java - addActionListener 中的递归？

Question

我正在尝试在Caballo方法中创建一个递归方法，这样我就可以移动骑士，但是我收到了一个N​​ullPointerException，并且递归不起作用，任何人都可以给我一些建议吗？我正在尝试制作一款类似于国际象棋的游戏。

package ratsuk;

import javax.swing.JFrame;
import javax.swing.JButton;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JPanel;
import java.util.Random;
import javax.swing.Icon;
import javax.swing.ImageIcon;
import javax.swing.JLabel;

/**
 *
 * @author Melvin
 */
public class Tablero {

    private JButton[][] mesa;
    private Random rad;
    private Icon image;
    private JPanel panel;

    public Tablero() {
        mesa = new JButton[8][8];
        panel = new JPanel(new GridLayout(8, 8, 0, 0));
        image = new ImageIcon(getClass().getResource("redKnight.gif"));
    }

    public void cuadriculado(JFrame ventana) {
        for (int i = 0; i < mesa.length; i++) {
            for (int j = 0; j < mesa[0].length; j++) {
                mesa[i][j] = new JButton();
                mesa[i][j].setPreferredSize(new Dimension(40, 40));
                panel.add(mesa[i][j]);

            }
        }
        for (int r = 0; r < mesa.length; r++) {
            for (int t = 0; t < mesa[0].length; t++) {
                if (r % 2 == 0 || r == 0) {
                    if (t % 2 == 0 || t == 0) {
                        mesa[r][t].setBackground(Color.BLACK);
                    } else {
                        mesa[r][t].setBackground(Color.WHITE);
                    }
                } else {
                    if (t % 2 == 0 || t == 0) {
                        mesa[r][t].setBackground(Color.WHITE);
                    } else {
                        mesa[r][t].setBackground(Color.BLACK);
                    }

                }

            }
        }
        ventana.setContentPane(panel);
        ventana.setSize(500, 500);
        ventana.setVisible(true);
    }

    public void Caballo(final int i, final int j) {

        final Tablero cab =new Tablero();

        mesa[i][j].setIcon(image);

        mesa[i][j].addActionListener(new ActionListener() {
                public void actionPerformed(ActionEvent e) {
                //Execute when button is pressed
                mesa[i][j].setBackground(Color.RED);

                }
        });

        mesa[i+2][j-1].addActionListener(new ActionListener() {
                public void actionPerformed(ActionEvent d) {
                //Execute when button is pressed
                if (cab.tienebotton(i+2, j-1)==true){
                mesa[i+2][j-1].setIcon(image);
                mesa[i][j].setIcon(null);

                cab.Caballo(i+2,j-1);
                }
                }
        });
        panel.repaint();
    }

    public boolean tienebotton(int k, int w) {
        for(int i = 0; i < mesa.length; i++) {
            for(int j = 0; j < mesa[i].length; j++) {
                if(mesa[k][w] == mesa[i][j]) return true;
            }
        }
        return false;
    }

}

空指针异常的行是

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at ratsuk.Tablero.Caballo(Tablero.java:76)
    at ratsuk.Tablero$2.actionPerformed(Tablero.java:93)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6505)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
    at java.awt.Component.processEvent(Component.java:6270)
    at java.awt.Container.processEvent(Container.java:2229)
    at java.awt.Component.dispatchEventImpl(Component.java:4861)
    at java.awt.Container.dispatchEventImpl(Container.java:2287)
    at java.awt.Component.dispatchEvent(Component.java:4687)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
    at java.awt.Container.dispatchEventImpl(Container.java:2273)
    at java.awt.Window.dispatchEventImpl(Window.java:2719)
    at java.awt.Component.dispatchEvent(Component.java:4687)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:729)
    at java.awt.EventQueue.access$200(EventQueue.java:103)
    at java.awt.EventQueue$3.run(EventQueue.java:688)
    at java.awt.EventQueue$3.run(EventQueue.java:686)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
    at java.awt.EventQueue$4.run(EventQueue.java:702)
    at java.awt.EventQueue$4.run(EventQueue.java:700)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:699)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

这是其余代码:

package ratsuk;

import java.util.Random;
import javax.swing.JFrame;

/**import javax.swing.JFrame;
import javax.swing.SwingUtilities;
import javax.swing.UIManager;
 */
public class Ratsuk extends JFrame {


    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
    Random rad;
    JFrame ventana;
     ventana= new JFrame ("Juego de Ratsuk");   
    rad= new Random();
    int azar= rad.nextInt(8);
    int azar2 = rad.nextInt(8);

    ventana.setVisible(true);
    ventana.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    Tablero newtablero= new Tablero();

    newtablero.cuadriculado( ventana);
    newtablero.Caballo(azar,azar2);



    }
}

Answer 1

以下是 Cabalo 的工作原理:

