：7093 次

发布时间：2019-06-28

本文共 17174 字，大约阅读时间需要 57 分钟。

前言：==和equals这个两个东西，经常放在一块做比较，下面我也给出一个简单的例子，对他俩进行一个简单的比较，先看例子，然后在看结论。（实验环境：win7+jdk7）

1：==和equals简单比较的实例代码如下

public class BooleanTestMain {    public static void main(String [] args){        Boolean _Boolean1 = true;        Boolean _Boolean2 = true;        Boolean _Boolean3 = new Boolean(true);        Boolean _Boolean4 = new Boolean(true);        //输入对象的hashCode和identityHashCode，当对应的类没有重写hashCode()方法时，hashCode和identityHashCode是一致的        System.out.println("_Boolean1.hashCode() is : "+_Boolean1.hashCode()+"   System.identityHashCode(_Boolean1) is : "+ System.identityHashCode(_Boolean1));        System.out.println("_Boolean2.hashCode() is : "+_Boolean2.hashCode()+"   System.identityHashCode(_Boolean2) is : "+ System.identityHashCode(_Boolean2));        System.out.println("_Boolean3.hashCode() is : "+_Boolean3.hashCode()+"   System.identityHashCode(_Boolean3) is : "+ System.identityHashCode(_Boolean3));        System.out.println("_Boolean4.hashCode() is : "+_Boolean4.hashCode()+"   System.identityHashCode(_Boolean4) is : "+ System.identityHashCode(_Boolean4));        //==比较的是对象引用地址值        System.out.println("_Boolean1==_Boolean2 is : "+(_Boolean1==_Boolean2));        System.out.println("_Boolean3==_Boolean4 is : "+(_Boolean3==_Boolean4));        System.out.println("_Boolean1==_Boolean3 is : "+(_Boolean1==_Boolean3));        //equals比较的是对象的值，如果对象没有重写equals的话，其作用和==一样，因为Object类的equals也是通过==来比较的        System.out.println("_Boolean1.equals(_Boolean2) is : "+(_Boolean1.equals(_Boolean2)));        System.out.println("_Boolean3.equals(_Boolean4) is : "+(_Boolean3.equals(_Boolean4)));        System.out.println("_Boolean1.equals(_Boolean3) is : "+(_Boolean1.equals(_Boolean3)));    }}

上例运行的结果如下：

"C:\Program Files\Java\jdk1.7.0_71\bin\java" -Didea.launcher.port=7532 "-Didea.launcher.bin.path=C:\Program Files (x86)\JetBrains\IntelliJ IDEA 2016.1.3\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.7.0_71\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jce.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jfxrt.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\resources.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\rt.jar;D:\workspace_test\hello-world\out\production\hello-world;C:\Program Files (x86)\JetBrains\IntelliJ IDEA 2016.1.3\lib\idea_rt.jar" com.intellij.rt.execution.application.AppMain com.jd.test.equ.BooleanTestMain_Boolean1.hashCode() is : 1231   System.identityHashCode(_Boolean1) is : 1969344019_Boolean2.hashCode() is : 1231   System.identityHashCode(_Boolean2) is : 1969344019_Boolean3.hashCode() is : 1231   System.identityHashCode(_Boolean3) is : 1746959769_Boolean4.hashCode() is : 1231   System.identityHashCode(_Boolean4) is : 756415799_Boolean1==_Boolean2 is : true_Boolean3==_Boolean4 is : false_Boolean1==_Boolean3 is : false_Boolean1.equals(_Boolean2) is : true_Boolean3.equals(_Boolean4) is : true_Boolean1.equals(_Boolean3) is : trueProcess finished with exit code 0

