java基础--HashCode与identityHashCode底层究竟发生了什么

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wx63ee08b3cead4 2023-02-20 00:44:25 博主文章分类：java基础 ©著作权

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Object #hashCode（）方法，其中@see java.lang.Object#equals(java.lang.Object)；@see java.lang.System#identityHashCode；说明equals，hashCode，identityHashCode之间存在着一定联系。

Object#hashCode()

/**
 * Returns a hash code value for the object. This method is
 * supported for the benefit of hash tables such as those provided by
 * {@link java.util.HashMap}.
 * <p>
 * The general contract of {@code hashCode} is:
 * <ul>
 * <li>Whenever it is invoked on the same object more than once during
 *     an execution of a Java application, the {@code hashCode} method
 *     must consistently return the same integer, provided no information
 *     used in {@code equals} comparisons on the object is modified.
 *     This integer need not remain consistent from one execution of an
 *     application to another execution of the same application.
 * <li>If two objects are equal according to the {@code equals(Object)}
 *     method, then calling the {@code hashCode} method on each of
 *     the two objects must produce the same integer result.
 * <li>It is <em>not</em> required that if two objects are unequal
 *     according to the {@link java.lang.Object#equals(java.lang.Object)}
 *     method, then calling the {@code hashCode} method on each of the
 *     two objects must produce distinct integer results.  However, the
 *     programmer should be aware that producing distinct integer results
 *     for unequal objects may improve the performance of hash tables.
 * </ul>
 * <p>
 * As much as is reasonably practical, the hashCode method defined by
 * class {@code Object} does return distinct integers for distinct
 * objects. (This is typically implemented by converting the internal
 * address of the object into an integer, but this implementation
 * technique is not required by the
 * Java™ programming language.)
 *
 * @return  a hash code value for this object.
 * @see     java.lang.Object#equals(java.lang.Object)
 * @see     java.lang.System#identityHashCode
 */
public native int hashCode();

System#identityHashCode(Object x);

/**
 * Returns the same hash code for the given object as
 * would be returned by the default method hashCode(),
 * whether or not the given object's class overrides
 * hashCode().
 * The hash code for the null reference is zero.
 *
 * @param x object for which the hashCode is to be calculated
 * @return  the hashCode
 * @since   JDK1.1
 */
public static native int identityHashCode(Object x);

identityHashCode(Object x)和hashCode（）

identityHashCode(Object x)和hashCode（）都是native方法，native方法请参考本地方法；以java8为例,JDK中代码样式可以看出，两个java方法使用的是同一个c++的方法JVM_IHashCode()，所以结果是一样的。

Object#hashCode

static JNINativeMethod methods[] = {
    {"hashCode",    "()I",                    (void *)&JVM_IHashCode},
    {"wait",        "(J)V",                   (void *)&JVM_MonitorWait},
    {"notify",      "()V",                    (void *)&JVM_MonitorNotify},
    {"notifyAll",   "()V",                    (void *)&JVM_MonitorNotifyAll},
    {"clone",       "()Ljava/lang/Object;",   (void *)&JVM_Clone},
};

System#identityHashCode

JNIEXPORT jint JNICALL
Java_java_lang_System_identityHashCode(JNIEnv *env, jobject this, jobject x)
{
    return JVM_IHashCode(env, x);
}

接下还是一段代码的展示equal,“==”,hashCode(),identityHashCode()的区别，选取Object类，原始类型boolean，int，原始类型包装类Boolean类,Integer，以及String类进行说明。

Object

/***************************************************************************
 * @Title: EqualsDome
 * @Package com.base
 * @Description: 实验比较equals
 * @author shimingda
 * @date 2020/1/9  
 * @version V1.0
 ****************************************************************************/
public class EqualsDome
{
   public static void main(String[] args)
   {
      Object a1="a";
      Object a2="a";
      Object b1=new Object();
      Object b2=new Object();

      System.out.println("a1==a2:"+(a1==a2));
      System.out.println("b1==b2:"+(b1==b2));

      System.out.println("a1.equals(a2):"+(b1.equals(b2)));
      System.out.println("b1.equals(b2):"+(b1.equals(b2)));

