I’m learning how to work with Java Swing for the first time by following a tutorial on YouTube. I have reached the section covering buttons and have been following the code to the T. However, while attempting to test the button so it would print out the word "test" when I’m using the actionPerformed method, my button would not print out the word.
You can find the original code for that test here:
Main.java
package com.learnjava;
public class Main {
public static void main(String[] args) {
new MyFrame();
}
}
MyFrame.java
package com.learnjava;
import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class MyFrame extends JFrame implements ActionListener{
JButton button;
MyFrame() {
JButton button = new JButton();
button.setBounds(200, 100, 100, 50);
button.addActionListener(this);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.setSize(500, 500);
this.setLayout(null);
this.setVisible(true);
this.add(button);
}
@Override
public void actionPerformed(ActionEvent e) {
if(e.getSource() == button) {
System.out.println("test");
}
}
}
Whenever I pressed the button using the previous code, it wouldn’t print out the word "test". If I used a lambda expression, it would work.
MyFrame.java (updated with lambda expression)
package com.learnjava;
import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class MyFrame extends JFrame implements ActionListener{
JButton button;
MyFrame() {
JButton button = new JButton();
button.setBounds(200, 100, 100, 50);
button.addActionListener(e -> {System.out.println("test");}); // updated with lambda expression
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.setSize(500, 500);
this.setLayout(null);
this.setVisible(true);
this.add(button);
}
}
I’ve been trying to find a reason online as to why the former code does not work. I’ve seen one question but it was left unanswered. While I could settle for using the lambda expression instead, I would still like to understand how to write the former code correctly if I was doing something wrong. Thank you in advanced!
Note: If it is relevant for some reason, the IDE I am using is IntelliJ and my JDK is version 12.
>Solution :
Change:
JButton button = new JButton();
To:
button = new JButton();
The first is a local variable that shadows the class attribute.
The 2nd class has the same problem, but since it (button creation, adding the listener, adding it to the GUI) is all done in the one code section, it is not obvious that the class attribute is never used.