Java adding a 2D arraylist to a list of 2D arraylists

Question

I am creating a map system for my game which requires me at a few stages to replace the old 2D array map with another. What I wanted to do is add all the 2D arraylist maps into a List of these 2D arraylists. However, what I found difficult was that when I wanted to retrieve said map, the .get method would not give me the correct data, as it you would try to pull the first item of that arraylist which in this case would be null. Thus the program threw me nullpointerexceptions.

private int[][] mapArray;
private List<int[][]> maps = new ArrayList<>();

    mapArray = new int[][]{
    {0, 0, 1, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
    {0, 1, 1, 1, 0, 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, 3, 0, 0, 0, 0, 0, 0},
    {0, 1, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
    {0, 1, 9, 1, 1, 7, 7, 7, 7, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
    {7, 7, 7, 7, 5, 8, 8, 8, 4, 6, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
    {4, 4, 4, 4, 4, 8, 8, 8, 4, 4, 6, 7, 7, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
    {4, 4, 4, 4, 4, 8, 8, 8, 4, 4, 4, 4, 4, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
   };
   maps.add(mapArray);

When I sout maps.get(0) I get the following:

[[I@58838f3f

If I have two items, it will show up like this:

[[[I@58838f3f, [[I@59a60155]

Ideally I want it to be added like this:

[I@58838f3f, I@58838f3f]

I am just unsure how to do this. Any help would be appreciated!
I might have overlooked something obvious, probably is the case!

Much obliged

Answer 1

You seem to be confusing the output of converting your map (or the list of maps) to string (which is what happens when you try to print them) - with what actual value there are there.

Because ArrayList like most Java container formats have nice toString() implementations that get automatically called when trying to print them, you'd get something that "looks like an array" in the output. So for example when printing an array list containing two strings, it might look like this: [a, b] .

Arrays don't have such nice formatters, because they aren't really objects (they are "primitive types") and so print them gets you their address and some Java byte code that describes their type (if you can understand Java byte code), so an array of integers will look like this: [I@<address> while an two dimensional array of bytes will look like this: [[B@<address> , and so on.

In regards to your original question, the code seems to work well for me. Here is an example program:

import java.util.ArrayList;

public class Test2D {

    public static void main(String... args) {
        ArrayList<int[][]> maps = new ArrayList<>();
        maps.add(new int[][] {
            {1,2,3},
            {4,5,6},
            {7,8,9},
        });
        maps.add(new int[][] {
            {10,20,30},
            {40,50,60},
        });
        int[][] map = maps.get(0);
        System.out.println(map[0][0]);
    }
}

The output will print correctly 1 , showing that there is no null pointer exception.