Why this object gets null in actionPerformed

[arun mahajan]

Hello Everybody,
I was trying to develop following simple program but have some questions. Could you please help me solve same..

Q1: Why this mp gets null in actionPerformed? Moment I am clicking it on "Changeuser/login" option under File Menu it sends error on console.
Q2: Can you help me to wrtie this code in better fashion?
Q3: Is this style of coding will manage the system Memory or this wil take more than required?
I would appreciate if some one will answer it pointwise to understand it better.

regards,
arun

 
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class test extends JFrame
{
JMenuBar menubar = null;
Container cont = null;
makePanel mp = null;
public test()
{
cont=getContentPane();
cont.setLayout(null);
menubar = new JMenuBar();
mp = new makePanel();
mp.setBounds(0,25,450,375);
cont.add(mp);
this.setJMenuBar(menubar);
makeMenu xx = new makeMenu(menubar,mp);
this.setSize(400,400);
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
        });
}
public static void main(String[] args)
{
test xx = new test();
xx.show();
}
}
class makeMenu extends JFrame
{
JMenuBar menubar = null;
JMenu menu = null;
JMenuItem menuitem = null;
makePanel mp = null;
makeMenu(JMenuBar menubar,makePanel mp)
{
this.menubar = menubar;
this.mp=mp;
buildFile();
}
public void buildFile()
{
menu = new JMenu("File");
menu.setMnemonic(KeyEvent.VK_F);
menubar.add(menu);
menuitem = new JMenuItem("Change User/Login",KeyEvent.VK_C);
        menuitem.setAccelerator(KeyStroke.getKeyStroke(
                KeyEvent.VK_1, ActionEvent.ALT_MASK));
        new myListener(menuitem,new myListener(),mp);
menu.add(menuitem);
menuitem = new JMenuItem("Disconnect",KeyEvent.VK_D);
        menuitem.setAccelerator(KeyStroke.getKeyStroke(
                KeyEvent.VK_2, ActionEvent.ALT_MASK));
        new myListener(menuitem,new myListener(),mp);
        menu.add(menuitem);
}
}
class myListener implements ActionListener
{
ActionListener ae;
makePanel mp =null;
public myListener()
{}
public myListener(AbstractButton comp,ActionListener ae,makePanel mp)
{
this.ae=ae;
this.mp=mp;
comp.addActionListener(ae);
}
public void actionPerformed(ActionEvent evt)
{
System.out.println("Entered into ActionListener");
Object o = evt.getSource();
String s= ""+o;
boolean flag=false;
if(s.indexOf("JMenuItem")>0)
{
s="";
JMenuItem source = (JMenuItem)(evt.getSource());
s = ""+source.getText();
}
else if(s.indexOf("JButton")>0)
{
s="";
JButton source = (JButton)(evt.getSource());
s = ""+source.getToolTipText();
}
//for File Menus
if("Change User/Login".compareToIgnoreCase(s)==0)
{
System.out.println("change user/login");
mp.login();
}
else if ("Disconnect".compareToIgnoreCase(s)==0)
{
System.out.println("Disconnect");
}
}
}
class makePanel extends JPanel
{
GridBagConstraints c = null;
GridBagLayout gridbag = null;
public makePanel()
{
super();
        gridbag = new GridBagLayout();
        c = new GridBagConstraints();
setLayout(gridbag);
setBackground(new Color(140,199,222));
}
public void login()
{
System.out.println("login called");
removeAll();
JLabel id = new JLabel();
        c.gridx = 0;
        c.gridy = 0;
        gridbag.setConstraints(id, c);
id.setText("Login :");
id.setForeground(new Color(255,255,255));
add(id);
JLabel pwd = new JLabel();
c.gridx = 0;
c.gridy = 1;
gridbag.setConstraints(pwd, c);
pwd.setText("Password :");
pwd.setForeground(new Color(255,255,255));
add(pwd);
repaint();
}
}

[Nathan Pruett]

OK, I looked over your class and patched it up... Here are some of the biggest problems I noticed...

You were adding the action listener in a really strange way... The usual way to do this is to create one instance of the class and call addActionListener( myActionListener ) on each button you want to listen to. I think that your reference to mp got lost due to the way you were using the actionlistener.

You don't need a seperate JFrame to build the menu bar... it is easier and better to do it inside the frame that is actually using the menu bar.

The way that you were figuring out which button or menuitem was pushed was strange... it is easier to use the action command of the button that was pressed to figure it out...

You need to call validate() instead of repaint() in your makePanel. Repaint should be called if only the graphics drawn on components are changed. If components are added or removed, you need to re - validate() the component.

You can make your actionlistener an inner class to prevent the reference to mp getting passed all over the place...

That's all for now...

Here's the modified code...

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class TestMP extends JFrame {
    
private JMenuBar menubar = null;
private Container cont = null;
private makePanel mp = null;
    
    public TestMP() {
cont=getContentPane();
cont.setLayout(null);
menubar = new JMenuBar();
mp = new makePanel();
mp.setBounds(0,25,450,375);
cont.add(mp);
buildFile();
        setJMenuBar( menubar );
this.setSize(400,400);
addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        });
    }
    
public void buildFile(){
JMenu menu = new JMenu("File");
menu.setMnemonic(KeyEvent.VK_F);
menubar.add(menu);
        myListener my = new myListener();
JMenuItem menuitem = new JMenuItem("Change User/Login",KeyEvent.VK_C);
        menuitem.setAccelerator(KeyStroke.getKeyStroke( 
               KeyEvent.VK_1, ActionEvent.ALT_MASK));
        menuitem.addActionListener( my );
menu.add(menuitem);
menuitem = new JMenuItem("Disconnect",KeyEvent.VK_D);
        menuitem.setAccelerator(KeyStroke.getKeyStroke(
                KeyEvent.VK_2, ActionEvent.ALT_MASK));
        menuitem.addActionListener( my );
        menu.add(menuitem);
}    
public static void main(String[] args){
TestMP xx = new TestMP();
xx.show();
}
    
    private class myListener implements ActionListener{
        
        public void actionPerformed( ActionEvent evt ){
            System.out.println("Entered into ActionListener");
            AbstractButton o = ( AbstractButton )evt.getSource();
            String s= o.getActionCommand();
            //for File Menus
            if("Change User/Login".compareToIgnoreCase(s)==0){
                System.out.println("change user/login");
                mp.login();
            }
            else if ("Disconnect".compareToIgnoreCase(s)==0){
                System.out.println("Disconnect");
            }
        }
    }    
}
 
class makePanel extends JPanel{
GridBagConstraints c = null;
GridBagLayout gridbag = null;
public makePanel(){
 super();
        gridbag = new GridBagLayout();
        c = new GridBagConstraints();
 setLayout(gridbag);
 setBackground(new Color(140,199,222));
}
    
public void login(){
System.out.println("login called");
removeAll();
JLabel id = new JLabel();
        c.gridx = 0;
        c.gridy = 0;
        gridbag.setConstraints(id, c);
id.setText("Login :");
id.setForeground(new Color(255,255,255));
add(id);
JLabel pwd = new JLabel();
c.gridx = 0;
c.gridy = 1;
gridbag.setConstraints(pwd, c);
pwd.setText("Password :");
pwd.setForeground(new Color(255,255,255));
add(pwd);
    validate();
}
}

-Nate
[ May 08, 2002: Message edited by: Nathan Pruett ]