1. 已创建 Tablero 的新实例

   final Tablero cab = new Tablero();
   

2. 构造函数初始化 mesa，但 JButton 为 null，因为 mesa 是由 填充的>夸德里库拉多

   mesa = new JButton[8][8];
   

3. 以下代码有效，因为 mesa[i][j] 不为 null，因为 newtablero.cuadriculado( ventana); 对于 this 在 Ratsuk#main 中调用

   mesa[i][j].setIcon(image);
   mesa[i][j].addActionListener(new ActionListener() {
           public void actionPerformed(ActionEvent e) {
           //Execute when button is pressed
           mesa[i][j].setBackground(Color.RED);
   
           }
   });
   

4. 以下代码可能会抛出 ArrayIndexOutOfBoundsException，因为 Ratsuk#main 中的 Random.nextInt(8) 可能返回 6,7或 i 为 8，j 为 0。

   mesa[i+2][j-1].addActionListener(new ActionListener() {
           public void actionPerformed(ActionEvent d) {
   

5. tienebotton 返回 true，因为正如我在第 2 点中提到的，所有“JButton”都是 null 并且 null = = null 返回true

   if (cab.tienebotton(i+2, j-1)==true){
   

6. 以下代码有效，请参阅第 3 点和第 4 点

   mesa[i+2][j-1].setIcon(image);
   mesa[i][j].setIcon(null);
   

7. 调用Caballo

   cab.Caballo(i+2,j-1);
   

8. 以下代码会导致 NPE，因为 mesa[i][j] == null。参见第 2 点

   mesa[i][j].setIcon(image);
   

这是我的解决方案

Ratsuk.java

package ratsuk;

import java.util.Random;
import javax.swing.JFrame;

public class Ratsuk extends JFrame {

    /**
    * @param args the command line arguments
    */
    public static void main(String[] args) {
        Random random= new Random();
        int row= random.nextInt(8);
        int column = random.nextInt(8);

        Tablero table= new Tablero();
        table.caballo(row, column);
    }
}

Tablero.java

package ratsuk;

import javax.swing.JFrame;
import javax.swing.JButton;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JPanel;
import javax.swing.Icon;
import javax.swing.ImageIcon;

/**
 *
 * @author Melvin
 */
public class Tablero {

    private static final int HEIGHT = 8;
    private static final int WIDTH = 8;

    private JButton[][] grid;
    private Icon image;
    private JPanel panel;

    public Tablero() {
        panel = new JPanel(new GridLayout(HEIGHT, WIDTH));
        image = new ImageIcon();

        createFrame();

        createGrid();

        paintGrid();
    }

    private void createFrame() {
        JFrame frame = new JFrame ("Juego de Ratsuk");
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        frame.setContentPane(panel);
        frame.setSize(500, 500);
        frame.setVisible(true);
    }

    private void createGrid() {
        grid = new JButton[HEIGHT][WIDTH];
        for (int row = 0; row < HEIGHT; row++) {
            for (int column = 0; column < WIDTH; column++) {
                JButton button = new JButton();
                button.setPreferredSize(new Dimension(40, 40));

                grid[row][column] = button;
                panel.add(button);
            }
        }
    }

    private void paintGrid() {
        for (int r = 0; r < HEIGHT; r++) {
            for (int t = 0; t < WIDTH; t++) {
                Color background = getBackgroundColor(r, t);

                grid[r][t].setBackground(background);
            }
        }
    }

    private Color getBackgroundColor(int r, int t) {
        Color background;
        if (r % 2 == 0 || r == 0) {
            if (t % 2 == 0 || t == 0) {
                background = Color.BLACK;
            } else {
                background = Color.WHITE;
            }
        } else {
            if (t % 2 == 0 || t == 0) {
                background = Color.WHITE;
            } else {
                background = Color.BLACK;
            }
        }
        return background;
    }

    public void caballo(final int row, final int column) {

        final JButton current = grid[row][column];

        current.setIcon(image);
        panel.repaint();

        addActionListeners(row, column, current);
    }

    private void addActionListeners(int row, int column, final JButton current) {
        current.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                current.setBackground(Color.RED);
            }
        });

        final int nextRow = row + 2;
        final int nextColumn = column - 1;
        if (tienebotton(nextRow, nextColumn)){
            JButton next = grid[nextRow][nextColumn];

            next.addActionListener(new ActionListener() {
                public void actionPerformed(ActionEvent d) {
                    current.setIcon(null);

                    caballo(nextRow, nextColumn);
                }
            });
        }
    }

    private boolean tienebotton(int row, int column) {
        return  row >= 0 && row < HEIGHT && column >=0 && column < WIDTH;
    }

}

为了清楚起见，我保留了 Ratsuk、Tablero、tienebotton 和 caballo 标识符，但是最好follow code conventions .