equals()和hashCode()方法

Student.java代码如下：

package com.bean;

public class Student {
	int num;
	String name;
	public Student(int num,String name) {
		this.num = num;
		this.name = name;
	}

	public boolean equals(Object obj){
		if(this == obj){
			return true;
		}
		if(obj instanceof Student){
			Student s = (Student)obj;
			if(this.name.equals(s.name)&&(this.num==s.num)){
				return true;
			}
		}
		return false;		
	}
	
	public int hashCode(){
		return num*name.hashCode();
	}

	public String toString(){
		return num+":"+name;
	}

}

Department.java代码如下：

package com.bean;

/**
 * 设计这个类的原因是想让两个对象的hashCode相同
 * @author yangjianzhou
 *
 */
public class Department {
	
	private int number;
	
	private int id;
	
	public Department(int number,int id){
		this.number = number;
		this.id = id;
	}
	
	public int hashCode(){
		return id+number;
	}
	
	public boolean equals(Object obj){
		if(this == obj){
			return true;
		}
		if(obj instanceof Department){
			Department dept = (Department)obj;
			if(this.number == dept.number&&this.id==dept.id){
				return true;
			}
		}
		return false;
	}

}

Test.java代码如下：

package com.test;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

import com.bean.Department;
import com.bean.Student;

/**
 * equals()与hashcode()
 * @author yangjianzhou
 * 两个对象 obj1和obj2,如果obj1.equals(obj2)返回true，那么obj1与obj2的hashcode()方法返回值一定相同。
 * 但是如果obj1和obj2的hashcode()方法返回值相同，则obj1.equals(obj2)返回值不一定为true。
 * 如果obj1和obj2的hashcode()不同，那么obj1.equals(obj2)一定返回false
 * 向set里面添加元素时，是根据hashcode和equals来添加的，如果hashcode相同，就判断equals，如果equals相同，就不重复添加
 * 如果quals不同，就添加
 */
public class Test {

	public static void main(String[] args) {
        testEqualsAndHashcode();
        testHashSet();
        testHashCode();
	}
	
	public static void testEqualsAndHashcode()
	{
		
        String s1 = new String("1234");
        String s2 = new String("1234");
        System.out.println(s1==s2);
        System.out.println(s1.equals(s2));
        System.out.println(s1.hashCode());
        System.out.println(s2.hashCode());
        Set set = new HashSet();
        set.add(s1);
        set.add(s2);
        System.out.println(set.size());
        System.out.println("========================");
	}
	public static  void testHashSet(){
		HashSet hs = new HashSet();
		Student s1 = new Student(1, "zhangsan");
		Student s2 = new Student(2, "lisi");
		Student s3 = new Student(3, "wangwu");
		Student s4 = new Student(1, "zhangsan");
		hs.add(s1);
		hs.add(s2);
		hs.add(s3);
		hs.add(s4);
		System.out.println(s1.hashCode());
		System.out.println(s4.hashCode());
		Iterator it = hs.iterator();
		System.out.println(hs.size());
		while(it.hasNext()){
			System.out.println(it.next());
		}
		System.out.println("========================");
	}
	
	/**
	 * 这个方法验证了向set里面添加元素的原则为
	 * 1.先验证hashCode
	 * 2.在验证equals
	 * 3.只有通过以上两个验证之后就会将该元素添加到set里面
	 */
	public static void testHashCode(){
		HashSet set = new HashSet();
		Department dept1 = new Department(1, 2);
		Department dept2 = new Department(2, 1);
		set.add(dept1);
		set.add(dept2);
		System.out.println(set.size());
		System.out.println("========================");
	}
}

运行结果：

false
true
1509442
1509442
1
========================
1149414639
1368554401
4
1:zhangsan
2:lisi
1:zhangsan
3:wangwu
========================
2
========================