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I have a situation where I need to have a key-value pair data structure. But it should allow multiple keys and values corresponding to that.

It would be like :

a - 1

b - 2

a - 3

So, when retrieving, I can have getFirstValueOfKey(key) and get 1...something like that.

Is there something existing or I need to implement this?

If I need to implement this, I am thinking to proceed as :

Create a class, which a=can hold K-V pairs, and add them to a list. And write corresponding API's required. Is this right approach? Shall I continue like this?

EDIT : I actually want multiple entries of keys in the data structure.

EDIT : The thing is, I want to maintain order in which new entries(keys) were made, i.e. I want to have sequence of keys how they are put, (a and then b and then a). If array of values is used, this sequence is lost.

Example :

a -> 1 (Time 0) b -> 2 (Time 1) a -> 3 (Time 2)

These time stamps are also required.

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3
  • commons.apache.org/proper/commons-collections/javadocs/… should do the trick Commented Apr 3, 2014 at 15:37
  • Could you also explain why do you need multiple keys? Do you want to rely on order of keys? Commented Apr 3, 2014 at 15:47
  • @ProblemFactory - yes, I want to rely on order of keys. :) Commented Apr 8, 2014 at 3:49

2 Answers 2

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6

You can have a look at MultiMap by Guava library from Google

There are two ways to think of a Multimap conceptually: as a collection of mappings from single keys to single values:

a -> 1 a -> 2 a -> 4 b -> 3 c -> 5 or as a mapping from unique keys to collections of values:

a -> [1, 2, 4] b -> 3 c -> 5

Though you have a jar dependency, Guava collections are normally more concise and efficient

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4 Comments

I want to rely on order of keys. Edited question.
then probably you need to use LinkedListMultiMap. Also see this discussion. Though I haven't tested these myself.
This is what was required. :)
glad that it helped :-) would you then consider choosing it as the correct answer? cheers!
2

It looks like you are looking for Map<Character, List<Integer>>.

Since you edited question: to rely both on keys order and values per each key,

  • you should use list of Map.Entry implementation, (for example Pair from Apache Commons), where: List<Map.Entry<Character, List>> struct = new ArrayList<Pair<Character, ArrayList>>();

  • or if you don't want to add additional libraries, instead of Pair you could use just Map (witch will always contains 1 key - your letter): List<Map<Character, List>>.

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4 Comments

It's like, 1 key having multiple values, this is not required. I actually want multiple entries of keys.
@Batty I don't see the difference. It all depends on how you access it. If you want a getFirstValueOfKey operation, use the map get to get the list and then get the first element of the list. It is not hard to get the program to see (key, [val1, val1, val3...]) as (key, val1), (key, val2), (key, val3) ...
@Batty, I don't understand your problem, you want have multiple keys and also keep order of values, my solution solve that problem. You can't rely on keys order in Map.
I want to rely on order of keys. Edited question.

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