java - 需要帮助在我的 Java 应用程序中查找 Nullpointer 异常

Question

我在这里收到一个空指针异常，我不确定为什么。

import cs251.lab2.GomokuGUI;
import cs251.lab2.GomokuModel;

public class Gomoku implements GomokuModel {

public Square[][] gameBoard;

public static void main(String[] args) {
    Gomoku game = new Gomoku(); 
    if (args.length > 0) {
        game.setComputerPlayer(args[0]);
    }
    GomokuGUI.showGUI(game);//This calls newGame().
    }

@Override
public void newGame() {        //Generic constant values.
    this.gameBoard = new Square[NUM_HSQUARES][NUM_VSQUARES];

    // For every block
    for (Square[] s : gameBoard)
        // And every element of that block
        for (Square ss : s)
            // Reset to empty.
            {ss = Square.EMPTY;}

}

@Override
public String boardString() {
    String output = ""; 

    //This is where the problem is!!
    for (Square[] s : this.gameBoard) // For every block
    {
        //output appends either " ", "x" or "o" accordingly.
        for (Square ss : s)//For every element of the block.
            {output += ss.toChar();}

        // Must be time for a new row already!
        output += "\n";
    }

    //output should now be some big nasty string of love xoxo\n baby.
    return output;
}
}

调用 newGame()

存在以下几行，并通过 GomokuGUI.class 调用

public static void showGUI(final GomokuModel model) {
        // For thread safety, invoke GUI code on event-dispatching thread
        SwingUtilities.invokeLater(new Runnable() {
                @Override
                public void run() {
                    new GomokuGUI(model);
                }
            });
    }

..........

      private GomokuGUI ( GomokuModel gomokuModel ) {
        model = gomokuModel;
        model.newGame ( );
        //Buncha GUI code...
      }

如果我删除这个。引用我仍然收到错误。也许我误解了这个？我想通过在 newGame() 中创建大小，我创建了一个与这个游戏实例相关联的唯一对象 gameBoard，并且通过调用 game.gameBoard(这是函数内的 this.gameBoard)我可以查看它的元素。

完整的 GomokuGUI.class 文件

package cs251.lab2;

import java.awt.*;
import java.awt.event.*;

import javax.swing.*;

/**
 * This is the graphical user interface for the Gomoku game.
 */
public class GomokuGUI {

    /**
     * Class used to display the Gomoku game board
     */
    private class BoardPanel extends JPanel {

        /** Used to shut up the compiler */
        private static final long serialVersionUID = 1L;

        /**
         * Draws the x's and o's on the game board in the right places.
         * @param g Graphics context for the component
         */
        public void drawBoard ( Graphics2D g ) {

            // Read the string from the model so I know what to draw,
            // convert to lowercase just in case.
            String s = model.boardString().toLowerCase();

            // Split board into rows
            String[] rowStrings = s.split("\\n");

            // Figure out number of rows and columns
            int numRows = rowStrings.length;
            int numCols = rowStrings[0].length();

            // Use a somewhat wide pen based on size of box
            int offset = SQUARE_SIZE / 5;
            int x2offset = 2 * offset;
            g.setStroke ( new BasicStroke ( offset ) );

            // Draw the signs on the board (within the squares)
            for ( int row = 0; row < numRows; row++ ) {
                for ( int col = 0; col < numCols; col++ ) {

                    // Choose what to do depending on the character in the string I got
                    switch ( rowStrings[row].charAt(col) ) {

                    case 'o': // Draw a ring
                        g.setColor ( Color.BLUE );
                        g.drawOval ( col * SQUARE_SIZE + offset,
                                     row * SQUARE_SIZE + offset,
                                     SQUARE_SIZE - x2offset,
                                     SQUARE_SIZE - x2offset );
                        break;

                    case 'x': // Draw an X
                        g.setColor ( Color.RED );
                        g.drawLine ( col * SQUARE_SIZE + offset,
                                     row * SQUARE_SIZE + offset,
                                     ( col + 1 ) * SQUARE_SIZE - offset,
                                     ( row + 1 ) * SQUARE_SIZE - offset );
                        g.drawLine ( ( col + 1 ) * SQUARE_SIZE - offset,
                                     row * SQUARE_SIZE + offset,
                                     col * SQUARE_SIZE + offset,
                                     ( row + 1 ) * SQUARE_SIZE - offset );
                        break;
                    }
                }
            }
        }

        /**
         * Mainly used to redraw the grid on the board, so that it's always there.
         */
        @Override
        public void paintComponent ( Graphics g ) {
            g.setColor ( Color.BLACK );
            int width = getWidth ( );
            int height = getHeight ( );
            for ( int row = 0; row <= GomokuModel.NUM_VSQUARES; row++ ) {
                int line = row * SQUARE_SIZE;

                // Draw the vertical lines
                g.drawLine ( 0, line, width, line );

                // Draw the horizontal lines
                g.drawLine ( line, 0, line, height );
            }

            // Then draw the contents
            drawBoard ( (Graphics2D) g );
        }
    }

    /** 
     * Handles a mouse click on the board.
     * @param x X position of the click
     * @param y Y position of the click
     */ 
    private void doMouseClick(int x, int y) {
        int row = y / SQUARE_SIZE;
        int col = x / SQUARE_SIZE;

        boolean gameOver = true;

