NullPointerException in GridWorld

Question

I know this type of question has been asked before, but in my case my code is different and I have no idea where this NullPointer is coming from.

I am recreating Conway's Game of Life using GridWorld, and I am getting this NullPointerException. I have no clue where it is coming from because looking at the code, it should be working. The act method of this code runs every time GridWorld moves forward a "step". In the first step, the cells are removed and born fine. However, when the second iteration comes, there is a NullPointerException when I try to remove a cell from the grid, when there is a cell there.

int count=0;
    public void act() {
        if (!isDying) {
            count++;
            for (Location loc : this.getGrid().getOccupiedLocations()) {
                int nCount = this.getGrid().getOccupiedAdjacentLocations(loc).size();
                if (nCount == 2 || nCount == 3) {
                    //Do nothing
                } else if (nCount == 1 || nCount == 0) {
                    //Die of loneliness
                    if(!deathRow.contains(loc)) {
                        deathRow.add(loc);
                    }
                } else if(nCount>=4) {
                    //Die of overcrowding
                    if(!deathRow.contains(loc)) {
                        deathRow.add(loc);
                    }
                }
                candidateNeighbors = this.getGrid().getEmptyAdjacentLocations(loc);
                for (Location candidate : candidateNeighbors) {
                    int cCount = this.getGrid().getOccupiedAdjacentLocations(candidate).size();
                    if(cCount==3){
                        if(!birthList.contains(candidate)) {
                            birthList.add(candidate);
                        }
                    }
                }
            }
            isDying=true;
        }
        else{
            count++;
            System.out.println("\nAct: "+count);
            if(deathRow.size()!=0) {
                for (Location death : deathRow) {
                    System.out.println("Death List: "+deathRow);
                    System.out.println("Cell to Die: "+death);
                    this.getGrid().get(death).removeSelfFromGrid();
                }
                deathRow.clear();
            }
            if(birthList.size()!=0) {
                for (Location birth : birthList) {
                    System.out.println("Birth List: "+birthList);
                    System.out.println("Cell to be born: "+birth);
                    Cell newCell = new Cell();
                    newCell.putSelfInGrid(this.getGrid(), birth);
                }
                birthList.clear();
            }
            isDying=false;
        }
    }

When I run the code, this is my console output:

Act: 2
Death List: [(9, 10), (11, 10)]
Cell to Die: (9, 10)
Death List: [(9, 10), (11, 10)]
Cell to Die: (11, 10)
Birth List: [(10, 11), (10, 9)]
Cell to be born: (10, 11)
Birth List: [(10, 11), (10, 9)]
Cell to be born: (10, 9)

Act: 2
Death List: [(10, 9), (10, 11)]
Cell to Die: (10, 9)
Death List: [(10, 9), (10, 11)]
Cell to Die: (10, 11)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at apcs.life.Cell.act(Cell.java:54)

Line 54 is this.getGrid().get(death).removeSelfFromGrid(); Thank you for your guys' help!

Please read the duplicate before re-asking the question – Hovercraft Full Of Eels Jan 20 '19 at 18:22 — Hovercraft Full Of Eels, Jan 20 '19 at 18:22

score 0 · Answer 1 · answered Jan 20 '19 at 18:20

You don't check the output of the method this.getGrid().get(death) this may return a null reference. I suggest you to check for null references before calling object methods

Grid grid = this.getGrid().get(death);

if(grid != null):
   grid.removeSelfFromGrid();

NullPointerException in GridWorld

1 Answers1