Java Swing 代码中的 java.lang.NullPointerException

Question

谁能告诉我为什么下面的代码会抛出一个空指针异常？该行抛出异常 numberJTextField.addMouseListener(new MyMouseAdapter(numberJTextField)); 在类 InputJFrame1.java 中。

import java.awt.*;

public class InputJFrame1 extends javax.swing.JFrame
{
    private javax.swing.JTextField numberJTextField;

    public InputJFrame1()
    {     
         numberJTextField.addMouseListener(new MyMouseAdapter(numberJTextField));
    }

    private void initComponents() 
    {
        numberJTextField = new javax.swing.JTextField();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        getContentPane().add(numberJTextField);
    }

    public static void main(String args[])
    {
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
        java.util.logging.Logger.getLogger(InputJFrame1.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
        java.util.logging.Logger.getLogger(InputJFrame1.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
        java.util.logging.Logger.getLogger(InputJFrame1.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
        java.util.logging.Logger.getLogger(InputJFrame1.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }

        java.awt.EventQueue.invokeLater(new Runnable()
                                    {
                                        @Override
                                        public void run()
                                        {
                                            new InputJFrame1().setVisible(true);
                                        }
                                    });
    }
}



import java.awt.Color;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.JTextField;

public class MyMouseAdapter extends MouseAdapter
{
    private JTextField jTextField;

    MyMouseAdapter(JTextField jTextField)
    {
        this.jTextField=jTextField;
    }

    @Override
    public void mouseClicked(MouseEvent e)
    {
          jTextField.setForeground(Color.red);
    }
}

堆栈跟踪:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at javaapplication8.InputJFrame1.(InputJFrame1.java:9) at javaapplication8.InputJFrame1$1.run(InputJFrame1.java:73) at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251) at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:727) 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.awt.EventQueue.dispatchEvent(EventQueue.java:697) 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)

Answer 1

不要从构造函数中删除 initComponents。它确保所有组件在使用之前都已正确初始化，因此它应该是构造函数的第一行。

public InputJFrame1()
{     
     initComponents();
     numberJTextField.addMouseListener(new MyMouseAdapter(numberJTextField));
}