Storing abstract and concrete types in Map

Question

I have an interface and class like following:

public interface Message extends Wrappable
{
}

public class MessageImpl implements Message
{
}

I want to use Java generics to store these Classes into a Map with interface as key and impl as value as:

map.put(Message.class, MessageImpl.class);

I want it to be typesafe, where key must extends Wrappable and value must extend key class. Any help on this?

Answer 1

A basic Map can't do this type safely, but you can create a simple wrapper around a Map to enforce type safety. That might look something like

class MyMap {
  private final Map<Class<?>, Class<?>> map = new HashMap<>();
  public <A extends Wrappable, B extends A> void put(
      Class<A> interfaceClass, Class<B> implClass) {
    // you can't statically enforce that A is an interface 
    // and B is a concrete class, though you can do it at runtime
    assert interfaceClass.isInterface();
    assert !Modifier.isAbstract(implClass.getModifiers());
    map.put(interfaceClass, implClass);
  }

  public <A extends Wrappable> Class<? extends A> get(Class<A> interfaceClass) {
    return (Class<? extends A>) map.get(interfaceClass);
  }
}