null pointer exception Java with drawing 2D array list

Question

This might be a dumb question, but why am I getting a null pointer exception here? I am trying to paint a tile map for a java applet. I just pasted the problem areas.

private int[][] map;     
public void init()
{
   int map[][]={ {0,0,0},
                {1,1,0},
                {0,0,0} };

}
public void drawCaveTiles(Graphics g)
{
   for(int i = 0; i <= 3; i++)
   {
       for( int j = 0; j <= 3; j++)
       {
           if(map[i][j] == 1)
           {
               g.drawImage(snow_brick, i*64, j*64, this);
           }
           if(map[i][j] == 0)
           {
               g.drawImage(black, i*64, j*64, this);
           }  
       }
    }

}

I fixed it

map =new int[][] { {0,0,0},
          {1,1,0},
          {0,0,0} };

After fixing the null pointer exception you have another logical problem in your code. The way you are using your loop variables i and j inside drawImage is wrong. i is the row of your array and therefore is the y coordinate and j is the column and so the x coordinate. If you already consider the difference between row, x coordinate and column, y coordinate by working with a transposed array see this comment as neglectable. — halex
– halex, Commented Mar 12, 2014 at 6:08

Suresh Atta · Accepted Answer · 2014-03-12 05:59:42Z

1

  int map[][]={ {0,0,0},
                {1,1,0},
                {0,0,0} };

Here in the method init(), you are creating a new map which is local to the method init() and not initializing instance member map.

Change it to

   map ={ {0,0,0},
                {1,1,0},
                {0,0,0} };

And make sure you calling inti() method before calling drawCaveTiles() method.

or Since it is a static data. Move that line to declaring place.

answered Mar 12, 2014 at 5:59

Suresh Atta

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Comments

Christian Tapia · Accepted Answer · 2014-03-12 06:00:35Z

1

In your init() method, you are hiding the class field map, because you are declaring it again (in the local block of the method init()).

Also, make sure you are calling the init() method. It should be called in your constructor:

public class YourApplet {
    public YourAppler() {
        init();
    }

    public void init()
    {
       map[][]={ {0,0,0},
                 {1,1,0},
                 {0,0,0} };
    }
}

answered Mar 12, 2014 at 6:00

Christian Tapia

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Comments

Rahul · Accepted Answer · 2014-03-12 06:00:46Z

1

The local variable int map[][] in the init() is shadowing your instance variable private int[][] map;. Do not create a new local int map in the init() method. Just use the global map array in the init() method.

public void init() {
    map={ {0,0,0}, {1,1,0}, {0,0,0} };
}

answered Mar 12, 2014 at 6:00

Rahul

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