Java GUI DiceRoll 应用程序

Question

我目前正在开发一个简单的 GUI 应用程序，可以掷两个骰子。我目前使用两个类，一个称为“Game”，另一个称为“DiceRoll”。为了满足分配标准，我们必须使用多个类才能使程序正常运行。仅使用一个类会容易得多...无论如何，我成功地在“DiceRoll”类中调用“roll()”方法，该方法使用 switch 语句并按顺序实现“Graphics.drawImage()”方法绘制指定的“.png”图像。一切看起来都很好，在运行之前我没有任何错误。当我运行该程序时，会弹出 GUI 框架，并且我创建的所有按钮/菜单都可以正常工作。然而，一旦我按下“掷骰子”按钮，我就会不断收到多个运行时错误。这是我到目前为止所拥有的:

游戏类

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JOptionPane;
import javax.swing.JPanel;

public class Game extends JFrame implements ActionListener 
{
JPanel mainPanel;
JPanel optionPanel;
JPanel dicePanel;
JButton rollDice;
JMenu options;
JMenuItem quit;
JMenuItem explanation;
JMenuBar menuBar;
JLabel diceLabel;
JLabel diceLabel2;
DiceRoll dr;
Graphics die1;
Graphics die2;

public Game()
{
    setTitle("Rollin' Dice");
    this.setPreferredSize(new Dimension(600,600));

    mainPanel = new JPanel();
    optionPanel = new JPanel();
    dicePanel = new JPanel();
    rollDice = new JButton("Roll Dice");
    options = new JMenu("Options");
    quit = new JMenuItem("Quit");
    explanation = new JMenuItem("Explanation");
    menuBar = new JMenuBar();
    dr = new DiceRoll();
    diceLabel = new JLabel();
    diceLabel2 = new JLabel();

    options.add(quit);
    options.add(explanation);

    menuBar.add(options);

    optionPanel.add(menuBar);
    optionPanel.setPreferredSize(new Dimension(600,100));

    dicePanel.add(rollDice);

    dicePanel.add(diceLabel);
    dicePanel.add(diceLabel2);

    mainPanel.setPreferredSize(new Dimension(600,600));
    mainPanel.add(optionPanel);
    mainPanel.add(dicePanel);


    quit.addActionListener(this);
    explanation.addActionListener(this);
    rollDice.addActionListener(this);

    this.getContentPane().add(mainPanel);

    this.pack();
    this.setVisible(true);
}


public void actionPerformed(ActionEvent e) {
    if (e.getSource()== quit)
        System.exit(0);

    if (e.getSource() == explanation)
    {
        JOptionPane.showMessageDialog(mainPanel,
                "Win: Roll a sum that is an even number \nLose: Roll a sum that is an odd number" + dicePanel, "Rules", JOptionPane.INFORMATION_MESSAGE); 
    }

    if (e.getSource() == rollDice)
    {

        dr.roll(die1);
        dr.roll(die2);

        diceLabel.updateUI();

        dicePanel.updateUI();
    }

}

public static void main (String []args)
{
    Game dg = new Game();
}

}

DiceRoll 类

    import java.awt.Graphics;
    import java.awt.image.BufferedImage;
    import javax.imageio.*;
    import java.io.File;
    import java.io.IOException;
    import javax.swing.JComponent;

public class DiceRoll extends JComponent {

private BufferedImage die1;
private BufferedImage die2;
private BufferedImage die3;
private BufferedImage die4;
private BufferedImage die5;
private BufferedImage die6;


public DiceRoll()
{
    try {
        die1 = (ImageIO.read(new File("die1.png")));
        die2 = ImageIO.read(new File("die2.png"));
        die3 = ImageIO.read(new File("die3.png"));
        die4 = ImageIO.read(new File("die4.png"));
        die5 = ImageIO.read(new File("die5.png"));
        die6 = ImageIO.read(new File("die6.png"));
    } catch (IOException ex){
        System.err.println("That is invalid");
    }
}


public Graphics roll(Graphics g)
{
    int dieResult = (int)(6 * Math.random());

    switch(dieResult){
    case 1: 
        g.drawImage(die1, 0, 0, null);
        break;
    case 2:
        g.drawImage(die2, 0, 0, null);
        break;
    case 3:
        g.drawImage(die3, 0, 0, null);
        break;
    case 4:
        g.drawImage(die4, 0, 0, null);
        break;
    case 5:
        g.drawImage(die5, 0, 0, null);
        break;
    case 6:
        g.drawImage(die6, 0, 0, null);
        break;
    }
    return g;
}

}

我收到的错误

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at DiceRoll.roll(DiceRoll.java:51)
at Game.actionPerformed(Game.java:89)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

您在此处传递null:

dr.roll(die1);
dr.roll(die2);

由于您从未实例化 die1 和 die2，因此您会得到 NullPointerException。