2：Boolean类的部分源码如下所示，重写了Object的hashCode()和equals()方法

/**     * Returns a hash code for this {
    @code Boolean} object.     *     * @return  the integer {
    @code 1231} if this object represents     * {
    @code true}; returns the integer {
    @code 1237} if this     * object represents {
    @code false}.     */    public int hashCode() {        return value ? 1231 : 1237;    }    /**     * Returns {
    @code true} if and only if the argument is not     * {
    @code null} and is a {
    @code Boolean} object that     * represents the same {
    @code boolean} value as this object.     *     * @param   obj   the object to compare with.     * @return  {
    @code true} if the Boolean objects represent the     *          same value; {
    @code false} otherwise.     */    public boolean equals(Object obj) {        if (obj instanceof Boolean) {            return value == ((Boolean)obj).booleanValue();        }        return false;    }

3：Object类的equals()方法的源码如下所示，也是通过==来比较的

/**     * Indicates whether some other object is "equal to" this one.     *      * The {
    @code equals} method implements an equivalence relation     * on non-null object references:     * 
     
           * 
      It is reflexive: for any non-null reference value     *     {
    @code x}, {
    @code x.equals(x)} should return     *     {
    @code true}.     * 
      It is symmetric: for any non-null reference values     *     {
    @code x} and {
    @code y}, {
    @code x.equals(y)}     *     should return {
    @code true} if and only if     *     {
    @code y.equals(x)} returns {
    @code true}.     * 
      It is transitive: for any non-null reference values     *     {
    @code x}, {
    @code y}, and {
    @code z}, if     *     {
    @code x.equals(y)} returns {
    @code true} and     *     {
    @code y.equals(z)} returns {
    @code true}, then     *     {
    @code x.equals(z)} should return {
    @code true}.     * 
      It is consistent: for any non-null reference values     *     {
    @code x} and {
    @code y}, multiple invocations of     *     {
    @code x.equals(y)} consistently return {
    @code true}     *     or consistently return {
    @code false}, provided no     *     information used in {
    @code equals} comparisons on the     *     objects is modified.     * 
      For any non-null reference value {
    @code x},     *     {
    @code x.equals(null)} should return {
    @code false}.     * 
     
     *      * The {
    @code equals} method for class {
    @code Object} implements     * the most discriminating possible equivalence relation on objects;     * that is, for any non-null reference values {
    @code x} and     * {
    @code y}, this method returns {
    @code true} if and only     * if {
    @code x} and {
    @code y} refer to the same object     * ({
    @code x == y} has the value {
    @code true}).     * 
     * Note that it is generally necessary to override the {
    @code hashCode}     * method whenever this method is overridden, so as to maintain the     * general contract for the {
    @code hashCode} method, which states     * that equal objects must have equal hash codes.     *     * @param   obj   the reference object with which to compare.     * @return  {
    @code true} if this object is the same as the obj     *          argument; {
    @code false} otherwise.     * @see     #hashCode()     * @see     java.util.HashMap     */    public boolean equals(Object obj) {        return (this == obj);    }

4：为了认清==比较的本质，我又多实验了一把，如下所示

public class EquStringTestMain {    public static void main(String[]args){        String _String1 = "godtrue";        String _String2 = "godtrue";        System.out.println("_String1 value is : "+_String1+"\n_String2 value is : "+_String2);        System.out.println("_String1 identityHashCode is :"+System.identityHashCode(_String1)+"\n_String2 identityHashCode is :"+System.identityHashCode(_String2));        System.out.println("_String1==_String2 is : "+(_String1==_String2)+"\n");        String _String3 = new String("godtrue");        String _String4 = new String("godtrue");        System.out.println("_String3 value is : "+_String3+"\n_String4 value is : "+_String4);        System.out.println("_String3 identityHashCode is :"+System.identityHashCode(_String3)+"\n_String4 identityHashCode is :"+System.identityHashCode(_String4));        System.out.println("_String3==_String4 is : "+(_String3==_String4)+"\n");        String _String5 = _String1;        String _String6 = _String1;        System.out.println("_String5 value is : "+_String5+"\n_String6 value is : "+_String6);        System.out.println("_String5 identityHashCode is :"+System.identityHashCode(_String5)+"\n_String6 identityHashCode is :"+System.identityHashCode(_String6));        System.out.println("_String5==_String6 is : "+(_String5==_String6)+"\n");        String _String7 = _String3;        String _String8 = _String3;        System.out.println("_String7 value is : "+_String7+"\n_String8 value is : "+_String8);        System.out.println("_String7 identityHashCode is :"+System.identityHashCode(_String7)+"\n_String8 identityHashCode is :"+System.identityHashCode(_String8));        System.out.println("_String7==_String8 is : "+(_String7==_String8)+"\n");        String _String9 = _String1;        String _String0 = _String3;        System.out.println("_String9 value is : "+_String9+"\n_String0 value is : "+_String0);        System.out.println("_String9 identityHashCode is :"+System.identityHashCode(_String9)+"\n_String0 identityHashCode is :"+System.identityHashCode(_String0));        System.out.println("_String9==_String0 is : "+(_String9==_String0)+"\n");    }}

