Prevent moving into check, implement checkmate detection

src/Board.java

@@ -15,6 +15,8 @@ public class Board {
     // 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);
+    King blackKing;
+    King whiteKing;
     final DrawingPanel panel;
     final Graphics graphics;
 
@@ -50,7 +52,13 @@ public class Board {
                     } else if(j == 2 || j == 5){
                         set(j, i, new Bishop(i==0, panel));
                     } else if(j == 4){
-                        set(j, i, new King(i==0, panel));
+                        if (i == 0) {
+                            blackKing = new King(true, panel);
+                            set(j, i, blackKing);
+                        } else {
+                            whiteKing = new King(false, panel);
+                            set(j, i, whiteKing);
+                        }
                     } else {
                         set(j, i, new Queen(i==0, panel));
                     }
@@ -90,13 +98,15 @@ public class Board {
         set(coordinate.x, coordinate.y, piece);
     }
 
-    public void move(int fromX, int fromY, int toX, int toY) {
+    public Piece move(int fromX, int fromY, int toX, int toY) {
+        Piece captured = get(toX, toY);
         set(toX, toY, get(fromX, fromY));
         set(fromX, fromY, null);
+        return captured;
     }
 
-    public void move(BoardCoordinate from, BoardCoordinate to) {
+    public Piece move(BoardCoordinate from, BoardCoordinate to) {
-        move(from.x, from.y, to.x, to.y);
+        return move(from.x, from.y, to.x, to.y);
     }
 
     // Mouse down event handler
@@ -123,6 +133,19 @@ public class Board {
             for (BoardCoordinate legalMove : legalMoves) {
                 if (newCoordinate.equals(legalMove)) {
                     move(dragging, newCoordinate);
+                    // QUICK TESTING CODE
+                    Piece movedPiece = get(newCoordinate);
+                    King oppositeKing = movedPiece.black ? whiteKing : blackKing;
+                    BoardCoordinate oppositeKingPosition = null;
+                    boolean inCheck = false;
+                    for (BoardCoordinate move : movedPiece.getLegalMoves(newCoordinate, this)) {
+                        if (get(move) == oppositeKing) {
+                            oppositeKingPosition = move;
+                            inCheck = true;
+                            break;
+                        }
+                    }
+                    if (inCheck && oppositeKing.getLegalMoves(oppositeKingPosition, this).size() == 0) System.out.println("Checkmate");
                     break;
                 }
             }

src/Piece.java

@@ -1,5 +1,6 @@
 import java.awt.*;
 import java.awt.image.ImageObserver;
+import java.lang.reflect.Array;
 import java.util.ArrayList;
 
 public abstract class Piece {
@@ -16,6 +17,10 @@ public abstract class Piece {
     public abstract ArrayList<BoardCoordinate> getPossibleMoves(BoardCoordinate position, Board board);
 
     public ArrayList<BoardCoordinate> getLegalMoves(BoardCoordinate position, Board board) {
+        return getLegalMoves(position, board, true);
+    }
+
+    public ArrayList<BoardCoordinate> getLegalMoves(BoardCoordinate position, Board board, boolean doCheckChecks) {
         ArrayList<BoardCoordinate> legalMoves = getPossibleMoves(position, board);
         for (int i = 0; i < legalMoves.size(); i++) {
             BoardCoordinate possibleMove = legalMoves.get(i);
@@ -28,7 +33,11 @@ public abstract class Piece {
                     possibleMove.x < 0 ||
                     possibleMove.y < 0 ||
                     possibleMove.x >= Board.BOARD_SIZE ||
-                    possibleMove.y >= Board.BOARD_SIZE) {
+                    possibleMove.y >= Board.BOARD_SIZE ||
+
+                    // is in check
+                    (doCheckChecks && isInCheck(position, possibleMove, board))
+            ) {
                 legalMoves.remove(i);
                 i--;
             }
@@ -37,6 +46,28 @@ public abstract class Piece {
         return legalMoves;
     }
 
+    boolean isInCheck(BoardCoordinate from, BoardCoordinate to, Board board) {
+        boolean isInCheck = false;
+        Piece captured = board.move(from, to);
+        outer: for (int y = 0; y < Board.BOARD_SIZE; y++) {
+            for (int x = 0; x < Board.BOARD_SIZE; x++) {
+                Piece piece = board.get(x, y);
+                if (piece == null || piece.black == black) continue;
+                ArrayList<BoardCoordinate> legalMoves = piece.getLegalMoves(new BoardCoordinate(x, y), board, false);
+                for (BoardCoordinate move : legalMoves) {
+                    Piece pieceAtMove = board.get(move);
+                    if (pieceAtMove instanceof King) {
+                        isInCheck = true;
+                        break outer;
+                    }
+                }
+            }
+        }
+        board.move(to, from);
+        board.set(to, captured);
+        return isInCheck;
+    }
+
     // The Piece class doesn't store position,
     // so when drawing we need to be provided this along with a graphics context when drawing
     public void draw(Graphics graphics, ImageObserver observer, int x, int y) {