java map with duplicate keys

Question

I'm creating a program that needs to store key-value pairs. The program needs to accept requests in the form of keys, and return the respective values.

The problem is that there are sometimes multiple values for each key, and the map class doesn't allow for duplicate keys.

The values are numbers, so I can't meaningfully concatenate the values like I would with strings.

Is there any elegant way of accounting for the fact that there can be more than one numerical value for each key? I want each number to be returned, not just one at random.

Will Hartung · Accepted Answer · 2011-03-26T05:17:13.207

20

$ cat YourMap.java
public class YourMap extends HashMap<String, List<Integer>> {
    public void put(String key, Integer number) {
        List<Integer> current = get(key);
        if (current == null) {
            current = new ArrayList<Integer>();
            super.put(key, current);
        }
        current.add(number);
    }

    public static void main(String args[]) {
        YourMap m = new YourMap();
        m.put("a", 1);
        m.put("a", 2);
        m.put("b", 3);
        for(Map.Entry e : m.entrySet()) {
            System.out.println(e.getKey() + " -> " + e.getValue());
        }
    }
}

$ java map
b -> [3]
a -> [1, 2]

edited Mar 26 '11 at 05:17

answered Mar 26 '11 at 04:23

Will Hartung

115,893
19
128
203

don't forget to put current into the map after you create it. – MeBigFatGuy Mar 26 '11 at 04:24
Ah, yea, doh. Should I mention the code was untested? :) – Will Hartung Mar 26 '11 at 05:06
I've implemented this, but when I call the get() method, I always get null. Should we be overriding the get method? If so, how would that be done? If not, what should I do to make the get method return a non-null value? cheers. – oadams Mar 26 '11 at 05:08
@oadams: the code as originally posted would have acted this way -- it was missing the "super.put" line above. You probably don't have that line in your copy. – Ernest Friedman-Hill Mar 26 '11 at 05:12
Use guava's MultiMap as @ErnestFriedman-Hill pointed out – arnabmitra Dec 14 '12 at 20:46

score 11 · Answer 2 · answered Mar 26 '11 at 04:21

11

The structure you're looking for is called a "multimap". You can either use an ordinary map with ArrayLists as values, or you can use a multimap implementation. There's a nice one in Google's "guava" toolkit. See here.

answered Mar 26 '11 at 04:21

Ernest Friedman-Hill

80,601
10
150
186

score 4 · Answer 3 · answered Mar 26 '11 at 04:22

4

If you make the value a list then it will be able to store multiple values for a key

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

answered Mar 26 '11 at 04:22

krock

28,904
13
79
85

java map with duplicate keys

3 Answers3

Linked