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.

Anderson Vieira · Accepted Answer · 2015-04-23 00:27:16Z

3

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 ArrayLists, 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 ArrayLists 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 ArrayLists. This is not a good practice. You should use the diamond operator instead. For example:

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

edited Apr 23, 2015 at 0:27

answered Apr 22, 2015 at 23:48

Anderson Vieira

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4 Comments

Alex Glassman Over a year ago

Thanks for your help, however the ArrayLists now decrease in size by 1 every time (eg. if graph.size() is 5, graph.get(1).size() is 5, graph.get(2).size() is 4 etc. Really baffling me D:

Anderson Vieira Over a year ago

@AlexGlassman That's because you add null to all previous lists every time you create a new one. So the ones the were created first will have more nulls. I updated the answer with a suggestion to deal with this.

Alex Glassman Over a year ago

Nice workaround, however I have to change the type of my inner ArrayLists to Object. Any way around that?

Anderson Vieira Over a year ago

@AlexGlassman Yes, if you are using Java 7, you need pass a type parameter to nCopies(): new ArrayList<>(Collections.<Edge>nCopies(n, null));

Forseth11 · Accepted Answer · 2015-04-22 23:54:51Z

0

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.

answered Apr 22, 2015 at 23:54

Forseth11

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