Avoiding Returning Wildcard Types

Question

I have a class with a collection of Wildcard Types that is a singleton, something like:

public ObliviousClass{

    private static final ObliviousClass INSTANCE          


        

Answer 1

Here's a type-safe way to store multiple instances of a given type in a map. The key is that you need to provide a Class instance when retrieving values in order to perform runtime type-checking, because static type information has been erased.

class ObliviousClass {

  private final Map map = new HashMap();

  public Object put(Key key, Object value)
  {
    return map.put(key, value);
  }

  public  T get(Key key, Class type)
  {
    return type.cast(map.get(key)); 
  }

}

Usage would look like this:

oc.put(k1, 42);
oc.put(k2, "Hello!");
...
Integer i = oc.get(k1, Integer.class);
String s = oc.get(k2, String.class);
Integer x = oc.get(k2, Integer.class); /* Throws ClassCastException */