Java集合(二)HashSet的使用

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> student = new HashSet<Student>();
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<Student> 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;
}

}