Generic Graph using Adjacency matrix - Java

Question

The main purpose of representing Graph using adjacency matrix method is, to check the vertex and its neighbor's existence in constant time proportional to \$\mathcal{O}(n)\$.

In the various tutorials I have seen, Graphs contain only integer vertices and it becomes straight forward to represent them in a \$v \times v\$ integer 2D array to map the vertices.

In real world, we might have to store a custom type Object as the graph vertex. For this, I have created a Graph implementation with Adjacency matrix. Can you please let me know of any feedback / improvements?

//V - type of Object stored on graph vertices
public class GraphAM<V> {

    //Maps vertex with its adjacency matrix index. O(1) to retrieve index of a vertex
    private Map<V, Integer> vertices;
    //To get vertex using index at O(1) time
    private List<V> verticesLookup;

    //adjacency matrix
    private int[][] adj;

    private int index;

    public GraphAM(int numVertices) {
        adj = new int[numVertices][numVertices];
        index = 0;
        vertices = new HashMap<>();
        verticesLookup = new ArrayList<>();
    }

    public void addEdge(V from, V to) {
        addVertex(from);
        addVertex(to);

        int fromIndex = vertices.get(from);
        int toIndex = vertices.get(to);
        adj[fromIndex][toIndex] = 1;
    }

    private void addVertex(V v) {
        if(!vertices.containsKey(v)) {
            vertices.put(v, index);
            verticesLookup.add(index, v);
            index++;
        }
    }

    public void bfs(V start) {
        Queue<V> queue = new LinkedList<>();
        boolean[] visited = new boolean[vertices.size()]; 

        queue.add(start);
        int index = vertices.get(start);
        visited[index] = true;

        while(!queue.isEmpty()) {
            V v = queue.poll();
            System.out.print(v + " ");

            List<V> adjacentVertices = getAdjacentVertices(v);
            for(V a : adjacentVertices) {
                int adjInd = vertices.get(a);
                if(!visited[adjInd]) {
                    queue.add(a);
                    visited[adjInd] = true;
                }
            }

        }

    }

    public void dfs(V start) {
        boolean[] visited = new boolean[vertices.size()];
        dfs(start, visited);
    }

    private void dfs(V v, boolean[] visited) {
        System.out.print(v + " ");
        int index = vertices.get(v);
        visited[index] = true;

        List<V> adjacentVertices = getAdjacentVertices(v);
        for(V a : adjacentVertices) {
            int aIndex = vertices.get(a);
            if(!visited[aIndex]) {
                dfs(a, visited);
            }
        }
    }

    private List<V> getAdjacentVertices(V v) {
        int index = vertices.get(v);
        List<V> result = new ArrayList<>();
        for(int i=0; i<adj[index].length; i++) {
            if(adj[index][i] == 1) {
                result.add(verticesLookup.get(i));
            }
        }
        return result;
    }

}

Main class

class Main {

  public static void main(String[] args) {
            GraphAM<Integer> g = new GraphAM<>(4);

            g.addEdge(0, 1);
            g.addEdge(0, 2);
            g.addEdge(1, 2);
            g.addEdge(2, 0);
            g.addEdge(2, 3);
            g.addEdge(3, 3);

            System.out.println("Following is Breadth First Traversal "+
                    "(starting from vertex 2)");

            g.bfs(2);

            System.out.println("\nFollowing is Depth First Traversal "+
                    "(starting from vertex 2)");

            g = new GraphAM<>(4);

            g.addEdge(0, 1);
            g.addEdge(0, 2);
            g.addEdge(1, 2);
            g.addEdge(2, 0);
            g.addEdge(2, 3);
            g.addEdge(3, 3);

            g.dfs(2);
    }

}

Answer 1

• You can simplify the mapping you are using to get from vertex to it's associated object. Instead of using a List<V>, which can only guarantee \$\mathcal{O}(n)\$ lookup time, you could just use an array.

• Comments restating what's known should be removed, comments that can be made obsolete through refactoring should prompt that refactoring:

  public class GraphAM<V> {
      private Map<V, Integer> vertexToIndex;
      private V[] indexToVertex;
  
      private int[][] adjacencyMatrix;
      private int index;
  

• vertices (aka vertexToIndex can be eagerly initialized.

• all private fields (apart from index) should be marked final.

• Instead of passing numVertices to the constructor, you should consider passing a Collection<V>, which removes the need to deal with adding vertices in addEdge.

• Finally instead of using a boolean[] for visited, a Set<V> is significantly more obvious. It might come with a minor performance penalty though.

• Finally the search functions do not search for anything... they're just performing an exhaustive traversal and are accordingly useless in their current form...