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();