How do I clone an object for the purposes of moving something and seeing if the move is valid? (Java, Chess)

Question

I am writing a simple chess-playing game. I won't post it all here, but I'll give you the necessary details.

I move by clicking a square with a piece on it, then the square becomes selected, and then clicking where I want to piece to move. Sometimes in chess, a move may fail to respond to check or create a check on one's own king and is therefore illegal. The best way, I've found, to decide whether a move is illegal is to make a move on an "ifBoard" (a clone of the board) and if I deem the move legal, set the real board equal to the ifBoard.

Here is a code snippet of me responding to mouse clicks (board is the real board, destination is the clicked square, selectedSquare is the square previously selected(if not null))

    public void mousePressed(MouseEvent e){
            Square selectedSquare = board.selectedSquare();
            Square destination = board.getSquare(e.getX(), e.getY());
            board.deselect();
            if(destination == null){
                repaint();
                return;
            }

            if(selectedSquare == null){
                System.out.println("SelectedSquare is null");
                if(destination.occupiedByTeam(turn)){
                    System.out.println("destination is occupied by right team and is null");
                    board.select(destination);
                }
            }
            else{
                if(!selectedSquare.occupiedByTeam(turn)){
                    System.out.println("SelectedSquare not occupied by correct team");
                    repaint();
                    return;
                }

                if(destination.occupiedByTeam(turn)){
                    System.out.println("ChosenSquare occupied by same team");
                    board.select(destination);
                    repaint();
                    return;
                }

                //move on a dummy board and check for conflicts
                Board ifBoard = (Board)board.clone();

                System.out.println(ifBoard.toString());
                System.out.println(board.toString());
                //check if you can't move due to piece movement limitations
//.place() is a coordinate of the square on the tile system (A-H and 1-8)
                if(
                !ifBoard.move((int)selectedSquare.place().getX(), (int)selectedSquare.place().getY(), (int)destination.place().getX(), (int)destination.place().getY())
                ){
                    repaint();
                    return;
                }

                //if moving results in self-check
                if(ifBoard.check(turn)){
                    //don't move
                    repaint();
                    return;
                }
                else{
                    //move
                    System.out.println("Board moved!");
                    board = new Board(ifBoard);
                    cycleTurns();
                }
            }

            repaint();
        }

The toString calls are registering differently, but I have narrowed the problem down to the ifBoard.move() call actually moving the real board.

Here is the board class, or part of it.

import java.awt.Color;
import java.lang.Cloneable;
import java.awt.geom.*;
import java.awt.Graphics;
import java.awt.Graphics2D;

public class Board implements Cloneable{
    private Square[][] squares;

    private Rectangle2D squareWrap;
    private Rectangle2D boardBorder;

    private Square selectedSquare;

    public Board(){
        squares = new Square[8][8];
        for(int i = 0; i < 8; i++){
            for(int j = 0; j < 8; j++){
                squares[i][j] = new Square(new Point2D.Double(i, j));
            }
        }

        boardBorder = new Rectangle2D.Double(Constants.boardX,
                                             Constants.boardY,
                                             Constants.borderWidth * 2 + Constants.boardSide,
                                             Constants.borderHeight * 2 + Constants.boardSide);

        squareWrap = new Rectangle2D.Double(Constants.boardX + Constants.borderWidth,
                                            Constants.boardY + Constants.borderHeight,
                                            Constants.boardSide,
                                            Constants.boardSide);

        selectedSquare = null;
    }

    public Object clone() {
        Board obj = new Board();
        obj.setSquares(this.squares);
        obj.setSelectedSquare(this.selectedSquare);

        return obj;
    }...

Am I cloning incorrectly? Is there a better way? Thank you in advance.

Answer 1

Am I cloning incorrectly? Is there a better way?

The clone method should always start by calling super.clone() for reasons that I won't go into in this post.

Also, you're not cloning the attributes of the object (you're doing a shallow copy instead of a deep copy). Thus a cloned Board would share the same reference to the squares structure. (Changing the cloned Board would change the original Board.)

(Many people argue that you should avoid clone and Cloneable all together though.)

If I were you, I would strongly consider making the Board class immutable and perhaps go with some copy-on-write mechanism. That would save you a lot of headache I believe.