I am writing a prim algorithm in java. To enable a good extendibility, I used some generics. But there's a generics mismatch that bothers me a lot. I summarize it here and show my code below.

the class of LazyPrimMST is to calculate a graph's mst. So I define a generics to present the type of all kinds of WeightedGraph. In my implementation, the part of the LazyPrimMST generics is >, and there are two subclass of WeightedGraph and they are DenseWeightedGraph and SparseWeightedGraph. In my test main function, I want to calculate the mst for a DenseWeightedGraph instance, so I instantiate the generics of the part of LazyPrimMST like DenseWeightedGraph, but I get a weird error which is Bound mismatch: The type DenseWeightedGraph<Double> is not a valid substitute for the bounded parameter <Graph extends WeightedGraph<Weight>> of the type LazyPrimMST<Weight,Graph>

Error info

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
    Bound mismatch: The type DenseWeightedGraph<Double> is not a valid substitute for the bounded parameter <Graph extends WeightedGraph<Weight>> of the type LazyPrimMST<Weight,Graph>

related code, part of LazyPrimMST class

public class LazyPrimMST<Weight extends Number & Comparable<Weight>, Graph extends WeightedGraph<Weight>> {
    private Graph G;
    private MinHeap<Edge<Weight>> pq;
    private boolean[] marked; 
    private List<Edge<Weight>> mst;
    private Weight mstWeight;

    public LazyPrimMST(Graph graph) {
        this.G = graph;
        pq = new MinHeap<>(G.E());
        marked = new boolean[G.V()];
        mst = new ArrayList<>();

part of my test main which contains the error line

DenseWeightedGraph<Double> g2 = new DenseWeightedGraph<>(8, false);
ReadWeightedGraph readGraph2 = new ReadWeightedGraph(g2, filename);
System.out.println("test g1 in Dense Graph:");
g2.show();

System.out.println();

LazyPrimMST<Double, DenseWeightedGraph<Double>> mst = new LazyPrimMST<>(g2); // error line
for (Edge<Double> edge: mst.mstEdges()) {
    System.out.println("(" + edge + ") ");
}

part of DenseWeightedGraph

public class DenseWeightedGraph<Weight extends Number & Comparable> implements WeightedGraph {
    private int n;  
    private int m;  
    private boolean directed;
    private Edge<Weight>[][] g;

    public DenseWeightedGraph(int n, boolean directed) {
        this.n = n;
        this.m = 0;
        this.directed = directed;

        g = new Edge[n][n];
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                g[i][j] = null;
            }
        }

    }

1 Answers

2
OLEGSHA On

The declaration of DenseWeightedGraph states that it implements WeightedGraph, not WeightedGraph<Weight>. This ought to have caused the error.

As @Tom mentioned in the comment to your question, you have also not specified that Weight extends Comparable<Weight> and not just Comparable in general. While this compiles (a backwards-compatibility feature), the use of raw types in strongly discouraged.