CollectionSet&HashSet&TreeSet(HashMap实现，去重特性)

  /**
     * Adds the specified element to this set if it is not already present.
     * More formally, adds the specified element e to this set if
     * this set contains no element e2 such that
     * (e==null ? e2==null : e.equals(e2)).
     * If this set already contains the element, the call leaves the set
     * unchanged and returns false.
     *
     * @param e element to be added to this set
     * @return true if this set did not already contain the specified
     * element
     */
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;    // 重复返回false
    }

public static void main(String[] args) {
    TreeSet t1= new TreeSet();
        t1.add(new Err());
        t1.add(new Err());
        //Exception in thread "main" java.lang.ClassCastException:

    ｝
    class Err{
}

public static void main(String[] args) {
        TreeSet t2 = new TreeSet();
        t2.add("hello");
        t2.add(1);
        //java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer

import java.util.*;

/*
 class Person implements Comparable
 {
 private String name;
 private int age;
 public Person(String name, int age)
 {
 this.name = name;
 this.age = age;
 }
 @Override
 public int compareTo(Object o) {
 // TODO Auto-generated method stub
 if (o instanceof Person == false) return 0;
 Person p = (Person)o;
 int t = this.name.compareTo(p.name);
 if(t != 0)return t;
 return this.age - p.age;
 }

 public String toString(){
 return name + ":" + age;
 }
 }
 */
class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String toString() {
        return name + ":" + age;
    }
    
    public String getName(){
        return this.name;
    }
    
    public int getAge(){
        return this.age;
    }
}

class K implements Comparator {

    @Override
    public int compare(Object a, Object b) {
        // TODO Auto-generated method stub
        if (a instanceof Person == false || b instanceof Person == false)
            return 0;// 因为如果a,b都不是Person类，那么已经超出了该方法判定的范围，我们只能假定其相等，0表示相等
        Person p1 = (Person) a;
        Person p2 = (Person) b;
        int t = p2.getName().compareTo(p2.getName());
        if (t != 0) return t;
        return p1.getAge() - p2.getAge();
    }

}

public class TreeSetTEST {
    public static void main(String[] args) {
        // Set a = new TreeSet();
        Set a = new TreeSet(new K());
        a.add(new Person("hao", 10));
        a.add(new Person("hao", 20));
        a.add(new Person("hao", 30));
        a.add(new Person("hao", 10));
        a.add(new Person("zhang", 10));
        a.add(new Person("zhang", 11));
        a.add(new Person("zhang", 20));
        a.add(new Person("li", 40));

        System.out.println(a);
    }

}