Map集合的方法和遍历

1 Map接口概述

l  Map中的集合，元素是成对存在的(理解为夫妻)。每个元素由键与值两部分组成，通过键可以找对所对应的值。

l  Collection中的集合称为单列集合，Map中的集合称为双列集合。

l  需要注意的是，Map中的集合不能包含重复的键，值可以重复；每个键只能对应一个值。

Map中常用的集合为HashMap集合、LinkedHashMap集合。

Map集合的遍历：

1.keySet

public class Demo01 {
    public static void main(String[] args) {
        Map<Integer,String> map=new HashMap<Integer,String>();
        //向map中添加键值对
        map.put(1, "熊大");
        map.put(2, "光头强");
        map.put(3, "熊二");
        map.put(2, "光头强");
        //根据key获取value
        System.out.println(map.get(3));
        //删除
        map.remove(1);
        //遍历方式1：keySet方法
        Set<Integer>set=map.keySet();
        for(Integer key:set){
            System.out.println(map.get(key));
        }
        Iterator<Integer>it=set.iterator();
        while(it.hasNext()){
            System.out.println(map.get(it.next()));
        }  
    }

2.entrySet

public class Demo02 {
    public static void main(String[] args) {
        Map<Integer,String> map=new HashMap<Integer,String>();
        //向map中添加键值对
        map.put(1, "熊大");
        map.put(2, "光头强");
        map.put(3, "熊二");
        map.put(2, "光头强");
        //遍历方式2：Entry
        Set<Map.Entry<Integer, String>>set=map.entrySet();
        for(Map.Entry<Integer, String>entry:set){
            System.out.println(entry.getKey()+"\t"+entry.getValue());
        }
    }
}

练习：

Iractor,for和entrySet,keySet遍历Map集合

public class Demo04 {
    public static void main(String[] args) {
        Map<GirlFriend,String>map=new HashMap<GirlFriend,String>();
        //创建对象
                GirlFriend g1=new GirlFriend("吴宣仪",24,166);
                GirlFriend g2=new GirlFriend("杨超越",21,168);
                GirlFriend g3=new GirlFriend("朴孝敏",28,167);
                GirlFriend g4=new GirlFriend("罗玉凤",34,165);
                //向map集合中添加key和value
                map.put(g1, "张三");
                map.put(g2, "李四");
                map.put(g3, "王五");
                map.put(g4, "李四");
            //遍历entrySet+for
                Set<Map.Entry<GirlFriend,String>>set=map.entrySet();
                for(Map.Entry<GirlFriend, String> entry:set){
                    System.out.println(entry.getKey()+"\t"+entry.getValue());
                }
                System.out.println("===============================");
            //entry+iterator
                Iterator<Map.Entry<GirlFriend, String>>it=set.iterator();
                while(it.hasNext()){
                    Map.Entry<GirlFriend, String>ent=it.next();
                    System.out.println(ent.getKey()+"\t"+ent.getValue());
                }
                
    }
}

public class Demo03 {
    public static void main(String[] args) {
        Map<GirlFriend,String>map=new HashMap<GirlFriend,String>();
        //创建对象
                GirlFriend g1=new GirlFriend("吴宣仪",24,166);
                GirlFriend g2=new GirlFriend("杨超越",21,168);
                GirlFriend g3=new GirlFriend("朴孝敏",28,167);
                GirlFriend g4=new GirlFriend("罗玉凤",34,165);
                //向map集合中添加key和value
                map.put(g1, "张三");
                map.put(g2, "李四");
                map.put(g3, "王五");
                map.put(g4, "李四");
            //KeySet+for
            Set<GirlFriend>set=map.keySet();
            for(GirlFriend g:set){
                System.out.println(g+"\t"+map.get(g));
            }
            System.out.println("===================================================");
            //KeySet+iterator
            Iterator<GirlFriend>it=set.iterator();
            while(it.hasNext()){
                GirlFriend gg=it.next();
                System.out.println(gg+"\t"+map.get(gg));
            }
    }
}