java - GUI 编程(续) - NullPointer 异常作为猜数游戏的输出

Question

我最近开始 GUI 编程，并一直致力于这个高低猜谜游戏。这是我的代码:

package experiment10;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.InputMismatchException;

public class mainFrame extends JFrame implements ActionListener
{
    public static int MAX = 500;    // maximum number to guess
    private JButton submitBtn;      // Submit button
    private JButton clearBtn;  // Clear Input button
    private JTextField inputNumber;  //input guessed number
    private int magicNumber;  //randomly generated number

    public static void main(String[] arg)    {
        //Displays frame
        mainFrame frame = new mainFrame();
        frame.setVisible(true);
    }

    public mainFrame()  {
        setSize(400, 400);
        setResizable (true);
        setTitle("Let's Play Hi Lo");
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        magicNumber = (int)(Math.random()*MAX);

        Container gameView  = getContentPane();
        gameView.setLayout(new FlowLayout());

        JLabel imageLabel = new JLabel(new ImageIcon("cat.gif")); //creates image
        gameView.add(imageLabel); //adds image

        JLabel textLabel = new JLabel("Please enter your guess"); //creates JLabel
        gameView.add(textLabel); //adds JLabel

        JTextField inputNumber = new JTextField(); //creates JTextField
        gameView.add(inputNumber); //adds JTextField
        inputNumber.setColumns(30); //sets JTextField's length

        submitBtn = new JButton("Submit"); //create submit button
        clearBtn = new JButton("Clear"); //create clear button
        gameView.add(submitBtn); //adds submit button
        gameView.add(clearBtn); //adds clear button

        submitBtn.addActionListener(this); //registers this as ActionListener of Submit button
        clearBtn.addActionListener(this); //registers this as ActionListener of Clear button

    }

    @Override
    public void actionPerformed(ActionEvent e) {
        JButton clickedBtn = (JButton)e.getSource();

        try {
        switch(clickedBtn.getText()) {

        case "Submit" : inputNumber.getText(); //if user selects "Submit" button
        int guess = Integer.parseInt(inputNumber.getText()); //converts guess to integer
        if(guess<magicNumber) {
            JOptionPane.showMessageDialog(null,"Your number is too small");
        }
        if(guess>magicNumber) {
            JOptionPane.showMessageDialog(null,"Your number is too big");
        }
        if(guess==magicNumber) {
            JOptionPane.showMessageDialog(null,"Congratulations, you got it!");
        }
        break;

        case "Clear" : inputNumber.setText(" "); //if user selects "Clear" button
        break;
        }
    }
        catch(InputMismatchException ime) {
            JOptionPane.showMessageDialog(null,"You didn't enter an integer. Please try again.");
        }


}
}

我以为我已经接近完成了，但是当我输入一个数字并单击提交按钮时，我得到了一个 NullPointerException (以及其他一些行)。同样，当我单击清除按钮时，也会发生同样的事情。非常感谢任何帮助/提示。这是我单击两个按钮后的输出:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at experiment10.mainFrame.actionPerformed(mainFrame.java:56)
    at java.desktop/javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1967)
    at java.desktop/javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2308)
    at java.desktop/javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:405)
    at java.desktop/javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:262)
    at java.desktop/javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:279)
    at java.desktop/java.awt.Component.processMouseEvent(Component.java:6636)
    at java.desktop/javax.swing.JComponent.processMouseEvent(JComponent.java:3342)
    at java.desktop/java.awt.Component.processEvent(Component.java:6401)
    at java.desktop/java.awt.Container.processEvent(Container.java:2263)
    at java.desktop/java.awt.Component.dispatchEventImpl(Component.java:5012)
    at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2321)
    at java.desktop/java.awt.Component.dispatchEvent(Component.java:4844)
    at java.desktop/java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4918)
    at java.desktop/java.awt.LightweightDispatcher.processMouseEvent(Container.java:4547)
    at java.desktop/java.awt.LightweightDispatcher.dispatchEvent(Container.java:4488)
    at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2307)
    at java.desktop/java.awt.Window.dispatchEventImpl(Window.java:2762)
    at java.desktop/java.awt.Component.dispatchEvent(Component.java:4844)
    at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:772)
    at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
    at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:95)
    at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:745)
    at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:743)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
    at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:742)
    at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
    at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
    at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
    at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
    at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at experiment10.mainFrame.actionPerformed(mainFrame.java:68)
    at java.desktop/javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1967)
    at java.desktop/javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2308)
    at java.desktop/javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:405)
    at java.desktop/javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:262)
    at java.desktop/javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:279)
    at java.desktop/java.awt.Component.processMouseEvent(Component.java:6636)
    at java.desktop/javax.swing.JComponent.processMouseEvent(JComponent.java:3342)
    at java.desktop/java.awt.Component.processEvent(Component.java:6401)
    at java.desktop/java.awt.Container.processEvent(Container.java:2263)
    at java.desktop/java.awt.Component.dispatchEventImpl(Component.java:5012)
    at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2321)
    at java.desktop/java.awt.Component.dispatchEvent(Component.java:4844)
    at java.desktop/java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4918)
    at java.desktop/java.awt.LightweightDispatcher.processMouseEvent(Container.java:4547)
    at java.desktop/java.awt.LightweightDispatcher.dispatchEvent(Container.java:4488)
    at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2307)
    at java.desktop/java.awt.Window.dispatchEventImpl(Window.java:2762)
    at java.desktop/java.awt.Component.dispatchEvent(Component.java:4844)
    at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:772)
    at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
    at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:95)
    at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:745)
    at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:743)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
    at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:742)
    at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
    at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
    at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
    at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
    at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

Answer 1

据我所知，主要问题是您首先将 JTextField numberInput 声明为类变量，然后在 mainFrame< 中再次声明它 方法，本质上是创建一个新的本地引用。

当您在actionPerformed方法中引用变量时，它会查看类变量，该变量未赋值，因此会抛出空指针异常。

简单的修复方法是在分配给 numberInput 时删除第二个声明:

numberInput = new JTextField();

而不是

JTextField numberInput = new JTextField();