How to Handle events in Java?

Question

I just made my first event based GUI in java, but i don't understand where i am wrong. the code worked fine when there was no event handling applied..

Here's the code.

package javaapplication1;

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

 class Elem implements ActionListener{

    void perform(){
        JFrame frame = new JFrame();
        JButton button;
        button = new JButton("Button");
        frame.getContentPane().add(button) ;
        frame.setSize(300,300);
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        button.addActionListener(this);
     }

    public void actionPerformed(ActionEvent ev){
        button.setText("Clicked");
    }
}

 public class JavaApplication1 {

  public static void main(String[] args) {
   Elem obj = new Elem();
   obj.perform();
  }  
}

yes, it compiles and runs too, but as soon as i click the button, there are around 10 errors thrown :\ — Juzz Coding
– Juzz Coding, Commented Nov 12, 2013 at 10:49

ashatte · Accepted Answer · 2013-11-12 10:55:30Z

There is a problem with variable scope. Move the JButton button; definition outside of the perform() method so that actionPerformed() can access it.

JButton button;

void perform(){
    JFrame frame = new JFrame();
    button = new JButton("Button");
    frame.getContentPane().add(button) ;
    frame.setSize(300,300);
    frame.setVisible(true);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    button.addActionListener(this);
 }

When you define an object inside of a method (e.g. perform()), its scope is limited to that method (inside the curly braces). This means that other methods inside your class cannot access that variable.

By moving the object definition outside of the method, it now has class level scope. This means that any method within that class can access the variable. You're still defining its value inside of perform(), but now it can be accessed by other methods, including actionPerformed().

See here for more explanation.

Masudul · Accepted Answer · 2013-11-12 10:49:59Z

2

As you used button object inside actionPerformed method. So declare button globally.

class Elem implements ActionListener{
     JButton button;// Declare JButton here.
    void perform(){
        JFrame frame = new JFrame();

        button = new JButton("Button");
        frame.getContentPane().add(button) ;
        frame.setSize(300,300);
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        button.addActionListener(this);
     }

    public void actionPerformed(ActionEvent ev){
        button.setText("Clicked");
    }
}

 public class JavaApplication1 {

  public static void main(String[] args) {
   Elem obj = new Elem();
   obj.perform();
  }  
}

answered Nov 12, 2013 at 10:49

Masudul

22k5 gold badges45 silver badges60 bronze badges

1 Comment

Masudul Over a year ago

This means visibility. Declaration is not visible outside of block.

Collectives™ on Stack Overflow

How to Handle events in Java?

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related