Question

In the grid world case study foe my final project I am making a game. In the game if the player clicks the “W” key the shiftUp() method is called which makes all of the other actors of a certain instance move down to create the illusion of the player moving. This isn’t a finished implementation of the method but this should get all the actors in the grid and test if they are an instance of a class called Enemy which extends Actor. If so the actor should move up one space. When I call this method a NullPointerException is called on the line that calls enemy.moveTo(…); This shouldn’t happen because I check to see if it’s null. Can anyone help me with this? I get this:
Exception in thread “AWT-EventQueue-0” java.lang.NullPointerException

public void shiftUp()
{
    if (((GameGrid)getGrid()).getMinX() != 0)
    {
        Grid<Actor> grid = getGrid();
        if (grid != null)
        {
            for (int y = 0; y < getGrid().getNumRows(); y++)
                for (int x = 0; x < getGrid().getNumCols(); x++)
                {
                    Actor enemy = grid.get(new Location(y,x));
                    if (enemy != null && enemy instanceof Enemy)
                        enemy.moveTo(new Location(enemy.getLocation().getRow() - 1, enemy.getLocation().getCol()));
                }
        }
    }
}

Answer 1

Since you are checking for enemy != null beforehand, I’m guessing that enemy.getLocation() is returning null, which is causing a NullPointerException when you call null.getRow() and null.getCol().

If this is the case, then it looks like the problem is that your Actor is never given a proper Location in the grid. Make sure you are using the putSelfInGrid(Grid<Actor> grid, Location loc) method to place the Actor in the grid (not grid.put(Location loc, E obj)), as putSelfInGrid() sets your Actor‘s location accordingly.