Java的Object.hashCode()的返回值到底是不是对象内存地址?
关于这个问题,查阅了网上的资料,发现证明过程太繁琐,这里我用了反证法。
java.lang.Object.hashCode()
的返回值到底是不是对象内存地址?
hashCode契约
说到这个问题,大家的第一反应一定和我一样——去查Object.hashCode
的源码,但翻开源码,看到的却是这样的(Oracle JDK 8):
1 /** 2 * Returns a hash code value for the object. This method is 3 * supported for the benefit of hash tables such as those provided by 4 * {@link java.util.HashMap}. 5 * <p> 6 * The general contract of {@code hashCode} is: 7 * <ul> 8 * <li>Whenever it is invoked on the same object more than once during 9 * an execution of a Java application, the {@code hashCode} method 10 * must consistently return the same integer, provided no information 11 * used in {@code equals} comparisons on the object is modified. 12 * This integer need not remain consistent from one execution of an 13 * application to another execution of the same application. 14 * <li>If two objects are equal according to the {@code equals(Object)} 15 * method, then calling the {@code hashCode} method on each of 16 * the two objects must produce the same integer result. 17 * <li>It is <em>not</em> required that if two objects are unequal 18 * according to the {@link java.lang.Object#equals(java.lang.Object)} 19 * method, then calling the {@code hashCode} method on each of the 20 * two objects must produce distinct integer results. However, the 21 * programmer should be aware that producing distinct integer results 22 * for unequal objects may improve the performance of hash tables. 23 * </ul> 24 * <p> 25 * As much as is reasonably practical, the hashCode method defined by 26 * class {@code Object} does return distinct integers for distinct 27 * objects. (This is typically implemented by converting the internal 28 * address of the object into an integer, but this implementation 29 * technique is not required by the 30 * Java™ programming language.) 31 * 32 * @return a hash code value for this object. 33 * @see java.lang.Object#equals(java.lang.Object) 34 * @see java.lang.System#identityHashCode 35 */ 36 public native int hashCode();
Object.hashCode()
是一个native方法,看不到源码(Java代码,Oracle的JDK是看不到的,OpenJDK或其他开源JRE是可以找到对应的C/C++代码)。
上面这段注释指出了Object.hashCode()
在JRE(Java Runtime Library)中应该遵循的一些契约(contract):
(PS:所谓契约当然是大家一致达成的,各个JVM厂商都会遵循)
(1)一致性(consistent),在程序的一次执行过程中,对同一个对象必须一致地返回同一个整数。
(2)如果两个对象通过equals(Object)
比较,结果相等,那么对这两个对象分别调用hashCode
方法应该产生相同的整数结果。(PS:这里equals
和hashCode
说的都是Object
类的)
(3)如果两个对象通过java.lang.Object.equals(java.lang.Ojbect)
比较,结果不相等,不必保证对这两个对象分别调用hashCode()
也返回两个不相同的整数
因为每个对象的内存地址都是不一样的,如果hashCode()
的返回值是对象内存地址,那么两个对象分别调用hashCode()方法将会返回两个截然不同的整数,这与契约三矛盾了,因此java.lang.Object.hashCode()
的返回值不是对象内存地址!
在这里我觉得hashCode()方法的返回值是对象内存地址通过hash算法计算之后的哈希码值,因此才会出现两个对象分别调用hashCode()返回同一个值。