内部静态泛型 class <<Cannot be resolved to a type>>

Question

我正在尝试使用 HashMap 和 Generics.
创建相邻的 Graph 列表但是，我不明白为什么我会收到错误消息“Edge cannot be resolved to a type”

import java.util.*;
import java.io.*;

public class Main {
    public static void main(String[] args) throws IOException{
        Scanner sc=new Scanner(new File("input.txt"));
        PrintWriter pw=new PrintWriter(new File("output.txt"));
        int roads = sc.nextInt(); // How many Edges we have
        int vertices = sc.nextInt(); // How many vertices Graph has
        Graph<String> g = new Graph<String>(vertices); // Every Vertex is a String
        for (int i=0;i<roads;i++) {
            g.add(sc.next(), sc.next());
        }
        //Here i get an error
        Edge<String> e; //Edge cannot be resolved to a type
    }
    static class Graph<E> {
        HashMap<E,Edge<E>> m;

        public Graph(int vertices) {
            m = new HashMap<E,Edge<E>>(vertices);
        }

        public void add(E from,E to) {
            if (m.get(from)==null) m.put(from,new Edge<E>(to,null));
            else m.put(from, new Edge<E>(to,m.get(from)));
        }
        //Storing Graph as an adjacent list of edges
        static class Edge<E> {
            E to;
            Edge<E> prev;
            public Edge(E to, Edge<E> prev) {
                this.to = to;
                this.prev = prev;
            }
        }
        //
    }
}

请详细说明...
提前致谢

Answer 1

不要试图将 Edge 包含在 Graph 中。将其移动到并行 class，或外部 class。像

public class Main {
    public static void main(String[] args) throws IOException {
        Scanner sc = new Scanner(new File("input.txt"));
        PrintWriter pw = new PrintWriter(new File("output.txt"));
        int roads = sc.nextInt(); // How many Edges we have
        int vertices = sc.nextInt(); // How many vertices Graph has
        Graph<String> g = new Graph<String>(vertices); // Every Vertex is a
                                                        // String
        for (int i = 0; i < roads; i++) {
            g.add(sc.next(), sc.next());
        }
        // Here i get an error
        Edge<String> e; // Edge cannot be resolved to a type
    }

    static class Graph<E> {
        HashMap<E, Edge<E>> m;

        public Graph(int vertices) {
            m = new HashMap<E, Edge<E>>(vertices);
        }

        public void add(E from, E to) {
            if (m.get(from) == null)
                m.put(from, new Edge<E>(to, null));
            else
                m.put(from, new Edge<E>(to, m.get(from)));
        }
    }

    // Storing Graph as an adjacent list of edges
    static class Edge<E> {
        E to;
        Edge<E> prev;

        public Edge(E to, Edge<E> prev) {
            this.to = to;
            this.prev = prev;
        }
    }
}

Answer 2

Edge 是 Graph 中的内部 class，所以在 Graph 中你可以直接调用它 Edge... 但是 main 不在 Graph.

内

从 class Main 内部（包括方法 main），您可以将其称为 Graph.Edge（使用通用参数这将是 Graph.Edge<String>).

在 class Main 之外，您可以将其称为 Main.Graph.Edge（使用泛型，Main.Graph.Edge<String>）。

内部静态泛型 class <<Cannot be resolved to a type>>

Inner static generic class <<Cannot be resolved to a type>>

java

graph