Am I using repaint correctly?

[Larry Homes]

Hello

I don't have much gui experience so I am not sure I am using repaint correctly in this case. Here is some example code to demonstrate:

   
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JApplet;
import javax.swing.JButton;
import javax.swing.JPanel;

public class AppletTest extends JApplet 
{
	public void init()
	{
		this.getContentPane().add(new PanelA(this));
	}
}

class PanelA extends JPanel
{
	JApplet app = null;
	
	public PanelA(JApplet app)
	{
		this.app = app;
		JButton button = new JButton("Push me");
		button.addActionListener(new PanelSwitchToB());
		this.add(button);
	}
	
	class PanelSwitchToB implements ActionListener
	{
		public void actionPerformed(ActionEvent ae) 
		{
			app.getContentPane().removeAll();
			app.getContentPane().add(new PanelB(app));
			app.getContentPane().repaint();
		}
		
	}
}

class PanelB extends JPanel
{
	JApplet app = null;
	
	public PanelB(JApplet app)
	{
		this.app = app;
		JButton button = new JButton("Don't push me");
		button.addActionListener(new PanelSwitchToA());
		this.add(button);
	}
	
	class PanelSwitchToA implements ActionListener
	{
		public void actionPerformed(ActionEvent e) 
		{
			app.getContentPane().removeAll();
			app.getContentPane().add(new PanelA(app));
			app.getContentPane().repaint();
		}
	}
}

When repaint is called, the applet container just gets blanked instead of displaying the new panel, and then, after some waiting, shows the new Panel. But if, while the screen was blanked, if I force a repaint by minimizing and restoring, the new panel is show immediately. This leads me to beleive I am using repaint wrong. Any suggestions?


Thanks

[Larry Homes]

Well I solved it.

Here is my solution if anyone is interested. I called invalidate(), then validate() before the repaint. So my actionPerformed methods looked something like:

public void actionPerformed(ActionEvent ae) 
{
	app.getContentPane().removeAll();
	app.getContentPane().add(new PanelB(app));
	app.invalidate();
	app.validate();
	app.getContentPane().repaint();
}

I am still interested as to why this works and why just repaint() did not though. I just found this solution on another forum, so if anyone has an answer, I'd still like to hear it.

Thanks

[Rob Camick]

All repaint() does is repaint the components at there current size and location.

When you create a component is has a 0 size and location. The size and location is only determined once the layout manager has been invoked. Adding a component does not invoke the layout manager since you could be adding multiple components so it doesn't make sense to invoke the layout manager every time.

So generally all you need to do us use the validate() method (although I prefer to use revalidate() on Swing components). This will basically invoke the layout manager. In many cause this will also cause the panel to be repainted, but it not you can always add the repaint() to make sure.

In your case you should also look at using a CardLayout which allows you so swap panels easily and look after all the details behind the scenes.

[Larry Homes]

Hi

Thanks for the answer. I did look into cardlayout. Here is what I got:

import java.awt.CardLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JApplet;
import javax.swing.JButton;
import javax.swing.JPanel;

public class AppletTest extends JApplet 
{
	public void init()
	{
		JPanel cards = new JPanel(new CardLayout());
		cards.add(new PanelA(), "panelA");
		cards.add(new PanelB(), "panelB");
		this.getContentPane().add(cards);
	}
}

class PanelA extends JPanel
{	
	public PanelA()
	{
		JButton button = new JButton("Push me");
		button.addActionListener(new PanelSwitchToB());
		this.add(button);
	}
	
	class PanelSwitchToB implements ActionListener
	{
		public void actionPerformed(ActionEvent ae) 
		{
			CardLayout cl = (CardLayout)((JButton)ae.getSource()).getParent().getParent().getLayout();
			cl.show(((JButton)ae.getSource()).getParent().getParent(), "panelB");
		}
		
	}
}

class PanelB extends JPanel
{

	public PanelB()
	{
		JButton button = new JButton("Don't push me");
		button.addActionListener(new PanelSwitchToA());
		this.add(button);
	}
	
	class PanelSwitchToA implements ActionListener
	{
		public void actionPerformed(ActionEvent ae) 
		{
			CardLayout cl = (CardLayout)((JButton)ae.getSource()).getParent().getParent().getLayout();
			cl.show(((JButton)ae.getSource()).getParent().getParent(), "panelA");
		}
	}
}

While that works, is there a more elegant way to get the CardLayout using panel than a string of getParents()?


Thanks for the help

[Rob Camick]

Pass in the CardLayout as a parameter to the child panels when you create them.

[Larry Homes]

If I pass in the CardLayout, I would still need the panel instance it belongs to for the show() method. I couldn't find a way to get the owning component of a layout, but passing in the panel that uses CardLayout works just as well. So the constructor would be

public PanelA(JPanel cards)

and the switching in the actionlistener would be

CardLayout cl = (CardLayout)cards.getLayout();
cl.show(cards, "panelB");

Thanks for all your help