Design a Chess Game

Problem Statement: The problem is to design a Chess Game using Object Oriented Principles.

Asked In: Adobe, Amazon, Microsoft, etc.

Solution:
These type of questions are asked in interviews to Judge the Object-Oriented Design skill of a candidate. So, first of all we should think about the classes.

The main classes will be:

1. Spot: A spot represents one block of the 8×8 grid and an optional piece.
2. Piece: The basic building block of the system, every piece will be placed on a spot. Piece class is an abstract class. The extended classes (Pawn, King, Queen, Rook, Knight, Bishop) implements the abstracted operations.
3. Board: Board is an 8×8 set of boxes containing all active chess pieces.
4. Player: Player class represents one of the participants playing the game.
5. Move: Represents a game move, containing the starting and ending spot. The Move class will also keep track of the player who made the move.
6. Game: This class controls the flow of a game. It keeps track of all the game moves, which player has the current turn, and the final result of the game.

Let’s look at the details. These codes are self-explanatory. You can have a look at the properties/variables and methods of different classes.

Spot: To represent a cell on the chess board:








public class Spot {     private Piece piece;     private int x;     private int y;        public Spot(int x, int y, Piece piece)     {         this.setPiece(piece);         this.setX(x);         this.setY(y);     }        public Piece getPiece()     {         return this.piece;     }        public void setPiece(Piece p)     {         this.piece = p;     }        public int getX()     {         return this.x;     }        public void setX(int x)     {         this.x = x;     }        public int getY()     {         return this.y;     }        public void setY(int y)     {         this.y = y;     } }

Piece: An abstract class to represent common functionality of all chess pieces:








public abstract class Piece {        private boolean killed = false;     private boolean white = false;        public Piece(boolean white)     {         this.setWhite(white);     }        public boolean isWhite()     {         return this.white;     }        public void setWhite(boolean white)     {         this.white = white;     }        public boolean isKilled()     {         return this.killed;     }        public void setKilled(boolean killed)     {         this.killed = killed;     }        public abstract boolean canMove(Board board,                                   Spot start, Spot end); }

King: To represent King as a chess piece:








public class King extends Piece {     private boolean castlingDone = false;        public King(boolean white)     {         super(white);     }        public boolean isCastlingDone()     {         return this.castlingDone;     }        public void setCastlingDone(boolean castlingDone)     {         this.castlingDone = castlingDone;     }        @Override     public boolean canMove(Board board, Spot start, Spot end)     {         // we can't move the piece to a Spot that          // has a piece of the same color         if (end.getPiece().isWhite() == this.isWhite()) {             return false;         }            int x = Math.abs(start.getX() - end.getX());         int y = Math.abs(start.getY() - end.getY());         if (x + y == 1) {             // check if this move will not result in the king             // being attacked if so return true             return true;         }            return this.isValidCastling(board, start, end);     }        private boolean isValidCastling(Board board,                                       Spot start, Spot end)     {            if (this.isCastlingDone()) {             return false;         }            // Logic for returning true or false     }        public boolean isCastlingMove(Spot start, Spot end)     {         // check if the starting and          // ending position are correct     } }

Knight: To represent Knight as a chess piece








public class Knight extends Piece {     public Knight(boolean white)     {         super(white);     }        @Override     public boolean canMove(Board board, Spot start,                                              Spot end)     {         // we can't move the piece to a spot that has         // a piece of the same colour         if (end.getPiece().isWhite() == this.isWhite()) {             return false;         }            int x = Math.abs(start.getX() - end.getX());         int y = Math.abs(start.getY() - end.getY());         return x * y == 2;     } }

Similarly, we can create classes for other pieces like Queen, Pawns, Rooks, Bishops etc.

