Exception in thread "main" java.lang.NullPointerException

[albert kao]

//package graph;
import java.util.List;
import java.util.ArrayList;

public class Vertex {
    String color;    
    Vertex parent;
    List<Vertex> next;
    int key;
    int time;

    Vertex(int i) {
        color = "";
        parent = null;
        next = new ArrayList<Vertex>();
        key = i;
        time = 0;
    }
}

//package graph;

import java.util.PriorityQueue;

public class DFS {
    public Vertex[] vertex;
    public int num;
    public  int time;
    public  PriorityQueue<Vertex> queue = new PriorityQueue<Vertex>();
    final static int size = 6;
    public void DFS() {
        vertex = new Vertex[size];
        buildGraph();
    }

    public void DFSearch() {
        for (int i = 1; i <= num; i++) {
            vertex[i].color = "white";
            vertex[i].parent = null;
        }

        for (int i = 1; i <= num; i++) {
            Vertex u = vertex[i];
            if (u.color == "white") {
                DFS_visit(u);
            }
        }
    }

    private void DFS_visit(Vertex u) {
        u.color = "gray";
        System.out.print(u.key + " ");
        for (Vertex v : u.next) {
            if (v.color == "white") {
                v.parent = u;
                DFS_visit(v);
            }
        }
        u.color = "black";
        time = time + 1;
        u.time = time;
    }

    private void buildGraph() {
        Vertex v1 = addVertex(1);
        Vertex v2 = addVertex(2);
        Vertex v3 = addVertex(3);
        Vertex v4 = addVertex(4);
        Vertex v5 = addVertex(5);

        addEdge(v1, v2);
        addEdge(v3, v1);
        addEdge(v2, v4);
        addEdge(v1, v4);
        addEdge(v5, v4);
    }

    private Vertex addVertex(int key) {
        vertex[key] = new Vertex(key);
        num = num + 1;
        return vertex[key];
    }

    private void addEdge(Vertex v, Vertex next) {
        v.next.add(next);
    }

    public void toplogicalSort() {
 //       for (int i = 1; i < vertex.length; i++) {
        for (int i = 1; i < size; i++) {
             queue.add(vertex[i]);
        }
    }

    public static void main(String[] args) {
        DFS dfs = new DFS();
        dfs.DFSearch();
        System.out.println();

        dfs.toplogicalSort();
        while (!dfs.queue.isEmpty()) {
            System.out.print(dfs.queue.poll().key + " ");
        }
    }
}

$ javac Vertex.java
$ javac DFS.java
$ java DFS

Exception in thread "main" java.lang.NullPointerException
at DFS.toplogicalSort(DFS.java:71)
at DFS.main(DFS.java:80)

[Riaan Nel]

Albert, you forgot to ask a question.

Just out of curiosity, why are you only using positions 1 - 5 in your Vertex array? The first index in a Java array is 0.

[Nico Van Brandt]

A constructor does not have return type void in the signature.
It should be:

public DFS() { 
        vertex = new Vertex[size];  
        buildGraph();
}

[Campbell Ritchie]

Does the API description for this method help?