adding parameterized object in a Collection

[Mauricio Lopes]

On my way studing AI, I've trying to develop an heuristic search and I come up with the following problem:

I have a fringe in a superclass:

protected Collection<? extends Node<T>> fringe;

for each different search strategies I have a subclass that instantiate a different kind of fringe, for example:

List<Node<T>> findSolution(Node<T> initial) {
		fringe = new PriorityQueue<Node<T>>();
		fringe.add(initial);
		return findSolution();
	}

the add() has a compilation problem:

The method add(capture#2-of ? extends Node<T>) in the type Collection<capture#2-of ? extends Node<T>> is not applicable for the
arguments (Node<T>)

What's happening?

[Stephan van Hulst]

You are trying to add a Node<T> to a collection that holds "something that is a subclass of Node<T>". What if your fringe happens to be an ArrayList<SubNode<T>>? Clearly, you are not allowed to add some Node<T> when it expects a SubNode<T>. One solution is to remove the upper bound on your fringe: simply make it a Collection<Node<T>>.

[Mauricio Lopes]

Thanks Stephan!

I've not mentioned that the code was as you told to be a Collection<Node<T>>. But on design progress I needed a subclass of Node<T> called HeuristicNode<T>. Somewhere else I've gonna need to add that subclass object to fringe. That's why the fringe must be as it is declared.

Can imagine any other solution!?

[Stephan van Hulst]

I think the neatest way is to probably just make it a Collection<Node<T>> and then simply add HeuristicNode<T> to it when you need to. Then when you retrieve nodes, simply check whether the node is a HeuristicNode:

import java.util.*;

public class Test {
  protected Collection<Node<String>> fringe = new ArrayList<Node<String>>();

  public Test() {
    fringe.add(new Node<String>());
    fringe.add(new HeuristicNode<String>());

    for (Node<String> node: fringe) {
      if (node instanceof HeuristicNode) {
        HeuristicNode<String> hnode = (HeuristicNode<String>) node;
        System.out.print("Heuristic");
      }

      System.out.println("Node");
    }
  }

  public static void main(String... args) {
    new Test();
  }
}