Java EventHandling

Question

When I compile it show error in line 33 : Cannot find symbol. I am calling jbtNew.addActionListener(listener), so why it's unable to find jbtNew in

(e.getSource() == jbtNew) in line 33.

from code

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class AnonymousListenerDemo extends JFrame {

    public AnonymousListenerDemo() {
        // Create four buttons
        JButton jbtNew = new JButton("New");
        JButton jbtOpen = new JButton("Open");
        JButton jbtSave = new JButton("Save");
        JButton jbtPrint = new JButton("Print");
        // Create a panel to hold buttons
        JPanel panel = new JPanel();
        panel.add(jbtNew);
        panel.add(jbtOpen);
        panel.add(jbtSave);
        panel.add(jbtPrint);

        add(panel);
        // Create and register anonymous inner-class listener
        AnonymousListenerDemo.ButtonListener listener = new AnonymousListenerDemo.ButtonListener();
        jbtNew.addActionListener(listener);


    }

    class ButtonListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            if (e.getSource() == jbtNew) //Here it show the problem
            {
                System.out.println("Process New");
            }
        }
    }

    /** Main method */
    public static void main(String[] args) {
        JFrame frame = new AnonymousListenerDemo();
        frame.setTitle("AnonymousListenerDemo");
        frame.setLocationRelativeTo(null); // Center the frame
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    }
}

SLaks · Accepted Answer · 2012-07-08 14:38:34Z

3

That's a local variable.
It doesn't exist outside the constructor.

You need to make a field in the class.

answered Jul 8, 2012 at 14:38

SLaks

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1 Comment

duffymo Over a year ago

I didn't notice that at first; local is worse than private data member. As written, no one will have the use of it once you exit the ctor.

mKorbel · Accepted Answer · 2012-07-08 14:49:11Z

1

this could be work (in the form as you posted here) and @SLaks mentioned +1, with a few major changes

in the case that all methods will be placed into separated classes to use put/getClientProperty()

import java.awt.EventQueue;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class AnonymousListenerDemo {

    private static final long serialVersionUID = 1L;
    private JFrame frame = new JFrame("AnonymousListenerDemo");
    // Create four buttons
    private JButton jbtNew = new JButton("New");
    private JButton jbtOpen = new JButton("Open");
    private JButton jbtSave = new JButton("Save");
    private JButton jbtPrint = new JButton("Print");

    public AnonymousListenerDemo() {
        JPanel panel = new JPanel();// Create a panel to hold buttons
        panel.add(jbtNew);
        panel.add(jbtOpen);
        panel.add(jbtSave);
        panel.add(jbtPrint);
        // Create and register anonymous inner-class listener
        jbtNew.addActionListener(new ButtonListener());
        frame.add(panel);
        //frame.setTitle("AnonymousListenerDemo");
        frame.setLocationRelativeTo(null); // Center the frame
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    }

    class ButtonListener implements ActionListener {

        public void actionPerformed(ActionEvent e) {
            if (e.getSource() == jbtNew) {
                System.out.println("Process New");
            }
        }
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            public void run() {
                new AnonymousListenerDemo();
            }
        });
    }
}

edited Jul 8, 2012 at 14:49

answered Jul 8, 2012 at 14:43

mKorbel

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2 Comments

shuvo Over a year ago

can u explain what is EventQueue.invokeLater(new Runnable()

trashgod Over a year ago

@NishitShuvo: See also Initial Threads.

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