Set

package TestCollection;

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

public class TestSet {
    public static void main(String[] args) {
        Set set=new HashSet();
        
        //add
        set.add("aaa");
        set.add("bbb");
        set.add(new String("aaa"));
        
        System.out.println(set.size());  //2
        
        //remove
        set.remove("bbb");
        System.out.println(set.size());  //2
        
    }

}

package TestCollection;
import java.util.HashMap;

public class MySet {
    int size;
    HashMap map;
    private static final Object PRESENT=new Object();
    
    public int size() {
        return size;
    }
    
    //构造器
    public MySet() {
        map=new HashMap();
        
    }
    
    public void add(Object o) {
        map.put(o,PRESENT);//set的不可重复就是利用了map键对象的不可重复
        size++;
    }
    
    public static void main(String[] args) {
        MySet set=new MySet();
        set.add("aaa");
        
        System.out.println(set.size());
        
    }
    

}