CODEWITHSUNDEEP
javaarrays
ElDoggo
ElDoggo

Reputation: 33

Java Resetting and copying two dimensional arrays

I am having trouble resetting a two-dimensional array in java. I have a class that takes in a two-dimentional array. What I am trying to do is copy an existing array, edit that copy, use that copy in an instance of the class, and reset that array to a copy of the original array, all without modifying the original array. If any more information is needed just ask. Thanks in advance! 

public Iterable<Board> neighbors(){

    Stack<Board> neighbors = new Stack<Board>();

    for (int i = 0; i < N; i++){
        for (int j = 0; j < N; j++){
            if (tiles[i][j] == 0){

                int [][] copy = new int[N][N];
                System.arraycopy(tiles, 0, copy, 0, tiles.length);

                if (i != 0){
                    exch(copy, i, j, i - 1, j);
                    neighbors.push(new Board(copy));
                    copy = null;
                    System.arraycopy(tiles, 0, copy, 0, tiles.length);
                }
                if (i <= N - 2){
                    exch(copy, i, j, i + 1, j);
                    neighbors.push(new Board(copy));
                    copy = null;
                    System.arraycopy(tiles, 0, copy, 0, tiles.length);
                }
                if (j != 0){
                    exch(copy, i, j, i, j - 1);
                    neighbors.push(new Board(copy));
                    copy = null;
                    System.arraycopy(tiles, 0, copy, 0, tiles.length);
                }
                if (j <= N - 2){
                    exch(copy, i, j, i, j + 1);
                    neighbors.push(new Board(copy));
                    copy = null;
                    System.arraycopy(tiles, 0, copy, 0, tiles.length);
                }
            }
        }
    }
    return neighbors;

}

I changed my code to that shown above but i got this error 

Exception in thread "main" java.lang.NullPointerException
at java.lang.System.arraycopy(Native Method)
at Board.neighbors(Board.java:74)
at Board.main(Board.java:136)

Upvotes: 1

Views: 302

Answers (2)

Wormbo
Wormbo

Reputation: 4992

Remember that in Java multidimensional arrays are actually arrays referencing other arrays. Internally an array in Java can only have a single dimension.

To copy the content of an array, you can use the System.arraycopy() method. Note that for multidimensional arrays this will only copy the top-level array's references to the same inner arrays, so things are a little more complicated:

int[][] copy = new int[original.length][]; // new top-level array of same size
for (int i = 0; i < copy.length; i++) {
    copy[i] = new int[original[i].length]; // new inner array of same size
    System.arraycopy(original[i], 0, copy[i], 0, original[i].length); // copy values
}

Upvotes: 2

Neeraj Jain
Neeraj Jain

Reputation: 7730

int [][] copy = tiles;

This is not how you make a copy of an Array , in the above mentioned code copy is just another reference to tiles Array Object .

Use System.arraycopy

int[] src  = new int[]{1,2,3,4,5};
int[] dest = new int[5];

System.arraycopy( src, 0, dest, 0, src.length );

Here if you even change src , dest will remain the same

Upvotes: 0

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