由equals和hashCode想到的.....

Java的超类Object中有两个非常重要的方法,equals和hashCode,我想关于这两个方法的应用应该属于非常基础的用法了吧,今天看到一个有意思的代码,我想在老生重弹一下吧.
先上源代码:

写道

package panio.core;

import java.util.HashMap;
import java.util.Map;

class Test{
class TestObj{
private String msg;

public TestObj(String m){
this.msg = m;
}


public boolean equals(Object o){
return ((TestObj)o).msg == this.msg;
}


/**
* 返回一个你认为可行的hash算法
*/
// public int hashCode(){
// return msg.length();
// }
}

public void test(){
Map<TestObj,String> map = new HashMap<TestObj,String>();
map.put(new TestObj("kant"),"Demo");
map.put(new TestObj("olya"),"Demo");
map.put(new TestObj("jack"),"Demo");
map.put(new TestObj("kant"),"Demo");

System.out.println("Guess the length of the result: "+map.size());

}
public static void main(String[] args){
new Test().test();
}
}

 运行结果,比较注释TestObj对象中hashCode方法后的结果,发现注释后map的size为4,而不注释的结果为3,为什么呢?

 现在有点困了,先去睡觉,明天接着写...

 

评论

3 楼 hu7363 2008-12-26  

你必须要同时写equals和hashcode方法。

2 楼 oxromantic 2008-12-26  

别写了，你注释掉就错了
无论何时你得记住，如果你override了equals，必须保证obj equal成立的前提是hashCode equal.
你要是不拿HashMap来做测试就算了，忽略红色的对你的程序运行健康也没多大关系，可是。。
打住

引用

boolean java.lang.Object.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:

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

See Also:
hashCode()
java.util.Hashtable
Parameters:
obj the reference object with which to compare.
Returns:
true if this object is the same as the obj argument; false otherwise.

1 楼 kjchen 2008-12-26  

等待中.....