JavaSE_坚持读源码_Object对象_Java1.7

  /**
      * Returns a hash code value for the object. This method is
      * supported for the benefit of hash tables such as those provided by
      * {@link java.util.HashMap}.
      * <p>
      * The general contract of {@code hashCode} is:
      * <ul>
      * 1.<li>Whenever it is invoked on the same object more than once during
      *     an execution of a Java application, the {@code hashCode} method
      *     must consistently return the same integer, provided no information
      *     used in {@code equals} comparisons on the object is modified.
      *     This integer need not remain consistent from one execution of an
      *     application to another execution of the same application.
      * 2. <li>If two objects are equal according to the {@code equals(Object)}
      *     method, then calling the {@code hashCode} method on each of
      *     the two objects must produce the same integer result.
      * 3.<li>It is <em>not</em> required that if two objects are unequal
      *     according to the {@link java.lang.Object#equals(java.lang.Object)}
      *     method, then calling the {@code hashCode} method on each of the
      *     two objects must produce distinct integer results.  However, the
      *     programmer should be aware that producing distinct integer results
      *     for unequal objects may improve the performance of hash tables.
      * </ul>
      * <p>
      * As much as is reasonably practical, the hashCode method defined by
      * class {@code Object} does return distinct integers for distinct
      * objects. (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<font size="-2"><sup>TM</sup></font> programming language.)
      *
      * @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();

2.如果两个对象通过equal方法判断是相同的话，那么他们通过hashCode方法的返回值一定是一样的

3.两个对象如果通equal方法的返回值是false，他们通过hashCode方法的返回值不一定是不同的；然而，我们应该意识到对于不相等的对象返回不同的hashCode值能够提高hash表的性能