集合关系理解

 

虚框代表的是接口，接口不能实例化，需要用他们的子类来实例化

Map接口可以理解为python的字典（dict）

import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;


public class CollectionDemo {
    public static void main(String[] args) {
        Collection<String> list = new ArrayList<String>();
        list.add("小明");
        list.add("小王");

        Iterator<String>  iter=list.iterator();//生成迭代器
        while(iter.hasNext())
            System.out.println(iter.next());

    }
}

 

import java.util.HashMap;
import java.util.Map;

public class MapDemo {
    public static void main(String[] args) {
        Map<Integer, String> dict = new HashMap<Integer, String>();
        dict.put(1, "a");
        dict.put(2, "b");
        dict.put(3, "c");

        //遍历key
        for (Integer s : dict.keySet())
            System.out.println(s);

        //遍历value
        for (String s : dict.values())
            System.out.println(s);

        //同时遍历key和value
        for(Map.Entry<Integer,String> entry:dict.entrySet())
            System.out.println(entry.getKey()+":"+entry.getValue());

    }
}