2d-LinkedList java.lang.NullPointerException by filling the 2d-LinkedList

Question

I want to create a matrix with LinkedLists anf fill all the cells with 0.

private LinkedList<LinkedList<T>> matrix;
private int rows;
private int columns;


// constructor
public Matrix(int rows, int columns){
    this.rows = rows;
    this.columns = columns;
    for(int i=0; i<rows; i++){
        for(int j=0; j<columns; j++){ 
            matrix.get(i).add(0);  // here I get the NullPointerException 
        }
    }
}

What am I doing wrong?

@AniketSahrawat if OP does that the entire for-loop will not do anything. — luk2302, Jan 21 '18 at 16:28
@AniketSahrawat `matrix.get(i)` right now is `null` for every `i`, that means that doing something conditionally on it not being `null` will never happen. — luk2302, Jan 21 '18 at 16:33
@luk2302 Well, MDBAN should take care of it. I wrote the solution just to prevent the NPE :) — Aniket Sahrawat, Jan 21 '18 at 16:34
@AniketSahrawat which is not a proper solution at all, simply deleting the code will prevent the NPE in the exact same way. — luk2302, Jan 21 '18 at 16:35
@luk2302 I did not see the constructor thing, I understand what you were trying to say. Yeah, that's right. If OP is trying to initialize the `matrix`, then my code is redundant. — Aniket Sahrawat, Jan 21 '18 at 16:38

score 0 · Answer 1 · answered Jan 21 '18 at 16:34

You are not creating any linked lists at all, and invoke methods on null instead. What did you expect to happen?

private LinkedList<LinkedList<Integer>> matrix;
private int rows;
private int columns;

// constructor
public Matrix(int rows, int columns){
    this.rows = rows;
    this.columns = columns;
    this.matrix = new LinkedList<LinkedList<Integer>>();
    for(int i=0; i<rows; i++){
        LinkedList<Integer> row = new LinkedList<>();
        for(int j=0; j<columns; j++){ 
            row.add((Integer)0);
        }
        matrix.add(row);
    }
}

And what was T supposed to do? You are adding integer zeroes, this cannot work for anything except Integer.

2d-LinkedList java.lang.NullPointerException by filling the 2d-LinkedList

1 Answers1