Map with duplicate key but with different values?

Question

Is there any way to use map or any other collection which allow us to store duplicate keys
with different values...
rather then using a List to store multiple values for same key?

Not that I'm aware of. How would you look up the value of that key if the key pointed to multiple values? — Paul Richter
– Paul Richter, Commented Oct 22, 2013 at 13:32
I think the best question to ask here is. When you call map.get(key), and the map contains multiple of key. What do you expect to be returned? — Cruncher
– Cruncher, Commented Oct 22, 2013 at 13:33
Implement your own Map, but I don't see meaning in that. What's the logic behind what you seek, that Map with key and list cannot provide? — Matej
– Matej, Commented Oct 22, 2013 at 13:35
can you give a little view to the scenario you are facing , that brings you to have this requirement — Hussain Akhtar Wahid 'Ghouri'
– Hussain Akhtar Wahid 'Ghouri', Commented Oct 22, 2013 at 13:37

Abhijith Nagarajan · Accepted Answer · 2013-10-22 13:33:10Z

8

Use Google Guava's MultiMap. This allows multiple values with single key

http://guava-libraries.googlecode.com/svn/tags/release03/javadoc/com/google/common/collect/Multimap.html

answered Oct 22, 2013 at 13:33

Abhijith Nagarajan

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1 Comment

Cruncher Over a year ago

+1, but OP should realise that this is effectively rather then using a List to store multiple values for same key?

Suresh Atta · Accepted Answer · 2013-10-22 14:45:27Z

2

Map doesn't allow you to have duplicated keys. That even doesn't make sense.

Possible solution is having list(Collection) of values. Just go for it.If anything stopping you, let us know.

edited Oct 22, 2013 at 14:45

answered Oct 22, 2013 at 13:31

Suresh Atta

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3 Comments

Pankaj Tiwari Over a year ago

Ok,,Got it going with the approach having list of values with same key Just wanted to know wheather there is any approach better then this..

Paul Over a year ago

Not sure about 'any approach better'; this actually sounds like the right approach. Depending on context though, a Map<K,Set<V>> might be better sometimes. Does the order of the values matter? Are there duplicates (for any given key)? Using TreeSet (naturally ordered) or LinkedHashSet (insertion order) might be useful, sometimes, too.

Suresh Atta Over a year ago

@Paul Understood your intention :) list of values in the sense, Having a collection of values for single key. That selecting Collection is next step. +1. Nice point thought. The first Doubt of OP Pankaj is to go for a Collection or not.

Admit · Accepted Answer · 2013-10-22 13:34:53Z

2

There is a Multimap concept. For example in guava. Multimap in guava

But it's not a part of Collection framework.

If you would not like signature like this Map<String, List<Item>>, you could easily wrap it with object. E.g.

class Items {
   private List<Item> items;
   public void add(Item i) {}
}

Of course it would not be possible to add items through map instance as map.add("key", item)

answered Oct 22, 2013 at 13:34

Admit

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Comments

Paul Vargas · Accepted Answer · 2013-10-22 13:43:13Z

0

What about the next?:

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

Add values

// add "key1" and "value1"
if (!map.containsKey("key1")) {
    map.put("key1", new ArrayList<String>());
}
map.get("key1").add("value1");

// add "key1" and "value2"
if (!map.containsKey("key1")) {
    map.put("key1", new ArrayList<String>());
}
map.get("key1").add("value2");

Get values

List<String> values = map.get("key1");

This with string, but can be for any type. And you don't need additional libraries.

edited Oct 22, 2013 at 13:43

answered Oct 22, 2013 at 13:32

Paul Vargas

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Comments

user1585111 · Accepted Answer · 2013-10-23 07:41:36Z

0

you can use MultiMap from apache commons collections

http://commons.apache.org/proper/commons-collections/javadocs/api-3.2.1/org/apache/commons/collections/MultiMap.html

answered Oct 23, 2013 at 7:41

user1585111

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