java-图形中的NullPointerException

Question

I was trying to implement an undirected graph using a HashMap of ArrayList to represent the adjacency list. However, I got the null pointer exception error when I initialize the adjacency list, in which I was unable to identify the null pointer. Sorry for the stupid question but I'd appreciate it if you could explain why the error occurs. Thanks!

import java.util.HashMap;
import java.util.ArrayList;

public class UDGraph {      //ajacency list implementation
    private final int V;
    private int E;
    private HashMap<Integer, ArrayList<Integer>> adj;

    public UDGraph(int V) {
        this.V = V;
        this.E = 0;
        for (int v = 0; v < V; v++) {
            Integer vertice = new Integer(v);
            ArrayList<Integer> list = new ArrayList<Integer>();
            adj.put(vertice, list);        //LINE 15
        }
    }

    public static void main(String[] args) {
        UDGraph graph = new UDGraph(5);    //LINE 20
    }
}

And the error message:

Exception in thread "main" java.lang.NullPointerException
at UDGraph.<init>(UDGraph.java:15)
at UDGraph.main(UDGraph.java:20)

Answer 1

您只需要在您的哈希表上调用new HashMap<Integer, ArrayList<Integer>> adj = new HashMap<>();