How do I add a null object to an ArrayList inside an ArrayList

Question

I am trying to represent an Ad Hoc network using the adjacency matrix structure. To do this, I am creating an ArrayList inside another ArrayList.

When I add a new vertex to the graph, I create a new ArrayList (inside a super ArrayList) and I then have a loop to add a new null object to each ArrayList, however the size of the ArrayLists do not increase correctly and I can't figure out why.

Here is my code:

public class Matrix {

public ArrayList<ArrayList<Edge>> graph;
public ArrayList<Vertex> verticies;
public ArrayList<Edge> edges;

public Matrix() {
    graph = new ArrayList();
    verticies = new ArrayList();
    edges = new ArrayList();
}

public Matrix(ArrayList<Vertex> verticies, ArrayList<Edge> edges) {

    this.verticies = verticies;
    this.edges = edges;      
}

public void addVertex(Vertex v) {
    verticies.add(v);
    graph.add(new ArrayList());

    for(int i=0; i<graph.size()-1; i++ ) {
        graph.get(i).add(null);
    }
}

Any help would be greatly appreciated.

Answer 1

The initial size of graph is 0 , so the for loop in addVertex() runs one time less than it should:

public void addVertex(Vertex v) {
    verticies.add(v);
    graph.add(new ArrayList()); // graph now has size 1

    for (int i = 0; i < graph.size() - 1; i++) { // i = 0, 0 < 0 is false       
        graph.get(i).add(null); // this is not executed for the last added list
    }
}

The next time you call addVertex() it will add null to the previous ArrayList s, but not to the one you just added.

So you probably should do:

for (int i = 0; i < graph.size(); i++)

Even with this fix though, note that if you call addVertex() 5 times you will have something like this:

index             ArrayList
  0      [null, null, null, null, null]
  1      [null, null, null, null]
  2      [null, null, null]
  3      [null, null]
  4      [null]

This is probably not what you want. A better approach would be to add all your vertices first :

public void addVertex(Vertex v) {
    this.vertices.add(v);
}

And then create the ArrayList s for the adjacency matrix with the appropriate size:

public void initializeAdjacencyMatrix() {
    int n = this.vertices.size();
    for (int i = 0; i < n; i++) {
        List<Edge> edges = new ArrayList<>(Collections.nCopies(n, null));
        graph.add(edges);
    }
}

Also, you're using raw types when instantiating the ArrayList s. This is not a good practice. You should use the diamond operator instead. For example:

graph = new ArrayList<>();
graph.add(new ArrayList<>());

Answer 2

On this line:

for(int i=0; i<graph.size()-1; i++ ) {

remove the -1 . Because of the -1 when the size of the graph is one i will not be less than 0, so the loop does not run. If the loop does not run then your code which adds the null value can not run.