Thread: Unused Variable in ActionEvent()

Greetings All

I am a C++ programmer and currently learning Java.

While playing around with event management I always find that I have an unused variable in the ActionEvent(). For example:

 
import javax.swing.JFrame;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
 
 
public class ExampleProgram extends JFrame {
 
    ExampleProgram(){
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        initGui();
 
        return;
    }
 
    public static void main(String args[]){
 
        ExampleProgram w = new ExampleProgram();
        w.setVisible(true);
 
        return;
 
    }
 
    private void initGui(){
 
        JButton b = new JButton();
        b.setText("Click Me!");
 
        b.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent evt){
                doStuff(evt);
            }
        });
 
        this.add(b);
        this.setSize(300,200);
 
        return;
    }
 
    private void doStuff(ActionEvent evt){
        System.out.println("Yeah!");
 
        return;
    }
}
 
 
}

Variable evt is always unused. All my coding experience tells me that unused variables are a horrible thing to see and so should always be removed. At least this is what I do in C++.

Am I missing something, doing something fundamentally wrong or just being silly?

I welcome any and all advice.

Regards
Nikki

Did you read the API doc for ActionEvent? It holds information about the recently occurred event your code is handling. It's practically worthless if you're using the anonymous innner class pattern to handle button clicks - as you are - because you're almost guaranteed (unless the API docs explicitly guarantees!) that no other event will cause your handler method to fire.

One very common other way of handling AWT events in noddy applications is to implement ActionListener in whatever class of your own is the top-level Frame for the application. That way all ActionEvents are handled by a single actionPerformed in your extended Frame class, which must then examine ActionEvent.getSource() to discover which button / control etc is responsible for firing the event.

If you only care that the event happened and not what caused the event then there is no point in passing evt in the method doStuff(). In your example program evt will just be sitting around being unused, that's just how things are. However, when you go to make the program more complex you may need to use evt to get data about the event; such as ewt.getSource() as Sean4u stated.

Just a quick message to say thanks to everyone that responded.

I think I understand a little more now

Regards
Nikki