BFS graph traversal - Append new node to adjacency matrix

Question

I'm trying to create a program that let's you append and remove nodes from a graph, then run BFS and DFS traversals.

So I originally add and connect nodes at runtime... then I want to allow users to hit a add child to parent button which appends a child appropriately.

AddButton.addActionListener(new ActionListener() {   
        public void actionPerformed(ActionEvent e)
        {   
            Nodes nX = new Nodes("X", nodeX, nodeY, nodeWidth, nodeHeight);
            appendNode(rootNode, nX);
        }
   });  

}

When I try to append a new node, it seems to overwrite the current adjacency matrix and replace it with the single new node X as a child of A .

I think I know why it's doing that... because my appendNode() function tries to update the adjacency matrix by overwriting the old one and creating a new one with the new nodeList size:

public void appendNode(Nodes parent, Nodes child) {
    //add new node X to nodeList
    addNode(child);

    //loop through all nodes again to be connected, plus new one... then create new adjMatrix
    System.out.println(nodeList.size());
    size = nodeList.size();

    adjMatrix = null;
    adjMatrix = new int[size][size];

    int fromNode = nodeList.indexOf(parent);
    int toNode = nodeList.indexOf(child);
    adjMatrix[fromNode][toNode] = 1;
    adjMatrix[toNode][fromNode] = 0;

}

public void addNode(Nodes n) {
    nodeList.add(n);
}

But I don't know any other way to achieve what I want without overwriting it.

Any thoughts?

Thanks!

---

FYI, here's the connect node method:

public void connectNode(Nodes from, Nodes to)
{
    //if matrix is empty..
    if(adjMatrix == null)
    {
        //set matrix [row][col] size to size of nodesList list
        size = nodeList.size();
        //set dimensions of adj matrix... (6x6 nodesList)

        adjMatrix = new int[size][size];
    }

    int fromNode = nodeList.indexOf(from);
    int toNode = nodeList.indexOf(to);

    //connect node A to B and B to A, set that i,j position = 1
    adjMatrix[fromNode][toNode] = 1;
    adjMatrix[toNode][fromNode] = 0;

}

---

EDIT:

public void appendNode(Nodes parent, Nodes child) {
    //add new node X to nodeList
    addNode(child);

    //loop through all nodes again to be connected, plus new one... then create new adjMatrix
    int newSize = nodeList.size();

    //make a new adj matrix of the new size...
    int[][] adjMatrixCopy = new int[newSize][newSize];

    int fromNode = nodeList.indexOf(parent);
    int toNode = nodeList.indexOf(child);
    adjMatrixCopy[fromNode][toNode] = 1;
    adjMatrixCopy[toNode][fromNode] = 0;


    //copy adjMatrix data to new matrix...
    for (int i = 0; i < adjMatrix.length; i++) {    
        for (int j = 0; j < adjMatrix[i].length; j++) {
            adjMatrixCopy[i][j] = adjMatrix[i][j];
        }
    }

    //still need to add newly added node 

    //adjMatrix = null;
}

Answer 1

By blanking the adjacency matrix and replacing it with a new one with the updated size, all cells except (from, to) and (to, from) will be zero. You need to make a new adjacency matrix with the new size, copy the data from the old one, and only then overwrite the old one.

Answer 2

Why not an adjacency list? or an adjacency matrix using collections which can be incremented on the fly?

I think using an array will lead to this perennial problem. You should use a collection if you want to use an adjacency matrix. that said, if you are just doing a BFS, an adjacency list would work faster so I would suggest that.