HashMap实现保存两个key相同的数据

public static void main(String[] args) {
    String str1 = "key";
    String str2 = "key";
    System.out.println(str1==str2);
    Map<String, String> map = new IdentityHashMap<>();
    map.put(str1, "value1");
    map.put(str2, "value2");
    map.forEach((k,v)-> System.out.println(k+"->"+v));
}

public static void main(String[] args) {
    String str1 = new String("key");
    String str2 = new String("key");
    System.out.println(str1==str2);
    Map<String, String> map = new IdentityHashMap<>();
    map.put(str1, "value1");
    map.put(str2, "value2");
    map.forEach((k,v)-> System.out.println(k+"->"+v));
}

import java.util.ArrayList;
import java.util.HashMap; 
public class MapAndLink { 
    public static void main(String[] args){ 
        HashMap<Integer, ArrayList> map = new HashMap<>();
        put(1, 1, map);
        put(1, 3, map);
        put(2, 2, map);
        put(3, 4, map);
        put(1, 3, map);
        System.out.println(map.toString());
    }
 
    public static void put(Integer key, Integer str, HashMap<Integer, ArrayList> map){
        ArrayList<Integer> list = map.get(key);
        if(list == null)
            list = new ArrayList();
        for(int i = 0; i < list.size(); ++i){
            if(list.get(i).equals(str))
                return;
        }
        list.add(str);
        map.put(key, list);
    }
}