How to declare a map with variable generics?

Question

I have a Map whose keys are of generic type Key<T>, and values are of type List<T>. If the key is an instance of Key<String>, the value must be a List<String>, and the same rule applies to any other key-value pairs. I have tried the following but it does not compile:

Map<T, List<T>> map;

At present I have to declare it with "partial" generics:

Map<Object, List> map;

I know this is bad but I currently have no better choice. Is it possible to use generics in this situation?

UPDATE

Maybe I didn't express my problem clearly. I want a map that is able to:

map.put(new Key<String>(), new ArrayList<String>());
map.put(new Key<Integer>(), new ArrayList<Integer>());

And the following code should not compile:

map.put(new Key<String>(), new ArrayList<Integer>());

The key and value should always have the same generic type while the generic type can be any, and obviously extending a map does not meet my requirement.

space should not be the issue. It's likely the way it is used. OP need to show more code to fix. — Nishant, Jul 16 '12 at 04:33
With your actual scenario, you should use encapsulation to solve your problem as Nishant and Geoff states in their answers, just adapt it to your actual code. — Luiggi Mendoza, Jul 16 '12 at 04:43
Encapsulation does not work here because I have only one map. I don't want to declare `Test`, `Test`, etc. — Zhao Yi, Jul 16 '12 at 04:46

score 7 · Accepted Answer · answered Jul 16 '12 at 05:16

I'm not aware of any existing library that does precisely this but it is not too hard to implement yourself. I've done something similar a few times in the past. You cannot use the standard Map interface but you can use a hash map inside to implement your class. To start, it might look something like this:

public class KeyMap {
  public static class Key<T> { }

  private final HashMap<Object,List<?>> values = new HashMap<Object,List<?>>();

  public <T> void put(Key<T> k, List<T> v) {
    values.put(k, v);
  }

  public <T> List<T> get(Key<T> k) {
    return (List<T>)values.get(k);
  }

  public static void main(String[] args) {
    KeyMap a = new KeyMap();
    a.put(new Key<String>(), new ArrayList<String>());
    a.get(new Key<Integer>());
  }
}

You are actually wrapping my `Map> map` into another class. Seems that a simple map can't meet my requirement, so this may be the last choice. Thank you! — Zhao Yi, Jul 16 '12 at 05:58

Brad · Answer 2 · 2012-07-16T04:45:49.517

4

This is what you want:

public class Test<T> extends HashMap<T, List<T>>
{
}

If you don't want a HashMap as the super class then change it to whatever concrete class you want.

edited Jul 16 '12 at 04:45

answered Jul 16 '12 at 04:33

Brad

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But how could I use only one `Test` instance to store my data? – Zhao Yi Jul 16 '12 at 04:56
I'm not entirely sure what you mean. Could you expand on your question again? – Brad Jul 16 '12 at 05:09
The key and its associated value should always have the same generic type, such as `Key --> List` and `Key --> List`. With your solution, if I declare a `Test`, then I will not be able to put a `Key --> List` to it. – Zhao Yi Jul 16 '12 at 05:16
If that was the system you were trying to make, how would you get the correctly typed values from it? – Brad Jul 16 '12 at 05:18
I think can use a cast like `List list = (List) map.get(new Key())`, but before this, I must first be able to declare such a map. – Zhao Yi Jul 16 '12 at 05:30

How to declare a map with variable generics?

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