addEdge for Graph is return null pointer?

Question

So I am trying to understand hashmaps and graphs, when I run my code, I always get a null pointer exception. on line 25 with "adjVertices.get(v1).add(v2);" and I am sure the next line would throw it too if it would get to it. what am I missing? Thanks in advance.

import java.util.*;

public class Graph {

    private Map<Vertex, List<Vertex>> adjVertices;

    Graph() {
        this.adjVertices = new HashMap<Vertex, List<Vertex>>();
    }

    class Vertex {
        String label;
        Vertex(String label) {
            this.label = label;
        }
    }

    void addVertex(String label) {
        adjVertices.putIfAbsent(new Vertex(label), new ArrayList<>());
    }

    void addEdge(String label1, String label2) {
        Vertex v1 = new Vertex(label1);
        Vertex v2 = new Vertex(label2);
        adjVertices.get(v1).add(v2);
        adjVertices.get(v2).add(v1);
    }

}

Here is what my main looks like:

public class Main {

    public static void main(String args[]) {
        // Create a graph given in the above diagram
        Graph g = new Graph();
        g.addVertex("CS2010");
        g.addVertex("CS2370");
        g.addVertex("CS2381");
        g.addVertex("CS3221");
        g.addVertex("CS3600");
        g.addEdge("CS2010", "CS2370");
        g.addEdge("CS2370", "CS2381");
        g.addEdge("CS2370", "CS3600");
        g.addEdge("CS2381", "CS3221");

    }
}

Answer 1

I always get a null pointer exception. on line 25 with "adjVertices.get(v1).add(v2) ... what am I missing?

If adjVertices.get(v1) returns null, the expression, adjVertices.get(v1).add(v2) will throw NullPointerException because .add(v2) will be called on a null reference. Do it as follows:

void addEdge(String label1, String label2) {
    Vertex v1 = new Vertex(label1);
    Vertex v2 = new Vertex(label2);
    if (adjVertices.get(v1) != null) {
        adjVertices.get(v1).add(v2);
    }
    if (adjVertices.get(v2) != null) {
        adjVertices.get(v2).add(v1);
    }
}

Another problem in your code is that you haven't implemented hashCode and equals in Vertex. Make sure to implement hashCode and equals in the class whenever you are going to add the instances of this class into a HashMap. Given below is the complete code:

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Objects;

class Vertex {
    String label;

    Vertex(String label) {
        this.label = label;
    }

    @Override
    public int hashCode() {
        return Objects.hashCode(label);
    }

    @Override
    public boolean equals(Object obj) {
        return label.equals(((Vertex) obj).label);
    }
}

class Graph {

    public Map<Vertex, List<Vertex>> adjVertices;

    Graph() {
        this.adjVertices = new HashMap<Vertex, List<Vertex>>();
    }

    void addVertex(String label) {
        adjVertices.putIfAbsent(new Vertex(label), new ArrayList<>());
    }

    void addEdge(String label1, String label2) {
        Vertex v1 = new Vertex(label1);
        Vertex v2 = new Vertex(label2);
        if (adjVertices.get(v1) != null) {
            adjVertices.get(v1).add(v2);
        }
        if (adjVertices.get(v2) != null) {
            adjVertices.get(v2).add(v1);
        }
    }
}

public class Main {
    public static void main(String args[]) {
        // Create a graph given in the above diagram
        Graph g = new Graph();
        g.addVertex("CS2010");
        g.addVertex("CS2370");
        g.addVertex("CS2381");
        g.addVertex("CS3221");
        g.addVertex("CS3600");
        g.addEdge("CS2010", "CS2370");
        g.addEdge("CS2370", "CS2381");
        g.addEdge("CS2370", "CS3600");
        g.addEdge("CS2381", "CS3221");
        System.out.println(g.adjVertices);
    }
}

Output:

{Vertex@76fdc54d=[Vertex@76fdd14a], Vertex@76fe414e=[Vertex@76fdd16a], Vertex@76fdd14a=[Vertex@76fdc54d, Vertex@76fdd16a, Vertex@76fe5013], Vertex@76fdd16a=[Vertex@76fdd14a, Vertex@76fe414e], Vertex@76fe5013=[Vertex@76fdd14a]}