Java 里 null 的 hash code 是多少？

Object o = null;

// Exception in thread "main" java.lang.NullPointerException
// System.out.println(o.hashCode());

点进去看 hashCode() 方法的实现

/**
 * @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();

null 也是个对象，是对象就一定有一个 hash code 看 @see 里的 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);

果然 The hash code for the null reference is zero。

再去看 HashMap 里的 put(null, xxx) 呢？

/**
 * 果然 key == null 的时候返回的也是 0
 */
static final int hash(Object key) {
    int h;
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

结论：Java 里 null 的 hash code 是 0。