NullPointerException when trying to set an element in a multidimensional ArrayList

Question

I am having a hard time figuring how to add objects to an ArrayList of ArrayList.

When I try:

ArrayList<ArrayList<A>> matrix = new ArrayList<ArrayList<A>>();

matrix.add(new ArrayList<A>());
matrix.add(new ArrayList<A>());
matrix.add(new ArrayList<A>());

matrix.get(0).set(0, A);

I get a NullPointerException.

What am I doing wrong?

Answer 1

You are setting at position not present in arraylist, may be you need "add".

import java.util.ArrayList;

class A{}

public class arrlst {

    public static void main(String[] args) {
        ArrayList<ArrayList<A>> matrix = new ArrayList<ArrayList<A>>();

        matrix.add(new ArrayList<A>());
        matrix.add(new ArrayList<A>());
        matrix.add(new ArrayList<A>());

        /*matrix.get(0).set(1, new A());*/
        matrix.get(0).add(new A());

    }

}

Also you need to create object of class A, you can't set A.

Answer 2

I guess you might be getting java.lang.IndexOutOfBoundsException . And that is because after matrix.add(new ArrayList<A>()); newly added ArrayList's Size is 0. set method initially check the size of the AraayList before setting the element at the mentioned position.

public E set(int paramInt, E paramE) { RangeCheck(paramInt); Object localObject = this.elementData[paramInt]; this.elementData[paramInt] = paramE; return localObject; }

private void RangeCheck(int paramInt) { if (paramInt < this.size) return; throw new IndexOutOfBoundsException("Index: " + paramInt + ", Size: " + this.size); }

As newly added ArrayList has Size as 0, java.lang.IndexOutOfBoundsException is thrown.

So, before setting the element make sure you the elements to the ArrayList using add method.