hashCode
关于hashCode参考之前的文章,点击参考之前文章。
identityHashCode是System里面提供的本地方法,java.lang.System#identityHashCode。
/** * 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和hashCode的区别是,identityHashCode会返回对象的hashCode,而不管对象是否重写了hashCode方法。
public static void main(String[] args) { String str1 = new String("abc"); String str2 = new String("abc"); System.out.println("str1 hashCode: " + str1.hashCode()); System.out.println("str2 hashCode: " + str2.hashCode()); System.out.println("str1 identityHashCode: " + System.identityHashCode(str1)); System.out.println("str2 identityHashCode: " + System.identityHashCode(str2)); User user = new User("test", 1); System.out.println("user hashCode: " + user.hashCode()); System.out.println("user identityHashCode: " + System.identityHashCode(user)); }
输出结果:
str1 hashCode: 96354 str2 hashCode: 96354 str1 identityHashCode: 1173230247 str2 identityHashCode: 856419764 user hashCode: 621009875 user identityHashCode: 621009875
结果分析:
1、str1和str2的hashCode是相同的,是因为String类重写了hashCode方法,它根据String的值来确定hashCode的值,所以只要值一样,hashCode就会一样。
2、str1和str2的identityHashCode不一样,虽然String重写了hashCode方法,identityHashCode永远返回根据对象物理内存地址产生的hash值,所以每个String对象的物理地址不一样,identityHashCode也会不一样。
3、User对象没重写hashCode方法,所以hashCode和identityHashCode返回的值一样。
结论
hashCode方法可以被重写并返回重写后的值,identityHashCode会返回对象的hash值而不管对象是否重写了hashCode方法。