Why can't I display a JPanel?

[Fernando Sanz]

Hi there,
I don't want to implement a JFrame as my GUI, so I thought I'd use a JPanel.
Since I was having problems with my GUI, I thought I should start again, with something very simple:
// I didn't post the import, but they're in my code

public class MyGUI extends JPanel
{
public static void main(String[] args) 
{
System.out.println("Hello World!");
MyGUI t = new MyGUI();
t.setSize(t.getToolkit().getScreenSize()) ;
t.setVisible(true);
}

private void MyGUI()
{
JPanel buttonsPanel = new JPanel();
buttonsPanel.setLayout(new FlowLayout(FlowLayout.LEFT, 0,0));

JButton exitButton = new JButton("Exit");
JButton openButton = new JButton("Open");
JButton searchButton = new JButton("Search");
JButton textButton = new JButton("Text");

buttonsPanel.add(exitButton);
buttonsPanel.add(openButton);
buttonsPanel.add(searchButton);
buttonsPanel.add(textButton);
add(BorderLayout.SOUTH, buttonsPanel);
setVisible(true);
exitButton.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
System.exit(0) ;
}
}); // end of exitButton.addActionListener
} // end of init()
}  

What am I missing so that it doesn't display my GUI at all?
Thanks!

[Joe Ess]

Originally posted by Fernando Sanz:

What am I missing so that it doesn't display my GUI at all?

A top-level container like JWindow or JFrame which has a heavyweight peer that can be displayed on its own. JPanel is a lightweight container. It cannot be displayed alone.

[Jose Botella]

Notice that MyGUI is not a constructor because constructors do not have return type (void).

[Fernando Sanz]

OK, this time I'm trying with a JFrame. So I've created my own JPanel:

import java.awt.* ;
import javax.swing.* ;
public class MyButtonPanel extends JPanel
{
public void MyButtonPanel()
{
JButton b1 = new JButton("One") ;
JButton b2 = new JButton("Two") ;

add(b1, BorderLayout.NORTH);
add(b2, BorderLayout.SOUTH) ;
setVisible(true) ;
}
}

Which is used by:

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import javax.swing.event.*;
import java.awt.Color ;
public class SimpleUI extends JFrame
{
public SimpleUI()
{
super("Testing") ;
MyButtonPanel myPanel = new MyButtonPanel() ;
myPanel.setVisible(true) ;

JButton button = new JButton("Button") ;

getContentPane().add(myPanel, BorderLayout.CENTER) ;
getContentPane().add(button, BorderLayout.SOUTH) ;


//setSize(300, 300) ;
setSize(getToolkit().getScreenSize()) ;
setVisible(true) ;

button.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
// just close the system, and see if the program does
// finish for real on the PDA
System.exit(0) ;
}
}); // end of button1.addActionListener
}

public static void main(String args[])
{
SimpleUI application = new SimpleUI() ;
application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE) ;
}
}

But, the top JPanel doesn't display the buttons that it should.
What am I still doing wrong?
Thanks guys

[Wayne L Johnson]

A couple of things. First, look at what Jose said. In the class "MyButtonPanel" your constructor has a return type of "void". Constructors DO NOT have return types:

...
public MyButtonPanel() {  // no [B]void[/B]
  ...
}

Second, just before your call to "setSize()" you probably want to call "pack()". This lets the layout manager (BorderManager in your case) go actually place and size all of it's components.
If you call "pack()" before the the call to "setSize()", you'll get a full-screen frame. If you call "pack()" after the call to "setSize()", you'll get a small frame only large enough to fit the three buttons.
There is no need to call "setVisible()" in the "MyButtonPanel" class.

[Fernando Sanz]

Originally posted by Wayne L Johnson:
A couple of things. First, look at what Jose said. In the class "MyButtonPanel" your constructor has a return type of "void". Constructors DO NOT have return types:

...
public MyButtonPanel() {  // no [B]void[/B]
  ...
}

Second, just before your call to "setSize()" you probably want to call "pack()". This lets the layout manager (BorderManager in your case) go actually place and size all of it's components.
If you call "pack()" before the the call to "setSize()", you'll get a full-screen frame. If you call "pack()" after the call to "setSize()", you'll get a small frame only large enough to fit the three buttons.
There is no need to call "setVisible()" in the "MyButtonPanel" class.

Thanks a lot!
I haven't tried it yet, but it sounds like that's my problem

[Fernando Sanz]

Hi again,
finally I've managed to get it almost working.
I have a container that "holds" everything. At the SOUTH I added my buttons, and at the CENTER I thought I'd add all my different JPanels. Having something like:
c.add(twoPanel, BorderLayout.CENTER) ;
c.add(onePanel, BorderLayout.CENTER) ;
Then, by clicking on the appropiate button, I'd use something like nameOfJPanel.setVisible(true).
But, it doesn't work. The container will only display the last element to be added to any certain location. I mean, it'll display on the CENTER the last panel to be added. In my example, it'll display only the "onePanel".
Any other solution/idea to how to display different panels in the same location?
Thanks,
Fernando

[Joe Ess]

java.awt.CardLayout

[Fernando Sanz]

Originally posted by Joe Ess:
java.awt.CardLayout

I tried, but I'm not able to make my JPanel ocupy all the available space for the card.

[Joe Ess]

Try browsing through the Swing Tutorial, especially the part about Layout Managers. You need to understand how components get laid out on a GUI and how the different layout managers interact.