      System.out.println("a1.hashCode() is : "+a1.hashCode()+"   System.identityHashCode(a1) is : "+ System.identityHashCode(a1));
      System.out.println("a2.hashCode() is : "+a2.hashCode()+"   System.identityHashCode(a2) is : "+ System.identityHashCode(a2));
      System.out.println("b1.hashCode() is : "+b1.hashCode()+"   System.identityHashCode(b1) is : "+ System.identityHashCode(b1));
      System.out.println("b2.hashCode() is : "+b2.hashCode()+"   System.identityHashCode(b2) is : "+ System.identityHashCode(b2));
   }
}
-------------------------------------------------------
a1==a2:true
b1==b2:false
a1.equals(a2):false
b1.equals(b2):false
a1.hashCode() is : 97   System.identityHashCode(a1) is : 1163157884
a2.hashCode() is : 97   System.identityHashCode(a2) is : 1163157884
b1.hashCode() is : 1956725890   System.identityHashCode(b1) is : 1956725890
b2.hashCode() is : 356573597    System.identityHashCode(b2) is : 356573597

int和Integer

/***************************************************************************
 * @Title: EqualsDome
 * @Package com.base
 * @Description: 实验比较equals
 * @author shimingda
 * @date 2020/1/9  
 * @version V1.0
 ****************************************************************************/
public class EqualsDome
{
  public static void main(String[] args)
  {
    int a1=130;
    int a2=130;
    Integer b1=130;
    Integer b2=130;
    Integer c1=new Integer(130);
    Integer c2=new Integer(130);


    System.out.println("a1==a2:"+(a1==a2));
    System.out.println("a1==b1:"+(a1==b1));
    System.out.println("b1==b2:"+(b1==b2));
    System.out.println("bi==c1:"+(b1==c1));
    System.out.println("c1==c2:"+(c1==c2));

    System.out.println("b1.equals(b2):"+(b1.equals(b2)));
    System.out.println("bi.equals(c1):"+(b1.equals(c1)));
    System.out.println("c1.equals(c2):"+(c1.equals(c2)));

    System.out.println("System.identityHashCode(a1) is : "+ System.identityHashCode(a1));
    System.out.println("System.identityHashCode(a2) is : "+ System.identityHashCode(a2));
    System.out.println("b1.hashCode() is : "+b1.hashCode()+"   System.identityHashCode(b1) is : "+ System.identityHashCode(b1));
    System.out.println("b2.hashCode() is : "+b2.hashCode()+"   System.identityHashCode(b2) is : "+ System.identityHashCode(b2));
    System.out.println("c1.hashCode() is : "+c1.hashCode()+"   System.identityHashCode(c1) is : "+ System.identityHashCode(c1));
    System.out.println("c2.hashCode() is : "+c2.hashCode()+"   System.identityHashCode(c2) is : "+ System.identityHashCode(c2));
  }
}
--------------------------------------------------------------
结果：
a1==a2:true
a1==b1:true
b1==b2:false
bi==c1:false
c1==c2:false
b1.equals(b2):true
bi.equals(c1):true
c1.equals(c2):true
System.identityHashCode(a1) is : 1163157884
System.identityHashCode(a2) is : 1956725890
b1.hashCode() is : 130   System.identityHashCode(b1) is : 356573597
b2.hashCode() is : 130   System.identityHashCode(b2) is : 1735600054
c1.hashCode() is : 130   System.identityHashCode(c1) is : 21685669
c2.hashCode() is : 130   System.identityHashCode(c2) is : 2133927002

boolean和Boolean

package com.base;

import sun.applet.Main;

/***************************************************************************
 * @Title: EqualsDome
 * @Package com.base
 * @Description: 实验比较equals
 * @author shimingda
 * @date 2020/1/9  
 * @version V1.0
 ****************************************************************************/
public class EqualsDome
{
   public static void main(String[] args)
   {
      boolean a1=true;
      boolean a2=true;

      Boolean b1=true;
      Boolean b2=true;
      Boolean c1=new Boolean(true);
      Boolean c2=new Boolean(true);

