Question

我开始使用Swing创建一个桌面应用程序，我完全是新手。因为我没有在语法或结构中发现任何错误但它会引发错误。这只是一个活动演示程序。

package testEvent;

import java.awt.FlowLayout;
import java.awt.event.*;
import javax.swing.*;

public class EventDemo {

    JLabel jlab;
    EventDemo(){
        JFrame jfrm = new JFrame("An Event Example");
        jfrm.setLayout(new FlowLayout());
        jfrm.setSize(220,90);
        jfrm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JButton jbtnAlpha = new JButton("Alpha");
        JButton jbtnBeta = new JButton("Beta");
        jbtnAlpha.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent ae){
                jlab.setText("Alpha was Pressed.");
            }
        });

        jbtnBeta.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent ae){
                jlab.setText("Beta was Pressed.");
            }
        });

        jfrm.add(jlab);     
        jfrm.setVisible(true);
    }

    public static void main(String args[]){
        //Create the Frame on Event Dispatching thread
        SwingUtilities.invokeLater(new Runnable(){
            public void run(){
                new EventDemo();
            }
        });
    }
}

抛出以下错误

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at java.awt.Container.addImpl(Unknown Source)
at java.awt.Container.add(Unknown Source)
at javax.swing.JFrame.addImpl(Unknown Source)
at java.awt.Container.add(Unknown Source)
at testEvent.EventDemo.<init>(EventDemo.java:40)
at testEvent.EventDemo$3.run(EventDemo.java:49)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)*
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$200(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(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

您需要初始化JLabel jlab。像下面这样的东西应该做。

JLabel jlab = new JLabel();

Swing中线程“AWT-EventQueue-0”中的异常NullPointerException错误

1 个答案: