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


public class SetTest {
	public static void main(String[] args) {
		/*
		 *对于用户自定义类型的数据放在容器（Set）中
		 *务必重写equals和hashCode方法
		 *要不然stu1和stu2放在容器中，和认为是两个不同的元素
		 **/
		
		//set中存放的元素是无序的
		//set中存储的元素是不可以重复的（根据equals方法和hashCode方法判断）
		Set set = new HashSet();
		Student stu1 = new Student(1, "aaa");
		Student stu2 = new Student(1, "aaa");
		Student stu3 = new Student(2, "ccc");
		Student stu4 = new Student(8, "fff");
		
		set.add(stu1);
		set.add(stu2);
		set.add(stu3);
		set.add(stu4);
		
		System.out.println(set);
	}
}

class Student {
	private int id;
	private String name;
	
	public Student(int id, String name) {
		this.id = id;
		this.name = name;
	}
	
	@Override
	public String toString() {
		return this.id + " " + this.name;
	}
	
	
	@Override
	public boolean equals(Object obj) {		
		Student stu = (Student) obj;
		
		return this.id == stu.id && this.name.equals(stu.name);
	}
	
	
	@Override 
	public int hashCode() {
		return this.id*this.name.hashCode();
	}
}

输出结果：

[8 fff, 1 aaa, 2 ccc]

如果不重写hashCode和equals方法

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


public class SetTest {
	public static void main(String[] args) {
		/*
		 *对于用户自定义类型的数据放在容器（Set）中
		 *务必重写equals和hashCode方法
		 *要不然stu1和stu2放在容器中，和认为是两个不同的元素
		 **/
		
		//set中存放的元素是无序的
		//set中存储的元素是不可以重复的（根据equals方法和hashCode方法判断）
		Set set = new HashSet();
		Student stu1 = new Student(1, "aaa");
		Student stu2 = new Student(1, "aaa");
		Student stu3 = new Student(2, "ccc");
		Student stu4 = new Student(8, "fff");
		
		set.add(stu1);
		set.add(stu2);
		set.add(stu3);
		set.add(stu4);
 
		System.out.println(set);
	}
}

class Student {
	private int id;
	private String name;
	
	public Student(int id, String name) {
		this.id = id;
		this.name = name;
	}
	
	@Override
	public String toString() {
		return this.id + " " + this.name;
	}	
}

输出结果：

[1 aaa, 1 aaa, 8 fff, 2 ccc]