HashCode()和Equals()，Hash结构 关系详解

HashCode()和Equals()，Hash结构 关系详解

首先看下hashCode()的官方解释

public int hashCode()

Returns a hash code value for the object. This method is supported for the benefit of hashtables such as those provided by java.util.Hashtable.

The general contract of hashCode is:

o        Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in 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.

If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.

It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the 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 hashtables.

As much as is reasonably practical, the hashCode method defined by class 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^TM programming language.

再看下Equals()的官方解释：

public boolean equals(Object obj)

Indicates whether some other object is "equal to" this one.

The equals method implements an equivalence relation on non-null object references:

o        It is reflexive: for any non-null reference value x, x.equals(x) should return true.

o        It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.

o        It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.

o        It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.

o        For any non-null reference value x, x.equals(null) should return false.

The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns true if and only if x and y refer to the same object (x == y has the value true).

Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.

自己的理解：

第一个问题：这里需要解释为什么equals()相等，hashCode就一定要相等，而hashCode相等，却不要求equals相等?

其实这很好理解：主要从Hash的数据结构来解释就OK了，equals相等，意味着两个对象在Hash的存储中(采用链表法解决冲突问题)，存放在同一个位置，这就要求两个对象必须在数组的同一个index中。而index = (hashCode() % MODULE ) 得来的。故其HashCode必须相等。而反过来，两个HashCode相等，只是表明两个对象在数组的index 相同，但其可能是产生了冲突，这里采用链表法来解决，所以其在同一个index中，但在链表的不同位置上。

 

第二：为什么hashCode与Equals要同时实现呢？且实现的算法所利用的成员要一致呢？

假定你的Equals是基于内容来实现的。而你的HashCode不变,还是继承于Object，那么当两个对象相等时，即Equals相等，而你实现的HashCode不等，这就违反了第一个问题的规定。