        // Check current status and decide what to do
        GomokuModel.Outcome outcome = model.doClick ( row, col );

        // Better repaint so that the latest move shows up
        boardFrame.repaint ( );

        // If necessary present a dialog box
        switch ( outcome ) {
        case DRAW:
            JOptionPane.showMessageDialog ( null, "Draw Game!", "Game Over",
                                            JOptionPane.INFORMATION_MESSAGE );
            break;
        case CROSS_WINS:
            JOptionPane.showMessageDialog ( null, "Cross Wins!", "Game Over",
                                            JOptionPane.INFORMATION_MESSAGE );
            break;
        case RING_WINS:
            JOptionPane.showMessageDialog ( null, "Ring Wins!", "Game Over",
                                            JOptionPane.INFORMATION_MESSAGE );
            break;
        default:
            // No action to take. Either game is over or someone clicked on the wrong spot.
            gameOver = false;
            break;
        }

        if ( gameOver ) {
            // Question user about another game, and either quit or restart
            int choice = JOptionPane.showConfirmDialog ( null, "Play again?", "Play again?",
                                                         JOptionPane.YES_NO_OPTION );
            if ( choice == JOptionPane.NO_OPTION ) {
                System.exit ( 0 );
            } else {
                model.newGame();
            }
        }

        // Repaint after user choose a new game as otherwise we'll still see the old
        // contents of the board, and that's no fun.
        boardFrame.repaint();
    }

    /** Size in pixels of each square */
    public static final int SQUARE_SIZE = 20;

    /** Reference to the model class */
    private final GomokuModel model;

    /** Reference to my board */
    private final JFrame boardFrame;

    /**
     * Constructor for the GUI.
     * @param gomokuModel Reference to a model that correctly
     * implements the interface, so that I know that all methods are
     * there.
     */
    private GomokuGUI ( GomokuModel gomokuModel ) {
        model = gomokuModel;
        model.newGame ( );

        // Create the frame
        boardFrame = new JFrame ( );
        boardFrame.setTitle ( "Gomoku" );

        // Create the board
        BoardPanel boardPanel = new BoardPanel ( );
        boardPanel.setPreferredSize ( new Dimension (
                                        GomokuModel.NUM_HSQUARES * SQUARE_SIZE + 1,
                                        GomokuModel.NUM_VSQUARES * SQUARE_SIZE + 1 ) );
        boardPanel.addMouseListener ( new MouseAdapter() {

                @Override
                public void mouseClicked ( MouseEvent e ) {

                    int x = e.getX ( );
                    int y = e.getY ( );
                    doMouseClick(x, y);
                }
            });

        // Create status panel for new game and quit buttons
        JPanel statusPanel = new JPanel ( new FlowLayout ( ) );
        statusPanel.setPreferredSize ( new Dimension (
                                         GomokuModel.NUM_HSQUARES * SQUARE_SIZE, 50 ) );

        // Create a new New Game Button
        JButton newGameButton = new JButton ( "New Game" );
        newGameButton.addActionListener ( new ActionListener ( ) {
                public void actionPerformed ( ActionEvent e ) {
                    model.newGame ( );
                    boardFrame.repaint ( );
                }
            } );
        statusPanel.add ( newGameButton );

        // Create a nice quit button
        JButton quitButton = new JButton ( "Quit" );
        quitButton.addActionListener ( new ActionListener ( ) {
                public void actionPerformed ( ActionEvent e ) {
                    System.exit ( 0 );
                }
            } );
        statusPanel.add ( quitButton );

        // Add the panels to the frame
        boardFrame.add ( statusPanel, BorderLayout.NORTH );
        boardFrame.add ( boardPanel, BorderLayout.CENTER );

        // Make sure it looks good
        boardFrame.pack ( );
        boardFrame.setDefaultCloseOperation ( JFrame.EXIT_ON_CLOSE );
        boardFrame.setLocationRelativeTo ( null );
        boardFrame.setResizable ( false );
        boardFrame.setVisible ( true );
    }

    /**
     * Construct and display GUI for a Gomoku game
     * @param model Reference to a model that correctly
     * implements the interface, so that I know that all methods are
     * there.
     */
    public static void showGUI(final GomokuModel model) {
        // For thread safety, invoke GUI code on event-dispatching thread
        SwingUtilities.invokeLater(new Runnable() {
                @Override
                public void run() {
                    new GomokuGUI(model);
                }
            });
    }
}

Answer 1

// For every block
for (Square[] s : gameBoard)
    for (Square ss : s)
        // Reset to empty.
        {ss = Square.EMPTY;}

这个循环什么都不做。它等同于:

// For every block
for (Square[] s : gameBoard) {
    for(int i = 0; i < s.length; i++) {
        Square ss;
        ss = s[i];
        ss = Square.EMPTY;
    }
}

您可以看到，实际上并没有改变棋盘。

相反，这样做:

for(Square[] s : gameBoard)
    for(int i = 0; i < s.length; i++)
        s[i] = Square.EMPTY;

或者这个:(并导入 java.util.Arrays)

for(Square[] s : gameBoard)
    Arrays.fill(s, Square.EMPTY);