HashSet和TreeSet都实现了Collection下的Set接口，他们实现的原理分别是HashMap和TreeMap，基本的数据结构是哈希表和Red-Black Tree

package com.pason.hashset;

import java.util.HashSet;

import java.util.Iterator;

public class HashSetDemo {

public static void main(String[] args) {

HashSet student = new HashSet();

Student stu1 = new Student("Pason",123);

Student stu2 = new Student("Wade",456);

Student stu3 = new Student("Lee",789);

Student stu4 = new Student("Wong",123);

student.add(stu1);

student.add(stu2);

student.add(stu3);

student.add(stu4);

System.out.println(student.size());

//打印的都是Student对象的toString()方法

System.out.println("======================");

//forEach方法遍历

for (Student s: student)

System.out.println(s);

System.out.println("======================");

//直接打印hashSet对象

System.out.println(student);

System.out.println("======================");

//迭代器

Iterator stu = student.iterator();

while(stu.hasNext()){

System.out.println(stu.next());

}

System.out.println("======================");

//加入重复元素发现对象加不进去

student.add(stu3);

System.out.println(student);

//如何定义对象是否重复呢？

//需要重写equals()和hashCode()方法，一般在Eclipse中

//自动根据类中字段属性重写

//现在重写Student中的equals()和hashCode()方法

//仅根据id判断是否一致，这样stu4因为与stu1的id一致，所以只加入了3个

//如果修改某个hashset中元素的能够决定hashcode和equals()的因素，那么修改后该元素无法移除

//原因和HashMap有关

stu3.setId(111);

student.remove(stu3);

System.out.println("======================");

System.out.println(student);

System.out.println("发现无法移除，但该改变可见！");

}

}

package com.pason.hashset;

public class Student {

private String name;

private int id;

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

public int getId() {

return id;

}

public void setId(int id) {

this.id = id;

}

public String toString() {

return "Student [name=" + name + ", id=" + id + "]";

}

public Student(String name, int id) {

super();

this.name = name;

this.id = id;

}

@Override

public int hashCode() {

final int prime = 31;

int result = 1;

result = prime * result + id;

return result;

}

@Override

public boolean equals(Object obj) {

if (this == obj)

return true;

if (obj == null)

return false;

if (getClass() != obj.getClass())

return false;

Student other = (Student) obj;

if (id != other.id)

return false;

return true;

}

}