上例运行的结果，如下所示（每次运行identityHashCode都不同的哟）：

"C:\Program Files\Java\jdk1.7.0_71\bin\java" -Didea.launcher.port=7534 "-Didea.launcher.bin.path=C:\Program Files (x86)\JetBrains\IntelliJ IDEA 2016.1.3\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.7.0_71\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jce.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jfxrt.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\resources.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\rt.jar;D:\workspace_test\hello-world\out\production\hello-world;C:\Program Files (x86)\JetBrains\IntelliJ IDEA 2016.1.3\lib\idea_rt.jar" com.intellij.rt.execution.application.AppMain com.jd.test.equ.EquStringTestMain_String1 value is : godtrue_String2 value is : godtrue_String1 identityHashCode is :1748462787_String2 identityHashCode is :1748462787_String1==_String2 is : true_String3 value is : godtrue_String4 value is : godtrue_String3 identityHashCode is :247835561_String4 identityHashCode is :1401482194_String3==_String4 is : false_String5 value is : godtrue_String6 value is : godtrue_String5 identityHashCode is :1748462787_String6 identityHashCode is :1748462787_String5==_String6 is : true_String7 value is : godtrue_String8 value is : godtrue_String7 identityHashCode is :247835561_String8 identityHashCode is :247835561_String7==_String8 is : true_String9 value is : godtrue_String0 value is : godtrue_String9 identityHashCode is :1748462787_String0 identityHashCode is :247835561_String9==_String0 is : false

5：String类相对特殊一点，再来一个栗子尝尝

public class EquTestMain {    public static void main(String[]args){        int int1 = 9;        int int2 = 9;        System.out.println("int1 value is : "+int1+"\nint2 value is : "+int2);        System.out.println("int1 identityHashCode is :"+System.identityHashCode(int1)+"\nint2 identityHashCode is :"+System.identityHashCode(int2));        System.out.println("int1==int2 is : "+(int1==int2)+"\n");        Integer _Integer1 = 9;        Integer _Integer2 = 9;        System.out.println("_Integer1 value is : "+_Integer1+"\n_Integer2 value is : "+_Integer2);        System.out.println("_Integer1 identityHashCode is :"+System.identityHashCode(_Integer1)+"\n_Integer2 identityHashCode is :"+System.identityHashCode(_Integer2));        System.out.println("_Integer1==_Integer2 is : "+(_Integer1==_Integer2)+"\n");        Integer _Integer3 = new Integer(9);        Integer _Integer4 = new Integer(9);        System.out.println("_Integer3 value is : "+_Integer3+"\n_Integer4 value is : "+_Integer4);        System.out.println("_Integer3 identityHashCode is :"+System.identityHashCode(_Integer3)+"\n_Integer4 identityHashCode is :"+System.identityHashCode(_Integer4));        System.out.println("_Integer3==_Integer4 is : "+(_Integer3==_Integer4)+"\n");        Integer _Integer5 = int1;        Integer _Integer6 = int1;        System.out.println("_Integer5 value is : "+_Integer5+"\n_Integer6 value is : "+_Integer6);        System.out.println("_Integer5 identityHashCode is :"+System.identityHashCode(_Integer5)+"\n_Integer6 identityHashCode is :"+System.identityHashCode(_Integer6));        System.out.println("_Integer5==_Integer6 is : "+(_Integer5==_Integer6)+"\n");        Integer _Integer7 = _Integer1;        Integer _Integer8 = _Integer1;        System.out.println("_Integer7 value is : "+_Integer7+"\n_Integer8 value is : "+_Integer8);        System.out.println("_Integer7 identityHashCode is :"+System.identityHashCode(_Integer7)+"\n_Integer8 identityHashCode is :"+System.identityHashCode(_Integer8));        System.out.println("_Integer7==_Integer8 is : "+(_Integer7==_Integer8)+"\n");        Integer _Integer9 = _Integer3;        Integer _Integer0 = _Integer3;        System.out.println("_Integer9 value is : "+_Integer9+"\n_Integer0 value is : "+_Integer0);        System.out.println("_Integer9 identityHashCode is :"+System.identityHashCode(_Integer9)+"\n_Integer0 identityHashCode is :"+System.identityHashCode(_Integer0));        System.out.println("_Integer9==_Integer0 is : "+(_Integer9==_Integer0)+"\n");    }}

