Question

我有一个空指针错误。我尝试通过System.out.print打印按钮，它的工作原理。我真的不知道什么是错的。这是我的第一篇文章也很抱歉任何错误。这是代码：

import javax.swing.JFrame;
import javax.swing.JButton;
import javax.swing.JTextField;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import java.awt.FlowLayout;
import java.awt.Container;

class ChangeText implements ActionListener{
    JButton button;
    JTextField tfield;
    public ChangeText(){
        JFrame frame = new JFrame();

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setTitle("Change Text");
        frame.setLayout(new FlowLayout());


        JTextField tfield = new JTextField("old text");
        JButton button = new JButton("Change Text");
        button.addActionListener(this);

        frame.add(tfield);
        frame.add(button);

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

    public static void main(String[] args){
        ChangeText ct = new ChangeText();
    }


    public void actionPerformed(ActionEvent e){
        tfield.setText(null);
    }


}

编辑：这是我按下按钮时得到的

java.lang.NullPointerException
    at ChangeText.actionPerformed(ChangeText.java:42)
    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.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$000(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.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.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

此行正在创建JTextField的本地副本

JTextField tfield = new JTextField("old text");

成功

tfield = new JTextField("old text");

（以便你的tfield成员被实例化），一切都应该是好的：）

通过按钮更改文本字段中的文本

1 个答案: