切换到不同方法时如何修复数组的 NullPointerException

Question

我正在用 Java 创建一个国际象棋游戏，它需要创建一个 2D 对象数组来制作棋盘。数组在创建时显示为其中包含所有对象，但是当我从不同的对象调用不同的方法时，数组突然充满了空值。

这是在 Java 中，包括在 Eclipse 中运行的一些 JavaFX（我认为 JavaFX 不会影响任何东西，但它被标记以防万一）。我试过在多个位置打印数组，它只在 createBoard() 中有效，而不是 setBoard()（或 GUI 的 startGame() 方法。

public class ChessGame {
    public static void main(String[] args) {
        GUI gui = new GUI();
        Application.launch(GUI.class, args);
        Player player1 = new Player();
        Player player2 = new Player();
        gui.startGame(player1, player2);
    }
}

public class GUI extends Application {
    Board chessBoard = new Board();

    @Override
    public void start(Stage primaryStage) {
        GridPane chessBoardPane = chessBoard.createBoard();
        primaryStage.setScene(new Scene(chessBoardPane, 400, 400));
        primaryStage.show();
    }

    public void startGame(Player player1, Player player2) {
        //printing the array here still produces nulls.
        chessBoard.setBoard(player1, player2);
    }

}

public class Board {
    private BoardSquare[][] boardArray;
    public static int boardSize = 8;
    //private GridPane boardGrid = null;

    public Board() {
        boardArray = new BoardSquare[boardSize][boardSize];  
    }

    public GridPane createBoard() {
        GridPane chessBoard = new GridPane();
        for (int x = 0; x < boardSize; x++) {
            for (int y = 0; y < boardSize; y++) {
                StackPane square = new StackPane();
                String color;
                if ((x + y) % 2 == 0) {
                    color = "white";
                } else {
                    color = "black";
                }
                square.setStyle("-fx-background-color: " + color + ";");
                chessBoard.add(square, y, x);
                boardArray[x][y] = new BoardSquare(x, y, color, square, null);
            }
        }
        for (int i = 0; i < boardSize; i++) {
            chessBoard.getColumnConstraints().add(new ColumnConstraints(5, Control.USE_COMPUTED_SIZE, Double.POSITIVE_INFINITY, Priority.ALWAYS, HPos.CENTER, true));
            chessBoard.getRowConstraints().add(new RowConstraints(5, Control.USE_COMPUTED_SIZE, Double.POSITIVE_INFINITY, Priority.ALWAYS, VPos.CENTER, true));
        }
        //at this point, printing the x and y values of the array is possible.
        return chessBoard;
    }

    public void setBoard(Player player1, Player player2) {
        Player currentPlayer = player1;
        for (int x = 0; x < boardSize; x++) {
            for (int y = 0; y < boardSize; y++) {
                //if (boardArray[x][y] != null) {  <-- error occurs if removed
                    if (y == 0) {
                        if (x == 0 || x == 7) {
                            Rook rook = new Rook(currentPlayer);
                            boardArray[x][y].setPiece(rook); //<-- error
                        }
//etc.

在 createBoard 中打印 boardArray 的 x 和 y 值会打印出预期的 8x8 网格的坐标。移至 setBoard() 时，boardArray 仍应充满 BoardSquare 值，但现在突然间所有值都为空，并且尝试打印数组的 x 和 y 值无济于事。我不知道为什么数组突然空了。

Answer 1

您的 main 方法存在 2 个问题：

1. 您创建自己的GUI 类实例，该实例与Application.launch 调用启动的实例不同。 start 永远不会为您在 main 中创建的 GUI 实例调用。
2. Application.launch 阻塞，直到应用程序退出。到那时，传递任何要在 gui 中使用的信息都为时已晚。

Application.launch 创建的Application 实例用作应用程序的入口点。出于这个原因，您应该将这种初始化移至start 方法：

 @Override
public void start(Stage primaryStage) {
    GridPane chessBoardPane = chessBoard.createBoard();

    Player player1 = new Player();
    Player player2 = new Player();
    this.startGame(player1, player2);

    primaryStage.setScene(new Scene(chessBoardPane, 400, 400));
    primaryStage.show();
}