上例运行的结果，如下所示（每次运行identityHashCode都不同的哟）：

"C:\Program Files\Java\jdk1.7.0_71\bin\java" -Didea.launcher.port=7536 "-Didea.launcher.bin.path=C:\Program Files (x86)\JetBrains\IntelliJ IDEA 2016.1.3\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.7.0_71\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jce.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jfxrt.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\resources.jar;C:\Program Files\Java\jdk1.7.0_71\jre\lib\rt.jar;D:\workspace_test\hello-world\out\production\hello-world;C:\Program Files (x86)\JetBrains\IntelliJ IDEA 2016.1.3\lib\idea_rt.jar" com.intellij.rt.execution.application.AppMain com.jd.test.equ.EquTestMainint1 value is : 9int2 value is : 9int1 identityHashCode is :1401482194int2 identityHashCode is :1401482194int1==int2 is : true_Integer1 value is : 9_Integer2 value is : 9_Integer1 identityHashCode is :1401482194_Integer2 identityHashCode is :1401482194_Integer1==_Integer2 is : true_Integer3 value is : 9_Integer4 value is : 9_Integer3 identityHashCode is :1110594262_Integer4 identityHashCode is :1977385357_Integer3==_Integer4 is : false_Integer5 value is : 9_Integer6 value is : 9_Integer5 identityHashCode is :1401482194_Integer6 identityHashCode is :1401482194_Integer5==_Integer6 is : true_Integer7 value is : 9_Integer8 value is : 9_Integer7 identityHashCode is :1401482194_Integer8 identityHashCode is :1401482194_Integer7==_Integer8 is : true_Integer9 value is : 9_Integer0 value is : 9_Integer9 identityHashCode is :1110594262_Integer0 identityHashCode is :1110594262_Integer9==_Integer0 is : true

6：如上的代码，如果看完了，也理解了，基本上结论也出来了。（建议先看下）

6-1：==是比较运算符，当是基本数据类型时，比较的是变量的值，当是其他类型的对象时，用它比较的是两个对象的引用地址值是否相等

6-2：equals是一个方法，如果对应的类没有重现Object类的equals()方法，则和==是一样的作用

6-3：identityHashCode更能间接的代表一个对象的引用地址值

6-4：如果一个类没有重写Object类的hashCode方法那么hashCode和identityHashCode是一致的

6-5：hashCode相等时使用==比较，不一定是true

6-6：identityHashCode相等时使用==比较，一定是true

6-7：hashCode相等时使用equals比较，也不一定是true（通常可认为是true，我们自己可以随便定义的）

6-8：我们通常关心的是对象值是否相当（一个值或者多个值），所以对象之间的比较一定要使用equals方法，而非==

7：参看博文如下（观点一致，只是解释的思路或者栗子不同）

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