hashCode和identityHashCode 的关系

1：首先看一下JDk API的观点

1-1：hashCode方法相关

1-2：identityHashCode（）方法相关

2：此例的核心程序，对应的观点在注释中已经有所说明，请自己也动手实验一下看看！

import static java.lang.System.out;
/**
 * 一个对象的hashCode和identityHashCode 的关系：
 * 1：对象的hashCode，一般是通过将该对象的内部地址转换成一个整数来实现的
 * 2：当一个类没有重写Object类的hashCode()方法时，它的hashCode和identityHashCode是一致的
 * 3：当一个类重写了Object类的hashCode()方法时，它的hashCode则有重写的实现逻辑决定，此时的hashCode值一般就不再和对象本身的内部地址有相应的哈希关系了
 * 4：当null调用hashCode方法时，会抛出空指针异常，但是调用System.identityHashCode(null)方法时能正常的返回0这个值
 * 5：一个对象的identityHashCode能够始终和该对象的内部地址有一个相对应的关系，从这个角度来讲，它可以用于代表对象的引用地址，所以，在理解==这个操作运算符的时候是比较有用的
 *
 */

public class HashCodeTestMain
{
    /**
     * 输出对象重写的hashCode和唯一的hashCode
     * @param object
     */
    public static void printHashCodes(final Object object)
    {
        //输入对象的数据类型，以及调用toString()方法后返回的字符串形式，当对象为空时，此处输出null
        out.println("
The object type is  : " + (object != null ? object.getClass().getName() : "null") + "
The object value is : "+String.valueOf(object));
        //输出对象的hashCode值，当对象为空时，此处输出N/A
        out.println("Overridden hashCode : " + (object != null ? object.hashCode() : "N/A"));
        //输出对象的identityHashCode值，如果对应的类没有重写Object类的hashCode()方法，则和默认的hashCode值一致
        out.println("Identity   hashCode : " + System.identityHashCode(object));
    }

    /**
     * 主函数，程序执行的入口
     * @param arguments
     */
    public static void main(String[] arguments)
    {
        //基本数据类型的测试数据
        final byte _byte = 6;
        final char _char = 's';
        final short _short = 6;
        final int _int = 6;
        final long _long = 6L;
        final float _float = 6;
        final double _double= 6;
        final boolean _boolean = true;

        //包装类型的测试数据
        final Byte _Byte = 9;
        final Character _Character = 'S';
        final Short _Short = 9;
        final Integer _Integer = 9;
        final Long _Long = 9L;
        final Float _Float = 9F;
        final Double _Double = 9D;
        final Boolean _Boolean = false;

        //字符串类型的测试数据
        final String someString = "someString";
        //null
        final String nullString = null;
        //自定义的测试对象
        final User user = new User(666,"godtrue");

        //基本数据类型的测试数据
        out.println("
测试基本数据类型的数据");
        printHashCodes(_byte);
        printHashCodes(_char);
        printHashCodes(_short);
        printHashCodes(_int);
        printHashCodes(_long);
        printHashCodes(_float);
        printHashCodes(_double);
        printHashCodes(_boolean);

        //包装类型的测试数据
        out.println("
测试包装数据类型的数据");
        printHashCodes(_Byte);
        printHashCodes(_Character);
        printHashCodes(_Short);
        printHashCodes(_Integer);
        printHashCodes(_Long);
        printHashCodes(_Float);
        printHashCodes(_Double);
        printHashCodes(_Boolean);

       //字符串类型的测试数据
        out.println("
测试字符串类型的数据");
        printHashCodes(someString);

        //null
        out.println("
测试null空对象");
        printHashCodes(nullString);

        //自定义的测试对象
        out.println("
测试自定义对象，构造此类的时候没有重写它的hashCode()方法");
        printHashCodes(user);
    }
}

3：User简单的自定义类，比较简单，没什么可讲的，关键是默认继承Object类，且没有重写hashCode()方法，不过重写了toString()方法

public class User {
    private Integer id;
    private String name;

    public User() {
    }

    public User(Integer id ,String name) {
        this.id = id;
        this.name = name;
    }

    @Override
    public String toString() {
        final StringBuffer sb = new StringBuffer("{"User":{");
        sb.append(""id":"").append(id).append(""").append(",");
        sb.append(""name":"").append(name).append(""");
        sb.append("}}");
        return sb.toString();
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

