Question

I’m trying to create a 2 dimensional array of “Node” objects as follows

    public static void main(String[] args) throws IOException {
            length=getNumber("Enter the length of the field: ");
            breadth=getNumber("Enter the breadth of the filed: ");
            node n = new node();
            node [][] field = new node[length][breadth];
            for(i=0;i<=length;i++){
                for(j=0;j<=breadth;j++){
                    F =getNumber("Enter the F value");
                    field[i][j].setF(F);
                    System.out.println("  "+field[i][j].getF(F);
                }
            }

    }

in above code getNumber is a function wherein i print and accept the number
Here is my node class:

public class node {
public int F;
public int G;
public int H;
public boolean isVisited;
public boolean isCurrent;
public void node(int F,int G,int H,boolean isVisited, boolean isCurrent){
    this.F=F;
    this.G=G;
    this.H=H;
    this.isVisited=isVisited;
    this.isCurrent=isCurrent;

}
public int getF() {
    return G+H;
}
public void setF(int f) {
    F = f;
}
public int getG() {
    return G;
}
public void setG(int g) {
    G = g;
}
public int getH() {
    return H;
}
public void setH(int h) {
    H = h;
}
public boolean isVisited() {
    return isVisited;
}
public void setVisited(boolean isVisited) {
    this.isVisited = isVisited;
}
public boolean isCurrent() {
    return isCurrent;
}
public void setCurrent(boolean isCurrent) {
    this.isCurrent = isCurrent;
}

}

all i want to do is, to store/access various values of F,G,H etc in each of the node objects, the problem however is i’m getting java.lang.NullPointerException for field[i][j].setF(F);
i dont know where i’m going wrong, need some help.

Answer 1

You initialized the array, but you did not populate it.

Consider this line:

field[i][j].setF(F);

When you do

field[i][j]

you are accessing the array; i.e. getting what is in the array at that position. Since you didn’t put anything in the array, you get a null. But you immediately try to call setF.

I noticed you do

node n = new node();

outside the loop. You probably want to do that in the loop.

node n = new node();
n.setF(F);
field[i][j] = n;

This code creates a node instance, sets a value on it, and then puts it in the array at the specified position. A bit more fancy approach would be to do something like

node n = field[i][j];
if (n == null) { // initialize n at the position if it doesn't exist
  n = new node();
  field[i][j] = n;
}

field[i][j].setF(f);

Alternatively, you could loop over the array and put a new node at each position, right after you initialize the array.

Finally, in Java standard practice is to start class names with capital letters. node should be Node.