java - 监听器显示运行时错误

Question

我编写了一个可以完美编译但显示运行时错误的程序。我已经搜索过这些错误，但没有摆脱这个问题。

import javax.swing.JFrame;
import javax.swing.SwingUtilities;

class  swing11
{
    public static void main(String[] args) 
    {

        SwingUtilities.invokeLater(new Runnable(){
        public void run(){
            new mainframe();

            }
        });
    }
}



 import javax.swing.JFrame;
 import javax.swing.JTextArea;
 import java.awt.BorderLayout;
 import javax.swing.JButton;
 import java.awt.event.ActionListener;
 import java.awt.event.ActionEvent;

 public class mainframe extends JFrame{
    private JTextArea textarea;
    private JButton btn;

    public mainframe(){
        super("new program");
        setLayout(new BorderLayout());

    btn.addActionListener(new ActionListener(){
    public void actionPerformed(ActionEvent arg0){
    textarea.append("hello\n");
    }
        });

    textarea =new JTextArea();
    btn = new JButton("click me!");

    add(textarea, BorderLayout.CENTER);
    add(btn, BorderLayout.SOUTH);

    setSize(500,400);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setVisible(true);
   }
}

错误:

---------- run ----------

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at mainframe.<init>(mainframe.java:16)
    at swing11$1.run(swing11.java:11)
    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)

Output completed (1 sec consumed) - Normal Termination

Answer 1

您需要在addActionListener之前分配btn。