Board: To represent a chess board:








public class Board {     Spot[][] boxes;        public Board()     {         this.resetBoard();     }        public Spot getBox(int x, int y)     {            if (x < 0 || x > 7 || y < 0 || y > 7) {             throw new Exception("Index out of bound");         }            return boxes[x][y];     }        public void resetBoard()     {         // initialize white pieces         boxes[0][0] = new Spot(0, 0, new Rook(true));         boxes[0][1] = new Spot(0, 1, new Knight(true));         boxes[0][2] = new Spot(0, 2, new Bishop(true));         //...         boxes[1][0] = new Spot(1, 0, new Pawn(true));         boxes[1][1] = new Spot(1, 1, new Pawn(true));         //...            // initialize black pieces         boxes[7][0] = new Spot(7, 0, new Rook(false));         boxes[7][1] = new Spot(7, 1, new Knight(false));         boxes[7][2] = new Spot(7, 2, new Bishop(false));         //...         boxes[6][0] = new Spot(6, 0, new Pawn(false));         boxes[6][1] = new Spot(6, 1, new Pawn(false));         //...            // initialize remaining boxes without any piece         for (int i = 2; i < 6; i++) {             for (int j = 0; j < 8; j++) {                 boxes[i][j] = new Spot(i, j, null);             }         }     } }

Player: An abstract class for player, it can be a human or a computer.








public abstract class Player {     public boolean whiteSide;     public boolean humanPlayer;        public boolean isWhiteSide()     {         return this.whiteSide;     }     public boolean isHumanPlayer()     {         return this.humanPlayer;     } }    public class HumanPlayer extends Player {        public HumanPlayer(boolean whiteSide)     {         this.whiteSide = whiteSide;         this.humanPlayer = true;     } }    public class ComputerPlayer extends Player {        public ComputerPlayer(boolean whiteSide)     {         this.whiteSide = whiteSide;         this.humanPlayer = false;     } }

Move: To represent a chess move:








public class Move {     private Player player;     private Spot start;     private Spot end;     private Piece pieceMoved;     private Piece pieceKilled;     private boolean castlingMove = false;        public Move(Player player, Spot start, Spot end)     {         this.player = player;         this.start = start;         this.end = end;         this.pieceMoved = start.getPiece();     }        public boolean isCastlingMove()     {         return this.castlingMove;     }        public void setCastlingMove(boolean castlingMove)     {         this.castlingMove = castlingMove;     } }








public enum GameStatus {     ACTIVE,     BLACK_WIN,     WHITE_WIN,     FORFEIT,     STALEMATE,     RESIGNATION }

Game: To represent a chess game:








public class Game {     private Player[] players;     private Board board;     private Player currentTurn;     private GameStatus status;     private List<Move> movesPlayed;        private void initialize(Player p1, Player p2)     {         players[0] = p1;         players[1] = p2;            board.resetBoard();            if (p1.isWhiteSide()) {             this.currentTurn = p1;         }         else {             this.currentTurn = p2;         }            movesPlayed.clear();     }        public boolean isEnd()     {         return this.getStatus() != GameStatus.ACTIVE;     }        public boolean getStatus()     {         return this.status;     }        public void setStatus(GameStatus status)     {         this.status = status;     }        public boolean playerMove(Player player, int startX,                                  int startY, int endX, int endY)     {         Spot startBox = board.getBox(startX, startY);         Spot endBox = board.getBox(startY, endY);         Move move = new Move(player, startBox, endBox);         return this.makeMove(move, player);     }        private boolean makeMove(Move move, Player player)     {         Piece sourcePiece = move.getStart().getPiece();         if (sourcePiece == null) {             return false;         }            // valid player         if (player != currentTurn) {             return false;         }            if (sourcePiece.isWhite() != player.isWhiteSide()) {             return false;         }            // valid move?         if (!sourcePiece.canMove(board, move.getStart(),                                              move.getEnd())) {             return false;         }            // kill?         Piece destPiece = move.getStart().getPiece();         if (destPiece != null) {             destPiece.setKilled(true);             move.setPieceKilled(destPiece);         }            // castling?         if (sourcePiece != null && sourcePiece instanceof King             && sourcePiece.isCastlingMove()) {             move.setCastlingMove(true);         }            // store the move         movesPlayed.add(move);            // move piece from the stat box to end box         move.getEnd().setPiece(move.getStart().getPiece());         move.getStart.setPiece(null);            if (destPiece != null && destPiece instanceof King) {             if (player.isWhiteSide()) {                 this.setStatus(GameStatus.WHITE_WIN);             }             else {                 this.setStatus(GameStatus.BLACK_WIN);             }         }            // set the current turn to the other player         if (this.currentTurn == players[0]) {             this.currentTurn = players[1];         }         else {             this.currentTurn = players[0];         }            return true;     } }

Reference: http://massivetechinterview.blogspot.com/2015/07/design-chess-game-using-oo-principles.html

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