Question

I am having problems understanding how deep copying works. I have this 3d vector that I wish to copy.

int bands[][][] = new int[parameters.numberPredictionBands + 1][][];
int copy[][][] = new int [parameters.numberPredictionBands + 1][][];

Then I pass this vector to some method that changes bands

prepareBands(bands);

And finally I need to create a deep copy of bands, so when copy is changed bands remains the same and viceversa.

copy = copyOf3Dim(bands, copy);

I’ve tried these different methods but they don’t seem to work for me

Method 1:

private int[][][] copyOf3Dim(int[][][] array, int[][][]copy) {

    for (int x = 0; x < array.length; x++) {  
        for (int y = 0; y < array[0].length; y++) {  
            for (int z = 0; z < array[0][0].length; z++) {
                copy[x][y][z] = array[x][y][z];  
            }  
        }  
    } 
    return copy;
}

Method 2:

private int[][][] copyOf3Dim(int[][][] array, int[][][]copy) {

    for (int i = 0; i < array.length; i++) {

        copy[i] = new int[array[i].length][];
        for (int j = 0; j < array[i].length; j++) {
            copy[i][j] = Arrays.copyOf(array[i][j], array[i][j].length);
        }
    }
    return copy;
}

Method 3:

public int[][][] copyOf3Dim(int[][][] array, int[][][] copy) {

    for (int i = 0; i < array.length; i++) {
        copy[i] = new int[array[i].length][];
        for (int j = 0; j < array[i].length; j++) {
            copy[i][j] = new int[array[i][j].length];
            System.arraycopy(array[i][j], 0, copy[i][j], 0, array[i][j].length);
        }
    }
    return copy;
}    

I think my program crashes when doing array[i].length

Could you please tell me what may I be doing wrong?

Answer 1

A general trick for deep cloning I have successfully used several times is to serialize the object into a ByteArrayOutputStream and then immedieately deserialize it. It’s not a top performer, but it’s a simple two-three lines of code and works to any depth.

Arrays happen to be Serializable.

final ByteArrayOutputStream out = new ByteArrayOutputStream();
new ObjectOutputStream(out).writeObject(array);
final Spec clone = (int[][][]) 
     new ObjectInputStream(new ByteArrayInputStream(out.toByteArray())).