Making copies of Objects in Java

Question

I have this Java code:

boolean[][] defaultBoard = {{false, false, false},
                            {false, false, false},
                            {false, false, false}};
Board board = new Board(defaultBoard);
Board nextBoard = new Board(defaultBoard);
nextBoard.getBoard()[1][5] = true;
board.printBoard();

printBoard:

public void printBoard() {
    for (boolean row[] : board) {
        for (boolean cell : row) {
            System.out.print( cell ? "#" : "." );
        }
        System.out.println();
    }
    System.out.println();
}

But it returns

...
.#.
...

Which makes me think that nextBoard and board have got the same memory address. My question is: How do create a copy of nextBoard so that when nextBoard is edited, board isn't edited as well?

you can achieve the same thing with a `short` and bitwise operations. — Kevin, May 11 '13 at 20:54
`defaultBoard.clone()`. (But then you need to clone each row as well.) — Hot Licks, May 11 '13 at 20:57
If you have a 3 x 3 board how are you updating the 6th element i.e. `[5]` accessed the sixth element. — Peter Lawrey, May 11 '13 at 21:49

Joe F · Answer 1 · 2013-05-11T21:08:32.853

4

It's not board and nextBoard that are the same. It is the defaultBoard array you are passing to the constructor. You need to pass different arrays. Otherwise, you are just modifying the same array.

Better yet, to ensure instances of Board always contain a new array, I would recommend doing a deep copy of the array inside the constructor.

edited May 11 '13 at 21:08

answered May 11 '13 at 20:56

Joe F

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if `Board`'s constructor made a defensive copy of its input, then it would be harder to write a misbehaving client. – Mike Samuel May 11 '13 at 21:02

Daniel Pereira · Answer 2 · 2013-05-11T20:59:50.990

0

An array is an object, so you will pass a reference to the Board constructor. Since you are using the same array, both board and nextBoard are sharing the same object. To avoid this situation, use the clone method. This way, you will have a copy of the defaultBoard array and every instance you have their own board. It will look like this:

Board board = new Board(cloneBoard(defaultBoard));
Board nextBoard = new Board(cloneBoard(defaultBoard));

private boolean[][] cloneBoard(boolean[][] boardToClone) {
    boolean[][] clone = new boolean[boardToClone.length][];
    for(int i = 0; i < boardToClone.length; i++) {
        clone[i] = boardToClone[i].clone();
    }
    return clone;
}

edited May 11 '13 at 20:59

answered May 11 '13 at 20:53

Daniel Pereira

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clone doesn't perform a deep copy... meaning lines are referring to the same array, this wouldn't solve the problem – Francisco Spaeth May 11 '13 at 21:01
The method `cloneBoard` goes through every line an make a clone, so it is supposed to work correctly. – Daniel Pereira May 11 '13 at 21:05

score 0 · Answer 3 · answered May 11 '13 at 20:54

0

Use the .clone() method, present in any Java Object subclass, namely, all of them, far as I can tell.

answered May 11 '13 at 20:54

hd1

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clone doesn't perform a deep copy... meaning lines are referring to the same array, this wouldn't solve the problem – Francisco Spaeth May 11 '13 at 21:00

score 0 · Answer 4 · answered May 11 '13 at 21:50

In this case, I would have the board create the array base don a size

class Board {
    final boolean[] board;

    public Board(int width, height) {
        board = new[width][height];
    }

    public boolean[][] getBoard() { return board; }

    public void set(int x, int y, boolean value) { board[x][y] = value; }
}

Now your code looks like this

Board board = new Board(3, 3);
Board nextBoard = new Board(3, 3);
nextBoard.set(1, 2, true); // the last element is 2 not 5
board.printBoard();

score 0 · Answer 5 · answered May 13 '13 at 06:23

0

You need to create a new fresh array every time you need to pass it to object so that the object's see a new array rather then the same array .

answered May 13 '13 at 06:23

csk

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Making copies of Objects in Java

5 Answers5