up vote -1 down vote favorite

I have a 2d array which is of objects of the Node class. This is the Node class:

public class Node {
private boolean edge;
private int parent;

public Node() {
 edge = false;
    parent = 0;
}

public Node(boolean edge, int parent) {
    this.edge = edge;
    this.parent = parent;
}

public boolean isNode() {
    return edge;
}

public void setNode(boolean node) {
    this.edge = node;
}

public int getParent() {
    return parent;
}

public void setParent(int parent) {
    this.parent = parent;
}
}

And this is my 2d array:

private Node[][] adjMatrix = new Node[x][y];

In a method named addEdge I am trying to set the node at the points i,j in the array to true.

public void addEdge(int i, int j) {
        adjMatrix[i][j].setNode(true);
        adjMatrix[j][i].setNode(true);
}

However I am getting a nullpointerexception on this line and I do not know how to fix it.

adjMatrix[i][j].setNode(true);

I assume it's a simple answer that I haven't been able to find the answer to because I have been looking for awhile. So any help is appreciated.

Thanks a lot :)

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1  
adjMatrix[i][j] is null => you need, somewhere, to initialise it with a non null value: adjMatrix[i][j] = new Node(...);. – assylias Oct 17 '12 at 16:21
Yes sorry the answer was obvious. I previously had had a 2 dimensional array of booleans and as they are primitive types I did not have to initialize them, when I then changed the array to one of objects I forgot about then having to initialize them :) Thanks for the answers everyone! – DanMc Oct 18 '12 at 16:18

closed as too localized by assylias, AmitD, Vikdor, GrailsGuy, Graviton Oct 18 '12 at 3:17

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2 Answers

up vote 1 down vote accepted

You have not instantiated your Nodes inside the list.

Node[][] adjMatrix = new Node[x][y];

The above statement only initializes your array and does not instantiate the element in it.

You need to terate through the matrix using for loop, and for each element do: -

adjMatrix[i][j] = new Node();

You need to do this before using your matrix elements..

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up vote 0 down vote

The array is initialized to null, you have to allocate instances of Node to fill the array.

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