a subclass for specifying a generic shows bound mismatch

Question

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;
            }
        }

    }

Well, one issue is your raw usage of `Comparable` in `DenseWeightedGraph`. It might not solve your question here, but you should avoid raw types. — Tom, May 11 '19 at 14:53

score 2 · Answer 1 · answered May 11 '19 at 16:13

2

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.

answered May 11 '19 at 16:13

OLEGSHA

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3
13

bingo！your first suggest is right, I had not noticed that until you said. Because I hardly used generics in java before, I always forget avoiding the use of raw type. – Melvin Levett May 11 '19 at 16:37
I have my IDE set up so it issues warnings about raw types. I find it very useful so I'd advice you to do the same if you haven't done it already. – OLEGSHA May 11 '19 at 16:40
Okay, in fact my IDE remind it. But I ignore warnings as usual. – Melvin Levett May 11 '19 at 16:44
It really is worth the time and effort to resolve all warnings. They exist for a reason, after all. If you are OK with, say, not writing `@Override`s ([although you shouldn't be](https://stackoverflow.com/questions/94361/when-do-you-use-javas-override-annotation-and-why?rq=1)), reconfigure your IDE to omit these warnings. – OLEGSHA May 11 '19 at 16:48

a subclass for specifying a generic shows bound mismatch

1 Answers1