通常我们都会看到这样一段描述：重写equals方法是，必须同时重写hashCode方法。很多人都没有太过深入的了解过hashCode方法，觉得它实在太过于神秘。现在让我们一起解开hashCode的神秘面纱。
show me the code：

// 以下代码均出自于jdk(1.8)中

  /**
     *  ···
     * <p>
     * The general contract of {@code hashCode} is:
     * <ul>
     * <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.
     * <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.
     * <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&trade; programming language.)
     * ···
     */
    public native int hashCode();
    /**
     * ···
     * Note that it is generally necessary to override the {@code hashCode}
     * method whenever this method is overridden, so as to maintain the
     * general contract for the {@code hashCode} method, which states
     * that equal objects must have equal hash codes.
     *
     * ···
     */
    public boolean equals(Object obj) {
        return (this == obj);
    }

总结：

• Object类的hashCode方法将对象在内存中的地址转化为整数返回
• 如果两个对象的根据equals方法相等，则hashCode的返回值也必须一致
• 在JAVA程序执行期间，在同一对象上多次调用hashCode方法，只要不修改它在equals方法比较中使用的信息，必须始终返回相同的整数
• 同一应用程序的不同执行，hashCode不必保持一致
• 两个对象的equals方法返回不相等时，不要求hashCode方法返回不同的整数。但是编程人员应该意识到为不同的对象返回不同的hashCode将会提高哈希表的性能

拓展：
当我们需要提升equals方法的性能时，可以先通过hashCode（高性能）进行判断，再通过复杂、性能稍差的equals逻辑进行比较，可以大大提升性能。