Java Swing Textfield Error

Question

I'm new to Java and Java Swing, and I can't get it to work. I have 2 textfields, and the text in textfield1 needs to be passed on te textfield2 by clicking a button, but it crashes constantly.

Here is the code:

package javaapplication7;

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;


public class JavaApplication7 {

    public static void main(String[] args) {
        JFrame theFrame = new JFrame();
        theFrame.setTitle("Title");
        theFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        theFrame.setSize(400, 400);
        theFrame.setLocation(400, 400);

        theFrame.setContentPane(new Paneel());
        theFrame.setVisible(true);
    }
}

class Paneel extends JPanel{
            JButton button1;
            JTextField field1;
            JTextField field2;


    public Paneel(){
            JButton button1 = new JButton("Click");
            button1.addActionListener(new KH());
            add(button1);

            JTextField field1 = new JTextField(10);
            add(field1);

            JTextField field2 = new JTextField(10);
            add(field2);
    }


    class KH implements ActionListener{
        @Override
        public void actionPerformed(ActionEvent e){
            field2.setText(field1.getText());
        }
    }
}

It gives an enormous list of errors when I run it, has someone any idea?

Error list:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at javaapplication7.Paneel$KH.actionPerformed(JavaApplication7.java:54)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6505)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
    at java.awt.Component.processEvent(Component.java:6270)
    at java.awt.Container.processEvent(Container.java:2229)
    at java.awt.Component.dispatchEventImpl(Component.java:4861)
    at java.awt.Container.dispatchEventImpl(Container.java:2287)
    at java.awt.Component.dispatchEvent(Component.java:4687)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
    at java.awt.Container.dispatchEventImpl(Container.java:2273)
    at java.awt.Window.dispatchEventImpl(Window.java:2719)
    at java.awt.Component.dispatchEvent(Component.java:4687)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:729)
    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.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
    at java.awt.EventQueue$4.run(EventQueue.java:702)
    at java.awt.EventQueue$4.run(EventQueue.java:700)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:699)
    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

You are currently shadowing the variables button1 , field1 , field2 in Paneel , ie you are creating local instances of these components in the constructor of Paneel but the class member variables with these names remain null , resulting in the NPE .

By removing the JButton and JTextField type keywords you will assign the components to the intended class member variables:

public Paneel(){
   button1 = new JButton("Click");
   ...
   field1 = new JTextField(10);
   ...
   field2 = new JTextField(10);
   ...
}

Answer 2

您在构造函数中声明新的本地“ button1”，“ field1”和“ field2”变量，而不是初始化实例变量。

Answer 3

Use this code for Paneel constructor indeed.

        public Paneel(){
        JButton button1 = new JButton("Click");
        button1.addActionListener(new KH());
        add(button1);

        field1 = new JTextField(10);
        add(field1);

        field2 = new JTextField(10);
        add(field2);
       }

field1 and field2 that you have created in your constructor is local to constructor. The member fields ( field1 and field2 ) of Paneel class is still null. So in actionPerformed it is throwing NullPointerException .