Java HashCode和Equals关联和区别

 

 

经常看到面试题有这个问题，今天查了下资料mark一下。

    我觉得英文的注释写的特别好，总结一下：

    1）如果两个对象equals方法返回true，那么hashCode方法返回的值必然相同

    2）如果两个对象equals方法返回false，但是不要求hashCode方法返回的值不同。

  

    其实HashCode方法主要用于Set容器来提高性能，极大的减少调用equals方法的次数。

    Set容器中有A对象，然后放入B对象。

    如果B和A的hashCode不相同，可以确保A，B不相同，直接放入容器中。

    如果B和A的hashCode相同，那么再判断equals方法是否相同。

 

 /**
     * Returns a hash code value for the object. This method is 
     * supported for the benefit of hashtables such as those provided by 
     * <code>java.util.Hashtable</code>. 
     * <p>
     * The general contract of <code>hashCode</code> is: 
     * <ul>
     * <li>Whenever it is invoked on the same object more than once during 
     *     an execution of a Java application, the <tt>hashCode</tt> method 
     *     must consistently return the same integer, provided no information 
     *     used in <tt>equals</tt> 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 <tt>equals(Object)</tt>
     *     method, then calling the <code>hashCode</code> 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 <tt>hashCode</tt> 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 hashtables.
     * </ul>
     * <p>
     * As much as is reasonably practical, the hashCode method defined by 
     * class <tt>Object</tt> 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.util.Hashtable
     */
    public native int hashCode();