Chess/src/Board.java

import java.awt.Color;
import java.awt.Graphics;
import java.util.ArrayList;

public class Board {
    // Width and height of each board tile
    static final int TILE_SIZE = 64;
    // Width and height of board in tiles
    static final int BOARD_SIZE = 8;
    // Width and height of window in pixels
    static final int DIMENSION = TILE_SIZE * BOARD_SIZE;
    // Colors from Lost Century 24
    // https://lospec.com/palette-list/lost-century-24
    // new Color() takes in an integer representing the color
    // Colors are represented in hexadecimal, so we can write the hex literal by prefixing the color code with 0x
    static final Color BLACK = new Color(0x6c595c);
    static final Color WHITE = new Color(0xab9b8e);
    final DrawingPanel panel;
    final Graphics graphics;

    // The board is a two-dimensional array of nullable pieces
    Piece[][] board;
    // The current board coordinate that's being dragged
    BoardCoordinate dragging = null;

    public Board() {
        // Initialize DrawingPanel
        panel = new DrawingPanel(DIMENSION, DIMENSION);
        graphics = panel.getGraphics();

        // Connect up event handlers
        panel.onMouseDown(this::handleMouseDown);
        panel.onMouseDrag((x, y) -> {
            // We want to re-render with new mouse position when dragging pieces
            if (dragging != null) draw(x, y);
        });
        panel.onMouseUp(this::handleMouseUp);

        // Initialize board
        board = new Piece[BOARD_SIZE][BOARD_SIZE];

        // Initialize pieces
        set(1, 0, new Knight(true));
        set(6, 0, new Knight(true));
        set(1, 7, new Knight(false));
        set(6, 7, new Knight(false));
    }

    public boolean outOfBounds(int x, int y) {
        return x < 0 || y < 0 || x >= BOARD_SIZE || y >= BOARD_SIZE;
    }

    public boolean outOfBounds(BoardCoordinate coordinate) {
        return outOfBounds(coordinate.x, coordinate.y);
    }

    public Piece get(int x, int y) {
        if (outOfBounds(x, y)) return null;
        return board[y][x];
    }

    public Piece get(BoardCoordinate coordinate) {
        if (coordinate == null) return null;
        return get(coordinate.x, coordinate.y);
    }

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

    public void set(BoardCoordinate coordinate, Piece piece) {
        set(coordinate.x, coordinate.y, piece);
    }

    public void move(int fromX, int fromY, int toX, int toY) {
        set(toX, toY, get(fromX, fromY));
        set(fromX, fromY, null);
    }

    public void move(BoardCoordinate from, BoardCoordinate to) {
        move(from.x, from.y, to.x, to.y);
    }

    // Mouse down event handler
    // This sets the currently dragging piece
    void handleMouseDown(int x, int y) {
        // Get board coordinate of mouse click
        BoardCoordinate coordinate = new ScreenCoordinate(x, y).toBoard();
        // If there's no piece there, return
        if (get(coordinate) == null) return;
        // Set currently dragging piece to that coordinate
        dragging = coordinate;
        // Redraw with dragging
        draw(x, y);
    }

    void handleMouseUp(int x, int y) {
        // Get board coordinate of mouse release
        BoardCoordinate newCoordinate = new ScreenCoordinate(x, y).toBoard();
        // Only do something if new coordinate is different from the originating coordinate
        if (dragging != null && !newCoordinate.equals(dragging)) {
            // dragging is BoardCoordinate of piece being dragged
            Piece piece = get(dragging);
            ArrayList<BoardCoordinate> legalMoves = piece.getLegalMoves(dragging, this);
            for (BoardCoordinate legalMove : legalMoves) {
                if (newCoordinate.equals(legalMove)) {
                    move(dragging, newCoordinate);
                    break;
                }
            }
        }
        // Clear dragging
        dragging = null;
        // Redraw without dragging
        draw();
    }

    public void draw() {
        draw(null);
    }

    public void draw(int mouseX, int mouseY) {
        draw(new ScreenCoordinate(mouseX, mouseY));
    }

    public void draw(ScreenCoordinate mousePosition) {
        // Draw board
        graphics.setColor(WHITE);
        graphics.fillRect(0, 0, DIMENSION, DIMENSION);
        graphics.setColor(BLACK);
        for (int y = 0; y < BOARD_SIZE; y++)
            for (int x = y % 2; x < BOARD_SIZE; x += 2)
                graphics.fillRect(x * TILE_SIZE, y * TILE_SIZE, TILE_SIZE, TILE_SIZE);

        // Draw pieces
        forEachPiece((boardCoordinate, piece) -> {
            // If piece is the one being dragged, render it at the mouse position
            // Otherwise, render it at the center of the board tile
            piece.draw(graphics, boardCoordinate.equals(dragging) ? mousePosition : boardCoordinate.toScreen());
        });
    }

    // Functional interfaces for forEachPiece
    // Provide ability to run code for each piece on board without having to duplicate nested for loop boilerplate

    @FunctionalInterface
    interface PieceActionXY {
        void forEachTile(int x, int y, Piece piece);
    }

    @FunctionalInterface
    interface PieceActionCoordinate {
        void forEachTile(BoardCoordinate coordinate, Piece piece);
    }

    // Run code on each tile on board
    // Usage:
    // forEachPiece((x, y, piece) -> {
    //     // do something with piece
    // });
    public void forEachPiece(PieceActionXY tileAction) {
        for (int y = 0; y < BOARD_SIZE; y++)
            for (int x = 0; x < BOARD_SIZE; x++) {
                Piece piece = get(x, y);
                if (piece == null) continue;
                tileAction.forEachTile(x, y, piece);
            }
    }

    // Run code on each tile on board
    // Usage:
    // forEachPiece((coordinate, piece) -> {
    //     // do something with piece
    // });
    public void forEachPiece(PieceActionCoordinate tileAction) {
        forEachPiece((x, y, piece) -> tileAction.forEachTile(new BoardCoordinate(x, y), get(x, y)));
    }
}