Question

Purpose of the code : Create two Buttons(button1 and button2). When User clicks button1, change the text of button2. When User clicks button2, change the text of button1.

Here’s the code I’m using :

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

public class multiAL {
JButton button1;
JButton button2;
JFrame frame;
public static void main(String[] args) {
    multiAL setterAL = new multiAL();
    setterAL.go();
}

public void go() {
    button1 =  new JButton("Click me, I'm One");
    button2 =  new JButton("Click me, I'm Two");
    frame.setSize(500,500);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().add(BorderLayout.WEST, button1);
    frame.getContentPane().add(BorderLayout.EAST, button2);
    frame.setVisible(true);
    button1.addActionListener(new b1L());
    button2.addActionListener(new b2L());
}

class b1L implements ActionListener {
    public void actionPerformed(ActionEvent event) {
        button2.setText("What??, you clicked 1??");
    }
}

class b2L implements ActionListener {
    public void actionPerformed(ActionEvent event) {
        button1.setText("What??, you clicked 2??");
    }
  }
}

It compiles perfectly, but when I run it I receive following error :
Exception in thread "main" java.lang.NullPointerException
at multiAL.go(multiAL.java:17)
at multiAL.main(multiAL.java:11)

Till now, I’ve encountered only compile-time errors. So there are two question which I want to ask:

1) What’s wrong with the code?
2) How to track down runtime errors?

Answer 1

frame is not initialized, so it resolves to null and you can’t call methods on null objects. Like you initialized button1 and button2 you should also initialize frame.

frame = new JFrame();