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.

Answer 1

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>());
  }
}