The trap of System.identityHashCode()-优快云博客

本文深入探讨了Java中System.identityHashCode方法的工作原理，解释了它如何返回一个对象的默认hashCode值，无论该对象的类是否覆盖了hashCode方法。通过具体示例展示了当类覆盖hashCode方法时，identityHashCode和自定义hashCode之间的区别。

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Why a trap ?

First ,let's take a look at the comments of this API provided by Sun:

    /**
     * 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);

Pls,pay more and more attention to the default method hashCode() , do you know what is the default method hashCode()?? It is the hashCode() defined in Object class !!!

i.e. the one public native int hashCode(); The most confusing comment is “whether or not the given object's class overrides hashCode()" , this is definitely bullshit bullshit.....

That means if your class overrides hashCode(), System.identityHashCode() != your.hashCode()! Checkout below testcase:

import junit.framework.TestCase;

public class TestIdentityHashCode extends TestCase{
	
	public void testIdentityHahshCodeAlwaysReturnTheSameAsDefaultHashCode()
	{
		Object arbitaryObj = new Object();
		assertEquals(arbitaryObj.hashCode(),System.identityHashCode(arbitaryObj));
		
		ObjectWhichDoesNotOverrideHashCode objUsingDefaultHashCode = new ObjectWhichDoesNotOverrideHashCode();
		assertEquals(objUsingDefaultHashCode.hashCode(),System.identityHashCode(objUsingDefaultHashCode));
		
		ObjectWhichOverrideHashCode objUsingOverridedHashCode = new ObjectWhichOverrideHashCode();
		assertEquals(objUsingOverridedHashCode.hashCode(),System.identityHashCode(objUsingOverridedHashCode));			
	}
	
	public void testIdentityHashCodeOfNullIsZero()
	{
		assertEquals(0, System.identityHashCode(null));
	}
	
	private static class ObjectWhichDoesNotOverrideHashCode
	{		
	}
	private static class ObjectWhichOverrideHashCode
	{		
		public int hashCode()
		{
			return 1000;
		}
	}
}


Above testcase always fails as expected.. so pls pay attention the trap .

BTW: the default hashCode() of Sun's JVM always returns the memory address of the specific Object 
which guarantee it's unique. So System.identityHashCode() could be used to generate a unique key 
in a same JVM.