      System.out.println("a1==a2:"+(a1==a2));
      System.out.println("a1==b1:"+(a1==b1));
      System.out.println("b1==b2:"+(b1==b2));
      System.out.println("bi==c1:"+(b1==c1));
      System.out.println("c1==c2:"+(c1==c2));

      System.out.println("b1.equals(b2):"+(b1.equals(b2)));
      System.out.println("bi.equals(c1):"+(b1.equals(c1)));
      System.out.println("c1.equals(c2):"+(c1.equals(c2)));

      System.out.println("System.identityHashCode(a1) is : "+ System.identityHashCode(a1));
      System.out.println("System.identityHashCode(a2) is : "+ System.identityHashCode(a2));
      System.out.println("b1.hashCode() is : "+b1.hashCode()+"   System.identityHashCode(b1) is : "+ System.identityHashCode(b1));
      System.out.println("b2.hashCode() is : "+b2.hashCode()+"   System.identityHashCode(b2) is : "+ System.identityHashCode(b2));
      System.out.println("c1.hashCode() is : "+c1.hashCode()+"   System.identityHashCode(c1) is : "+ System.identityHashCode(c1));
      System.out.println("c2.hashCode() is : "+c2.hashCode()+"   System.identityHashCode(c2) is : "+ System.identityHashCode(c2));
   }
}
----------------------------------------------
a1==a2:true
a1==b1:true
b1==b2:true
bi==c1:false
c1==c2:false
b1.equals(b2):true
bi.equals(c1):true
c1.equals(c2):true
System.identityHashCode(a1) is : 1163157884
System.identityHashCode(a2) is : 1163157884
b1.hashCode() is : 1231   System.identityHashCode(b1) is : 1163157884
b2.hashCode() is : 1231   System.identityHashCode(b2) is : 1163157884
c1.hashCode() is : 1231   System.identityHashCode(c1) is : 1956725890
c2.hashCode() is : 1231   System.identityHashCode(c2) is : 356573597

String

/***************************************************************************
 * @Title: EqualsDome
 * @Package com.base
 * @Description: 实验比较equals
 * @author shimingda
 * @date 2020/1/9  
 * @version V1.0
 ****************************************************************************/
public class EqualsDome
{
   public static void main(String[] args)
   {
      String a1="a";
      String a2="a";
      String b1=new String("a");
      String b2=new String("a");


      System.out.println("a1==a2:"+(a1==a2));
      System.out.println("a1==b1:"+(a1==b1));
      System.out.println("b1==b2:"+(b1==b2));

      System.out.println("a1.equals(a2):"+(b1.equals(b2)));
      System.out.println("ai.equals(b1):"+(b1.equals(b1)));
      System.out.println("b1.equals(b2):"+(b1.equals(b2)));
      
      System.out.println("a1.hashCode() is : "+a1.hashCode()+"   System.identityHashCode(a1) is : "+ System.identityHashCode(a1));
      System.out.println("a2.hashCode() is : "+a2.hashCode()+"   System.identityHashCode(a2) is : "+ System.identityHashCode(a2));
      System.out.println("b1.hashCode() is : "+b1.hashCode()+"   System.identityHashCode(b1) is : "+ System.identityHashCode(b1));
      System.out.println("b2.hashCode() is : "+b2.hashCode()+"   System.identityHashCode(b2) is : "+ System.identityHashCode(b2));

   }
}
-----------------------------------------------------------------
a1==a2:true
a1==b1:false
b1==b2:false
a1.equals(a2):true
ai.equals(b1):true
b1.equals(b2):true
a1.hashCode() is : 97   System.identityHashCode(a1) is : 1163157884
a2.hashCode() is : 97   System.identityHashCode(a2) is : 1163157884
b1.hashCode() is : 97   System.identityHashCode(b1) is : 1956725890
b2.hashCode() is : 97   System.identityHashCode(b2) is : 356573597

对于以上实验进行总结

1.当一个类没有重写Object类的hashCode()方法时，它的hashCode和identityHashCode是一致的

2.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

3.identityHashCode值相等，"=="结果一定是true;但是“==”为true时，identityHashCode不一定相等，样例int大于127时。

4..identityHashCode值相等，equal()不一定是true;

5..hashCode值相等时，"=="结果不一定是true

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