Deep copy (clone) of an object with matrix (Java)

Question

i have some trouble with deep copying. I have this java project, chess, and I need to use the clone() method, because I need to try new configurations without changing the board.

    Board scacchiera = new Board();
    Initialization(scacchiera);
    Board clone = scacchiera.clone();
    System.out.println(scacchiera.toString());
    System.out.println(clone.toString());

I create an object, scacchiera, then I clone it. I think I have done correctly a deep copy, but when I change something in scacchiera, clone changes too. In object Board:

public class Board implements Cloneable{
//TODO
//rivedere se check e checkmate public o private;
//se private, costruire get e set;

public Pedine[][] board;
public boolean check;
public boolean checkmate;
//creating 2 lists for all the pieces; Neri=black, Bianchi=White
public ArrayList<Pedine> Neri;
public ArrayList<Pedine> Bianchi;

public Board(){

    this.board = new Pedine [8][8];
    this.check = false;
    this.checkmate = false;
    this.Neri = new ArrayList<Pedine>();
    this.Bianchi = new ArrayList<Pedine>();


}

...

@Override
public Board clone() throws CloneNotSupportedException{

    Board cloned = (Board) super.clone();
    cloned.board = (Pedine[][]) board.clone();
    return cloned;
}

I have this double array of Pedine, and I have to clone it too, so I do:

public class Pedine implements Cloneable{

private int x;
private int y;
private Piece pezzo;
private Colour colore;

...

@Override
public Pedine clone() throws CloneNotSupportedException{

    return (Pedine) super.clone();

}

Why it doesn't work?

I tried this code too, but it doesn't work.

@Override
public Board clone() throws CloneNotSupportedException{

    Board cloned = (Board) super.clone();
    cloned.board = (Pedine[][]) board.clone();
    for (int i=0; i<8; i++)
        for(int j=0; j<8; j++){
            cloned.board[i][j] = board[i][j].clone();
        }
    return cloned;
}

(Pedine extends Object)

What is `Piece`? What is `Colour`? Are they both Enums (as they should be) or are they something different? — tucuxi, May 28 '15 at 16:53

score 1 · Answer 1 · edited May 23 '17 at 10:30

1

deep cloning of multi dimension arrays should be custom coded, as is explained here

edited May 23 '17 at 10:30

Community

1
1

answered May 28 '15 at 13:37

Sharon Ben Asher

13,849
5
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I tried, but not working. I added the code in the first post. – Pleasant94 May 28 '15 at 13:45
If I modify the first object, the "deep-cloned" object change too, and it's not supposed to. – Pleasant94 May 28 '15 at 13:53
what modification? in the matrix? – Sharon Ben Asher May 28 '15 at 13:59
Yes. I change a piece on Matrix A and the piece is changed on Matrix A AND on Matrix B. I tried to copy withous using the clone() method too, just copying, but still doesnt' work. I'm starting to think that the problem is the set of the changes on the matrix... – Pleasant94 May 28 '15 at 14:05

score 1 · Accepted Answer · answered May 28 '15 at 13:49

The problem, as sharonbn indicates, is in the double-array. While you can clone it manually with a double-loop, your chess engine is going to suffer a performance penalty: you will be cloning a lot of boards, and you can benefit from making them a lot easier to copy around.

One option is to use a flat array and some clever addressing to speed things up:

private Piece[] board; // 64 Pieces in there
public Piece at(col, row) {
    if (row < 0 || row >= 8 || col < 0 || col >= 8) return null;
    return board[col + row*8];
}

Now, instead of accessing board[row][col] you use at(col, row). And copying and creating boards is a lot easier:

board = other.board.clone();

... should now work as expected.

I also strongly recommend having immutable pieces, with no state information whatsoever. Your current pieces have an x and y field, for example. What do they need those for? You should tell them their actual positions only while moving them; that way, you don't need to clone pieces at all -- because all pawns are exactly alike, and you can actually use the same "black pawn" for everything black-pawn related.

I tried, but still isn't working, the cloned object changes too. By the way, it's easier for me to work with a double array. — Pleasant94, May 28 '15 at 13:55
I can't change the pieces, because i use their field, like x, y and colour, for checking if their movement is legit. If i change, i had to rewrite everything :( — Pleasant94, May 28 '15 at 14:03
Programming an efficient chess engine is hard - this is only scratching the surface. There is something called "bitboards" that dramatically improve speeds in exchange for being quite hard to read. — tucuxi, May 28 '15 at 16:52

Deep copy (clone) of an object with matrix (Java)

2 Answers2