Question

I have been using generics for sometime, but only in a simple straightforward fashion.

Now I would like to do this, I have been reading and trying many things, such as using interfaces and wildcards but with no success. I would like to learn if there is a valid way to achieve this with Java generics, or if I have misunderstood they way in which generics should be used.

Lets say I want to make a generic collection class, an ArrayList, and I want to create such arraylists for various differing types BUT IMPORTANTLY differing types which are ASSURED to implement a certain method. I would then like to be able to call that method from within my generic arraylist.

The following code is very simplistic, and clearly will not work, I know that. I have tried plenty more sophisticated ideas than this, however I include the following code just to sum up what I am trying to do, and to provide an example for answers.

See the line of code in DemonstrateProblem()…I would like to be able to call that on the understanding that I will only ever use types in TestContainer which implement the method StartGame()

public class TestContainer<T> extends ArrayList<T> {
  public void DemonstrateProblem() {
    // assumes we have populated the collection with 5+ objects...
    // the line of code below is ultimately what I would like to acheive
    this.get(4).StartGame(); // <-- my goal!
  }
}

public abstract class Game () {
  public void StartGame() {}
  public void EndGame() {}
}

public class HockeyGame extends Game {
  // ... overrides here
}

public class BaseballGame extends Game  {
  // ... overrides here
}

public class BasketballGame extends Game  {
  // ... overrides here
}

Answer 1

You do not need to keep your container generic on T: you can instantiate an existing generic container on Game, like this:

public class TestContainer extends ArrayList<Game> {
    public void DemonstrateProblem() {
        this.get(4).StartGame(); // <-- should work
    }
}