Question

Why cant JLabel in java swing be declared inside the inner class like JMenu or JMenuBar

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

public class Chk extends JFrame 
{

private JLabel lbl ;

public Chk()
{

lbl = new JLabel("StatusBar");  
lbl.setBorder(BorderFactory.createEtchedBorder(EtchedBorder.RAISED));
add(lbl,BorderLayout.SOUTH);


JMenuBar menubar=new JMenuBar();
JMenu file = new JMenu("File");
JMenu view = new JMenu("View");

JCheckBoxMenuItem sbar= new JCheckBoxMenuItem("Status-Bar");
sbar.setState(true);
sbar.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent event)
{


if (lbl.isVisible())
{lbl.setVisible(false);}
else
{lbl.setVisible(true);}


}});




menubar.add(file);
view.add(sbar);
menubar.add(view);
setJMenuBar(menubar);

setSize(300,200);
setLocationRelativeTo(null);
setDefaultCloseOperation(EXIT_ON_CLOSE);
setVisible(true);
}

public static void main(String args[])
{new Chk();}

}

in the above program why do i have to put this line “private JLabel lbl ;”
Why Cant i use JLabel lbl = new JLabel(“Label”);

Answer 1

You can, but variables used in a closure need to be declared final.

    final JLabel lbl = new JLabel("StatusBar");
    lbl.setBorder(BorderFactory.createEtchedBorder(EtchedBorder.RAISED));
    add(lbl, BorderLayout.SOUTH);

This should work.

In case you are wondering, the closure is the part where you create an instance of an anonymous inner class and refer to a variable declared in an enclosing scope. In this case ‘lbl’ is referenced from within an anonymous ActionListener instance:

    sbar.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            if (lbl.isVisible()) {
                lbl.setVisible(false);
            } else {
                lbl.setVisible(true);
            }
        }
    });