Multidimensional arrays: Java. How does this code work?

Question

I've been following a tutorial about creating a basic tile-based tower defense game in Java and have encountered a piece of code I cannot wrap my brain around and would like some help. (My main question is at the bottom after the code)

At this point we are iterating through a multidimensional array of 0's and 1's that we pass to a constructor which has a method that assigns a grass tile for 0's and stone tile for 1's and then another method to draw them to the screen creating our game screen. Simple enough, right?

Here is the class:

package data;

import static helpers.Artist.*;

public class TileGrid {

public Tile[][] map;

public TileGrid(int[][] newMap){
    map = new Tile[20][15];
    for(int i = 0; i < map.length; i++){
        for(int j = 0; j <map[i].length; j++){
            switch(newMap[j][i]){
            case 0:
                map[i][j] = new Tile(row * 64, col * 64, 64, 64, TileType.GRASS);
                break;
            case 1:
                map[i][j] = new Tile(row * 64, col * 64, 64, 64, TileType.STONE);
                break;
            case 2:
                map[i][j] = new Tile(row * 64, col * 64, 64, 64, TileType.WATER);
                break;
            }
        }
    }
}

public void Draw(){
    for(int i = 0; i < map.length; i++){
        for(int j = 0; j < map[i].length; j++){
            Tile t = map[i][j];
            DrawQuadTex(t.getTexture(), t.getX(), t.getY(), t.getWidth(), t.getHeight());
        }
    }
}

}

And here is the array we are passing in:

int[][] map = { //20 tiles wide, 15 tiles high
            {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0},
            {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0},
            {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0},
            {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0},
            {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0},
            {0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0},
            {0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
            {0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
            {0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
            {0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
            {0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 2, 2, 2, 0, 0, 0, 0, 0, 0, 0},
            {0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 2, 2, 2, 0, 0, 0, 0, 0, 0, 0},
            {0, 0, 1, 0, 0, 0, 0, 0, 0, 2, 2, 2, 0, 0, 0, 0, 0, 0, 0, 0},
            {0, 0, 1, 0, 0, 0, 0, 0, 0, 2, 2, 2, 0, 0, 0, 0, 0, 0, 0, 0},
            {0, 0, 1, 0, 0, 0, 0, 0, 2, 2, 2, 2, 2, 0, 0, 0, 0, 0, 0, 0},

    };

So my main question has to do with the switch statement in the constructor for the TileGrid .

Why do the i and the j get switched newMap[j][i] when checking what it equals? I get that this code works, well, because it does and I understand nested for loops to iterate through a multidimensional array.

But why wouldn't newMap[i][j] work?

As well at the very beginning of that same constructor why do we create an array (of type Tile) with the dimension of map = new Tile[20][15] when we are passing in an array with the dimensions of map[15][20] ?

I have tried to figure this out and study this on my own and will continue to do so until I understand it but any help would be soooo appreciated!

Answer 1

You are passing to the TileGrid constructor a 2D array of 15 rows and 20 columns, but inside the constructor you create a 2D array of 20 rows and 15 columns. That's why map[i][j] corresponds with newMap[j][i] .

If the input int[][] map was also of 20 rows and 15 columns, you wouldn't have to switch the order of the indices inside the constructor.

Answer 2

This would give an index out of bounds error if it was really switched. However, there is a col variable in the second for loop that is not defined anywhere so it's hard to tell what's going on. This code will not compile.

If it is some code pulled off aa tutorial on some web page, I would just assume that it was a typo and continue on with that in mind. OR better yet, contact the author.