需要hashcode的情况

import java.util.HashMap;

/**
 * 验证即使有了equals,还需要hashcode的情况
 * @author microsoft
 *
 */
public class Person {
	
	private String id;

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public static void main(String args[]) {
        Person p = new Person();
        p.id = "123";
        Person pp = new Person();
        pp.id = "321";        
        HashMap map = new HashMap();
        map.put(p, "p1");
        map.put(pp, "p2");
        
        
        Person pNew = new Person();
        pNew.id = "123";
        System.out.print(map.get(pNew));
    }
    
    @Override
    public boolean equals(Object obj) {
    	// TODO Auto-generated method stub
    	if(this==obj) return true;
    	if(!(obj instanceof Person)){
    		return false;
    	}
    	return ((Person)obj).id.equals(id);  	
    }
    
    @Override
    public int hashCode() {
    	// TODO Auto-generated method stub
    	return Integer.parseInt(this.id);
    }


}

display result is:"p1"

if no funcation hashCode() ,the result is  null;
if no funcation equals(),the result alse is null;

so for the right result,these two funcations "equals()" and "hashcode()" is needed.