local variable updates globally in java

Question

I call function move with 2 dimensional array full of zeros at point [6, 5] which has value 0. The function increases the value to 1.

Then the same function call itself again move(x - 1, y, map, i), which means it is at point [5, 5] with value 0, which it increases to 1 and ends itself.

But why was the map variable updated also in the function, which was called first?

private static byte[10][10] myMap = {*all zeros*};

public static void main(String[] args) {
    move(6, 5, myMap, 0);
}

private static void move(int x, int y, byte[][] map, int i) {
    if (map[x][y] == 0) {
        map[x][y]++;
        i++;
    }
    if (i > 1) return;

    System.out.print(x + " " + y);
    // 6 5
    System.out.print(map[5][5] + " " + i);
    // 0 1
    move(x - 1, y, map, i);

    System.out.print(map[5][5] + " " + i);
    // 1 1 ... WTH? Shouldn't this return 0 1 like above?
}

And when it updates the map, why it doesn't update the i variable?

I'm struggling hours to find why, but still don't know:/ Thanks for help

Answer 1

This can be confusing at first sight, but it is very easy to understand when you understand passing by reference and passing by value.

Array variables consist of a reference to the actual array. Basically they are treated the same as objects. This means that you are updating the map that you give to the function.

int variables are primitive types (int, short, byte, long, char, float, double and of course boolean - note the initial lowercase character in the names), which are passed by value in Java. Basically a copy of the value is made. So you can never use such a variable to return any value. You need a return statement if you want to do that.

---

For example:

// using depth first, then width!!!
private static byte[][] myMap = new byte[10][10];

public static void main(String[] args) {
    move(6, 5, myMap, 0);
}

private static byte[][] cloneMap(byte[][] map) {
    byte[][] newMap = new byte[map.length][];
    for (int x = 0; x < map.length; x++) {
        newMap[x] = map[x].clone();
    }
    return newMap;
}

private static void printMap(byte[][] map) {
    for (int x = 0; x < map.length; x++) {
        for (int y = 0; y < map[0].length; y++) {
            System.out.printf("%3d ", map[x][y] & 0xFF);
        }
        System.out.printf("%n");
    }
}

private static int move(int x, int y, byte[][] map, int i) {
    if (map[x][y] == 0) {
        map[x][y]++;
        i++;
    }
    if (i > 1) return i;

    System.out.printf("x: %d, y: %d%n", x, y);
    // 6 5

    printMap(map);
    System.out.printf("i: %d%n", i);

    // -- note, you may still be missing some way of storing the maps
    map = cloneMap(map);
    i = move(x - 1, y, map, i);

    // System.out.println(map[5][5] + " " + i);
    printMap(map);
    System.out.printf("i: %d%n", i);
    return i;
}