为什么我不能(for)循环图形节点和边的输入?

这是我的代码:

import java.util.*;

public class Dijkstra {

// 1 = A
// 2 = B
// 3 = C
// 4 = D
// 5 = E
// 6 = F
// 7 = G
// 8 = H

private static final Graph.Edge[] GRAPH = null;

public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    Graph.Edge[] GRAPH = { 
            new Graph.Edge(sc.next(), sc.next(), sc.nextInt()),
            new Graph.Edge(sc.next(), sc.next(), sc.nextInt()),
            new Graph.Edge(sc.next(), sc.next(), sc.nextInt()),
            new Graph.Edge(sc.next(), sc.next(), sc.nextInt()),
            new Graph.Edge(sc.next(), sc.next(), sc.nextInt()),
            new Graph.Edge(sc.next(), sc.next(), sc.nextInt()),
            new Graph.Edge(sc.next(), sc.next(), sc.nextInt()),
            new Graph.Edge(sc.next(), sc.next(), sc.nextInt())
            };

    String START = sc.next();
    String END = sc.next();
    Graph g = new Graph(GRAPH);
    g.dijkstra(START);
    // print the shortest path using Dijkstra algorithm
    g.printPath(END);
    // g.printAllPaths();
    }
}

class Graph {
    public static Map<String, Vertex> graph; // mapping of vertex names to 
Vertex objects, built from a set of Edges

/** One edge of the graph (only used by Graph constructor) */
public static class Edge {
    public String v1, v2;
    public int dist;

    public Edge(String v1, String v2, int dist) {
        this.v1 = v1;
        this.v2 = v2;
        this.dist = dist;
    }
}

/** One vertex of the graph, complete with mappings to neighbouring vertices */
public static class Vertex implements Comparable<Vertex> {
    public String name;
    public int dist = Integer.MAX_VALUE; // MAX_VALUE assumed to be infinity
    public Vertex previous = null;
    public Map<Vertex, Integer> neighbours = new HashMap<Vertex, Integer>();

    public Vertex(String name) {
        this.name = name;
    }

    public void printPath() {
        if (this == this.previous) {
            System.out.println(this.name);
        } else if (this.previous == null) {
            System.out.printf("%s(unreached)", this.name);
        } else {
            this.previous.printPath();
            System.out.println(this.name);
        }
    }

    public int compareTo(Vertex other) {
        if (dist == other.dist)
            return name.compareTo(other.name);
        return Integer.compare(dist, other.dist);
    }
}

/** Builds a graph from a set of edges */
public Graph(Edge[] edges) {
    graph = new HashMap<String, Vertex>(edges.length);

    // one pass to find all vertices
    for (Edge e : edges) {
        if (!graph.containsKey(e.v1))
            graph.put(e.v1, new Vertex(e.v1));
        if (!graph.containsKey(e.v2))
            graph.put(e.v2, new Vertex(e.v2));
    }

    // another pass to set neighbouring vertices
    for (Edge e : edges) {
        graph.get(e.v1).neighbours.put(graph.get(e.v2), e.dist);
        graph.get(e.v2).neighbours.put(graph.get(e.v1), e.dist); // also do this for an undirected graph
    }
}

/** Runs dijkstra using a specified source vertex */
public void dijkstra(String startName) {
    if (!graph.containsKey(startName)) {
        System.err.printf("Graph doesn't contain start vertex \"%s\"\n", startName);
        return;
    }
    final Vertex source = graph.get(startName);
    NavigableSet<Vertex> q = new TreeSet<Vertex>();

    // set-up vertices
    for (Vertex v : graph.values()) {
        v.previous = v == source ? source : null;
        v.dist = v == source ? 0 : Integer.MAX_VALUE;
        q.add(v);
    }

    dijkstra(q);
}

/** Implementation of dijkstra's algorithm using a binary heap. */
private void dijkstra(final NavigableSet<Vertex> q) {
    Vertex u, v;
    while (!q.isEmpty()) {

        u = q.pollFirst(); // vertex with shortest distance (first iteration will return source)
        if (u.dist == Integer.MAX_VALUE)
            break; // we can ignore u (and any other remaining vertices) since they are unreachable

        // look at distances to each neighbour
        for (Map.Entry<Vertex, Integer> a : u.neighbours.entrySet()) {
            v = a.getKey(); // the neighbour in this iteration

            final int alternateDist = u.dist + a.getValue();
            if (alternateDist < v.dist) { // shorter path to neighbour found
                q.remove(v);
                v.dist = alternateDist;
                v.previous = u;
                q.add(v);
            }
        }
    }
}

/** Prints a path from the source to the specified vertex */
public void printPath(String endName) {
    if (!graph.containsKey(endName)) {
        System.err.printf("Graph doesn't contain end vertex \"%s\"\n", endName);
        return;
    }

    graph.get(endName).printPath();
}

/**
 * Prints the path from the source to every vertex (output order is not
 * guaranteed)
 */
public void printAllPaths() {
    for (Vertex v : graph.values()) {
        v.printPath();
        }
    }
}

在我的主要方法中,我尝试循环图形的输入:

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        Graph.Edge[] GRAPH = { 
                new Graph.Edge(sc.next(), sc.next(), sc.nextInt()),
                new Graph.Edge(sc.next(), sc.next(), sc.nextInt()),
                new Graph.Edge(sc.next(), sc.next(), sc.nextInt()),
                new Graph.Edge(sc.next(), sc.next(), sc.nextInt()),
                new Graph.Edge(sc.next(), sc.next(), sc.nextInt()),
                new Graph.Edge(sc.next(), sc.next(), sc.nextInt()),
                new Graph.Edge(sc.next(), sc.next(), sc.nextInt()),
                new Graph.Edge(sc.next(), sc.next(), sc.nextInt())
                };

        String START = sc.next();
        String END = sc.next();
        Graph g = new Graph(GRAPH);
        g.dijkstra(START);
        // print the shortest path using Dijkstra algorithm
        g.printPath(END);
        // g.printAllPaths();
    }

所以我试着让它像这样循环,我可以循环图形输入n次。然而,每当我这样做的时候,总会有一个问题

public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    for(int i = 0; i < n; i++) {
        Graph.Edge[] GRAPH = {
            new Graph.Edge(sc.next(),sc.next(),sc.nextInt())
        };
    }

错误是:

Exception in thread "main" java.lang.NullPointerException
at Graph.<init>(Dijkstra.java:80)
at Dijkstra.main(Dijkstra.java:26)

我想 知道 问题是什么,以及问题的解决方案。