System.identityHashCode()方法

    /**
     * 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);

 无论给定的x对象是否覆盖了hashCode()方法，都会调用默认的hashCode()方法返回hashCode,如果x == null, 返回0。
 这个默认的hashCode()方法就是Object类中的hashCode方法。

public native int hashCode();

 在其注释提到了一句：

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.

 这说明默认的hashCode方法是根据对象的地址转换所得到的。

应用场景

fastJson中的IdentityHashMap类的get方法为获取key的hash值使用了这个方法。版本：1.1.33-android。