如何在A *中模块化启发式（运行时启发式）

Question

I was wanting to use the strategy pattern to modularize the heuristic in my A* implementation but am having some trouble separating it. I tried to initialize my priority queue with a Comparator that uses my heuristic in the following way:

public AStarSearch(Graph g, AStarHeuristic heuristic) {
        this.heuristic = heuristic;
        this.open = new PriorityQueue<Node>(10, new Comparator<Node>(){
            @Override
            public int compare(Node n1, Node n2) {
                int fScoreOne = n1.getTotalPathWeight() + heuristic.calculate(n1, open);
                int fScoreTwo = n1.getTotalPathWeight() + heuristic.calculate(n1, open);
                if (fScoreOne < fScoreTwo)
                    return 1;
                else if (fScoreOne > fScoreTwo)
                    return -1;
                return 0;
            }
        });
    }

But I get: "Cannot refer to non-final variable heuristic inside and inner class defined in a different method."

I am running it on a weighted complete graph with the plan of using a basic heuristic of moving toward the closest node in the open set (I don't have a destination node, just a set of nodes that need to be visited). Of course, to find the least weight edge to a node in the open set, I need the list/queue of open nodes and the current node (which has a list of edges), so I made the Heuristic interface as follows:

public interface AStarHeuristic {
    public int calculate(Node curr, Queue<Node> open);
}

How can I separate out my heurisitc such that it can be used to sort my Queue at run-time?

Answer 1

The issue here is that you are creating an anonymous inner class and are trying to refer to a local variable in the calling function (here, heuristic ). In order to do this, Java requires that the variable be marked final . If you try changing the method to

public AStarSearch(Graph g, final AStarHeuristic heuristic) {
     // ... Same as before ...
}

the issue should go away.

Hope this helps!