Why does method change the boolean array parameter?

Question

I have a class for finding a path through a basic maze. A maze is passed to path along with starting and ending positions. I use recursion to find a path between the points. The path is returned as a boolean array. For some reason the initial maze passed to the Path() gets changed. So I tried making a copy of the maze array to avoid changing any values but it is still not working.

Why is path() making changes to open[][]? Might be the C programming confusing me here.

public static boolean[][] path(boolean[][] open, 
                               int start_i, int start_j, 
                               int end_i, int end_j) 
{
    int n = open.length;
    boolean[][] openCopy = new boolean[n][n]; //make a copy of open

    for(int i = 0; i < n; i++)
    {
        for(int j = 0; j < n; j++)
        {
            openCopy[i][j] = open[i][j]; 
        }
    } 
    return findPath(openCopy, start_i, start_j, end_i, end_j);
}

public static boolean[][] findPath(boolean[][] openCopy, int start_i, int start_j, int end_i, int end_j) 
{
    boolean[][] path = new boolean[openCopy.length][openCopy[0].length];

    if(openCopy[start_i][start_j] == false) //return false if current position is not open
        return path;
    else
        openCopy[start_i][start_j] = false;  //make current position false if not (to prevent infinite backtracking)

    if(start_i == end_i && start_j == end_j) //if end found return true
    {
        path[start_i][start_j] = true;
        return path;
    }

    path = findPath(openCopy, start_i+1, start_j, end_i, end_j); // Move North
    path = findPath(openCopy, start_i, start_j+1, end_i, end_j); // Move East
    path = findPath(openCopy, start_i-1, start_j, end_i, end_j); // Move South
    path = findPath(openCopy, start_i, start_j-1, end_i, end_j); // Move West

    return path;
}

Answer 1

It doesnt, I know this might not be exactly 'appropriate' input for the method, but it proves the point:

public class MainForm {


public static void main(String[] args) {
    boolean[][] array = new boolean[][] {{false, false, true, false, false, false}, 
            {false, false, true, false, false, false}, 
            {false, false, true, false, false, false}, 
            {false, false, true, false, false, false}};

    boolean[] [] another = path(array, 0, 0, 3, 5);

    for (boolean[] bArray : array) {
        for (boolean b : bArray) {
            System.out.println(b);
        }
    }
    System.out.println("***********************");
    for (boolean[] bArray : another) {
        for (boolean b : bArray) {
            System.out.println(b);
        }
    }
}

public static boolean[][] path(boolean[][] open, int start_i, int start_j, int end_i, int end_j) {
    int n = open.length;
    boolean[][] openCopy = new boolean[n][n]; // make a copy of open

    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            openCopy[i][j] = open[i][j];
        }
    }
    return findPath(openCopy, start_i, start_j, end_i, end_j);
}

public static boolean[][] findPath(boolean[][] openCopy, int start_i, int start_j, int end_i, int end_j) {
    boolean[][] path = new boolean[openCopy.length][openCopy[0].length];

    if (openCopy[start_i][start_j] == false) // return false if current position is not open
        return path;
    else
        openCopy[start_i][start_j] = false; // make current position false if not (to prevent infinite backtracking)

    if (start_i == end_i && start_j == end_j) // if end found return true
    {
        path[start_i][start_j] = true;
        return path;
    }

    path = findPath(openCopy, start_i + 1, start_j, end_i, end_j); // Move North
    path = findPath(openCopy, start_i, start_j + 1, end_i, end_j); // Move East
    path = findPath(openCopy, start_i - 1, start_j, end_i, end_j); // Move South
    path = findPath(openCopy, start_i, start_j - 1, end_i, end_j); // Move West

    return path;
}
}

output

false
false
true
false
false
false
false
false
true
false
false
false
false
false
true
false
false
false
false
false
true
false
false
false
***********************
false
false
false
false
false
false
false
false
false
false
false
false
false
false
false
false

Clearly the returned array and the one passed into path() are different in their contents, and the original values of the original array remain intact. I think the problem lies elsewhere in your application.

Answer 2

i think the problem is, that you are passing around references in your inner for-loop. Try instead of this:

for(int j = 0; j < n; j++)
        {
            openCopy[i][j] = open[i][j]; 
        }

this:

for (int j = 0; j < n; j++) {
    openCopy[i][j] = new Boolean(open[i][j]);
}

I think it should help.