java - 用于指定泛型的子类显示绑定(bind)不匹配

Question

我正在用java编写一个prim算法。为了实现良好的可扩展性，我使用了一些泛型。但泛型不匹配让我很困扰。我在这里总结并在下面显示我的代码。

LazyPrimMST 类用于计算图的 mst。所以我定义了一个泛型来表示各种WeightedGraph的类型。在我的实现中，LazyPrimMST泛型的部分是>，并且WeightedGraph有两个子类，它们是DenseWeightedGraph和SparseWeightedGraph。在我的测试主函数中，我想计算 DenseWeightedGraph 实例的 mst，因此我实例化了 LazyPrimMST 部分的泛型，如 DenseWeightedGraph，但我收到一个奇怪的错误，即 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>

错误信息

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>

相关代码，LazyPrimMST类的一部分

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<>();

我的测试主部分包含错误行

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 + ") ");
}

DenseWeightedGraph 的一部分

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;
            }
        }

    }

Answer 1

DenseWeightedGraph的声明声明它实现了 WeightedGraph ，不是WeightedGraph<Weight> 。这应该导致了错误。

正如 @Tom 在对您的问题的评论中提到的，您也没有指定 Weight延伸Comparable<Weight>不仅仅是Comparable一般来说。虽然此编译(向后兼容功能)，但在 strongly discouraged 中使用原始类型.