4：运行结果如下所示，不同的运行环境结果可能存在差异（win7+jdk7）：

"C:Program FilesJavajdk1.7.0_71injava" -Didea.launcher.port=7532 "-Didea.launcher.bin.path=C:Program Files (x86)JetBrainsIntelliJ IDEA 2016.1.3in" -Dfile.encoding=UTF-8 -classpath "C:Program FilesJavajdk1.7.0_71jrelibcharsets.jar;C:Program FilesJavajdk1.7.0_71jrelibdeploy.jar;C:Program FilesJavajdk1.7.0_71jrelibextaccess-bridge-64.jar;C:Program FilesJavajdk1.7.0_71jrelibextdnsns.jar;C:Program FilesJavajdk1.7.0_71jrelibextjaccess.jar;C:Program FilesJavajdk1.7.0_71jrelibextlocaledata.jar;C:Program FilesJavajdk1.7.0_71jrelibextsunec.jar;C:Program FilesJavajdk1.7.0_71jrelibextsunjce_provider.jar;C:Program FilesJavajdk1.7.0_71jrelibextsunmscapi.jar;C:Program FilesJavajdk1.7.0_71jrelibextzipfs.jar;C:Program FilesJavajdk1.7.0_71jrelibjavaws.jar;C:Program FilesJavajdk1.7.0_71jrelibjce.jar;C:Program FilesJavajdk1.7.0_71jrelibjfr.jar;C:Program FilesJavajdk1.7.0_71jrelibjfxrt.jar;C:Program FilesJavajdk1.7.0_71jrelibjsse.jar;C:Program FilesJavajdk1.7.0_71jrelibmanagement-agent.jar;C:Program FilesJavajdk1.7.0_71jrelibplugin.jar;C:Program FilesJavajdk1.7.0_71jrelib
esources.jar;C:Program FilesJavajdk1.7.0_71jrelib
t.jar;D:workspace_testhello-worldoutproductionhello-world;C:Program Files (x86)JetBrainsIntelliJ IDEA 2016.1.3libidea_rt.jar" com.intellij.rt.execution.application.AppMain com.jd.test.equ.HashCodeTestMain

测试基本数据类型的数据

The object type is  : java.lang.Byte
The object value is : 6
Overridden hashCode : 6
Identity   hashCode : 692191789

The object type is  : java.lang.Character
The object value is : s
Overridden hashCode : 115
Identity   hashCode : 748481924

The object type is  : java.lang.Short
The object value is : 6
Overridden hashCode : 6
Identity   hashCode : 1923976189

The object type is  : java.lang.Integer
The object value is : 6
Overridden hashCode : 6
Identity   hashCode : 1606535644

The object type is  : java.lang.Long
The object value is : 6
Overridden hashCode : 6
Identity   hashCode : 732674977

The object type is  : java.lang.Float
The object value is : 6.0
Overridden hashCode : 1086324736
Identity   hashCode : 397106541

The object type is  : java.lang.Double
The object value is : 6.0
Overridden hashCode : 1075314688
Identity   hashCode : 1937943358

The object type is  : java.lang.Boolean
The object value is : true
Overridden hashCode : 1231
Identity   hashCode : 129543857

测试包装数据类型的数据

The object type is  : java.lang.Byte
The object value is : 9
Overridden hashCode : 9
Identity   hashCode : 1842670188

The object type is  : java.lang.Character
The object value is : S
Overridden hashCode : 83
Identity   hashCode : 896176329

The object type is  : java.lang.Short
The object value is : 9
Overridden hashCode : 9
Identity   hashCode : 1732745092

The object type is  : java.lang.Integer
The object value is : 9
Overridden hashCode : 9
Identity   hashCode : 221024277

The object type is  : java.lang.Long
The object value is : 9
Overridden hashCode : 9
Identity   hashCode : 1755797876

The object type is  : java.lang.Float
The object value is : 9.0
Overridden hashCode : 1091567616
Identity   hashCode : 1122108505

The object type is  : java.lang.Double
The object value is : 9.0
Overridden hashCode : 1075970048
Identity   hashCode : 76255581

The object type is  : java.lang.Boolean
The object value is : false
Overridden hashCode : 1237
Identity   hashCode : 1727296255

测试字符串类型的数据

The object type is  : java.lang.String
The object value is : someString
Overridden hashCode : -1264993755
Identity   hashCode : 984217639

测试null空对象

The object type is  : null
The object value is : null
Overridden hashCode : N/A
Identity   hashCode : 0

测试自定义对象，构造此类的时候没有重写它的hashCode()方法

The object type is  : com.jd.test.equ.User
The object value is : {"User":{"id":"666","name":"godtrue"}}
Overridden hashCode : 1826809479
Identity   hashCode : 1826809479

Process finished with exit code 0

5：上面的代码和注释已经抛出了我的观点，代码相对简单，可能还有一些繁琐，不过经过对比更能体现出自己观点的正确性，原本是想写一篇关于==操作符相关的博文的，后来发现要能清楚某些知识点，就需要先弄明白另外一些知识点，经过试验和查阅资料就有了此篇博文了，后续再补上相关联的知识点小结。