一、Set

1.set特点

? ? ?①.不可重复?

②无序（每一次运行的结果都会不一样）

2.遍历方式 (与list相比少一种遍历方式 for)

代码如下（示例）：

①.foreach

	set.add(new stu("小白", 12));
	set.add(new stu("小黑", 15));
	set.add(new stu("小薛", 18));
	
	for (Object ob : set) {
		System.out.println(ob);
	}

代码如下（示例）：

②。Iterator（迭代器）

  Iterator<Object> it=set.iterator();
    while(it.hasNext()) {
    	System.out.println(it.next());
    }

二、Set集合去重

1.hashset 底层数据结构是哈希表

2.优先会调用hashcode方法对比地址
??然后调用equals方法对比值

代码如下（示例）：

public static void main(String[] args) {
?? ?Set<Object> set=new HashSet<>();
?? ?
?? ?set.add(new stu("小白", 12));
?? ?set.add(new stu("小黑", 15));
?? ?set.add(new stu("小薛", 18));
?? ?set.add(new stu("小黑", 15));
?? ?
?? ?for (Object ob : set) {
?? ??? ?System.out.println(ob);
?? ?}
?? ??? ??? ?
?? ?}

}

class stu{
?? ?private String name;
?? ?private int age;
?? ?public String getName() {
?? ??? ?return name;
?? ?}
?? ?public void setName(String name) {
?? ??? ?this.name = name;
?? ?}
?? ?public int getAge() {
?? ??? ?return age;
?? ?}
?? ?public void setAge(int age) {
?? ??? ?this.age = age;
?? ?}
?? ?public stu() {
?? ??? ?// TODO Auto-generated constructor stub
?? ?}
?? ?public stu(String name, int age) {
?? ??? ?super();
?? ??? ?this.name = name;
?? ??? ?this.age = age;
?? ?}
?? ?@Override
?? ?public String toString() {
?? ??? ?return "stu [name=" + name + ", age=" + age + "]";
?? ?}
?? ?@Override
?? ?public int hashCode() {
?? ??? ?System.out.println("hashCode 被调用");
?? ??? ?final int prime = 31;
?? ??? ?int result = 1;
?? ??? ?result = prime * result + age;
?? ??? ?result = prime * result + ((name == null) ? 0 : name.hashCode());
?? ??? ?return result;
?? ?}
?? ?@Override
?? ?public boolean equals(Object obj) {
?? ??? ?System.out.println("equals 被调用");
?? ??? ?if (this == obj)
?? ??? ??? ?return true;
?? ??? ?if (obj == null)
?? ??? ??? ?return false;
?? ??? ?if (getClass() != obj.getClass())
?? ??? ??? ?return false;
?? ??? ?stu other = (stu) obj;
?? ??? ?if (age != other.age)
?? ??? ??? ?return false;
?? ??? ?if (name == null) {
?? ??? ??? ?if (other.name != null)
?? ??? ??? ??? ?return false;
?? ??? ?} else if (!name.equals(other.name))
?? ??? ??? ?return false;
?? ??? ?return true;
?? ?}
?? ?
?? ?

?2.当HashCode相同时会增加equals的调用次数

三、TreeSet

1.自然排序

自然排序对象需要实现自然排序接口如果没实现会报类转换异常 user不能转换成Comparable接口

（Exception in thread "main" java.lang.ClassCastException: com.zy.set.user cannot be cast to java.lang.Comparable）

正确写法代码如下（示例）：

public class One3 {
	public static void main(String[] args) {
		Set<user> set=new TreeSet<>();
		set.add(new user("小白", 1200));
		set.add(new user("小黑",80000));
		set.add(new user("小薛", 17000));
		set.add(new user("小杨", 12000));
		for (user u : set) {
			System.out.println(u);
		}
	}

}

class user implements Comparable{
	private String name;
	private int money;
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	
	public int getMoney() {
		return money;
	}
	public void setMoney(int money) {
		this.money = money;
	}
	public user() {
		// TODO Auto-generated constructor stub
	}
	
	
	public user(String name, int money) {
		super();
		this.name = name;
		this.money = money;
	}
	@Override
	public String toString() {
		return "user [name=" + name + ", money=" + money + "]";
	}
	
	public int compareTo(Object ob) {
		//ob 是对比的那个值
		// TODO Auto-generated method stub
		user u=(user)ob;
		
		return this.money - u.money;
	}

运行结果会根据money来升序

?这样写有弊端（因为自然排序是写死了的，不灵活），因为不同的模块有不同的需求，对于一张表会有不同的排序规则，这个时候我们就要用到比较器排序

2.比较器排序

代码如下（示例）：

根据money降序

public class One4 {
	
	public static void main(String[] args) {
		Set<user> set=new TreeSet<>(new Comparator() {

			@Override
			public int compare(Object o1, Object o2) {
				// TODO Auto-generated method stub
				
				return ((user)o2).getMoney() -((user)o1).getMoney();
			}

			
		});
		set.add(new user("小", 15000));
		set.add(new user("白", 1200));
		set.add(new user("黑",80000));
		set.add(new user("薛", 17000));
		set.add(new user("杨", 12000));
		for (user u : set) {
			System.out.println(u);
		}
	}

}



class user {
	private String name;
	private int money;
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	
	public int getMoney() {
		return money;
	}
	public void setMoney(int money) {
		this.money = money;
	}
	public user() {
		// TODO Auto-generated constructor stub
	}
	
	
	public user(String name, int money) {
		super();
		this.name = name;
		this.money = money;
	}
	@Override
	public String toString() {
		return "user [name=" + name + ", money=" + money + "]";
	}

	
	
	

}

代码如下（示例）：

根据name的首字母进行排序


public class One5 {
	public static void main(String[] args) {
		Set<user> set=new TreeSet<>(new Comparator() {

			@Override
			public int compare(Object o1, Object o2) {
				// TODO Auto-generated method stub

				return ((user)o1).getName().compareTo(((user)o2).getName());
			}

			
		});
		set.add(new user("b", 15000));
		set.add(new user("a", 1200));
		set.add(new user("y",80000));
		set.add(new user("x", 17000));
		set.add(new user("r", 12000));
		for (user u : set) {
			System.out.println(u);
		}
	}


}