Create a collection like key value pair with duplicate keys?

Question

consider as,

HashMap<Integer, String> map = new HashMap<Integer, String>();

i want store like,

map.put(1, "Val-1");
map.put(2, "Val-2");
map.put(1, "Val-3");
map.put(1, "Val-4");
map.put(3, "Val-5");
.
.
. it goes like this

when I give a it should give all the values associated with a key. How can I solve it. Not one HashMap if you know other possibilities please to tell me

possible duplicate of [how do i import multimap for java?](http://stackoverflow.com/questions/14925329/how-do-i-import-multimap-for-java) — William Price, Feb 21 '15 at 05:31

Eran · Answer 1 · 2015-02-21T05:34:08.360

6

You can use a HashMap<Integer,List<String>>.

You group all the values sharing the same key in a list.

HashMap<Integer,List<String>> map = new HashMap<>();
List<String> l = new ArrayList<>();
l.add("Val-1");
l.add("Val-3");
l.add("Val-4");
map.put(1, l);
l = new ArrayList<>();
l.add("Val-2");
map.put(2, l);
l = new ArrayList<>();
l.add("Val-5");
map.put(3, l);

edited Feb 21 '15 at 05:34

answered Feb 21 '15 at 05:27

Eran

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score 1 · Answer 2 · answered Feb 21 '15 at 05:41

In Java, Maps do not allow duplicate keys. So, to associate multiple values you'll need to assign every key a List instead. Also, notice how I'm using an interface type on the reference side.

Map<Integer, List<String>> map = new HashMap<Integer, List<String>>();

Now you just have to create your own put() version that adds to these Lists instead.

public void add(Integer key, String val) {
    List<String> list = map.get(key);
    if (list == null) {
        list = new ArrayList<String>();
        map.put(key, list);
    }
    list.add(val);
}

Here's how you would add and display multiple values

add(1, "one");
add(2, "two");
add(2, "three");

for(Map.Entry<Integer, List<String>> entry: map.entrySet()) {
    System.out.println(entry.getKey() + " = " + entry.getValue());
}

Output :

1 = [one]
2 = [two, three]

Create a collection like key value pair with duplicate keys?

2 Answers2