making a scroll pane widget

[jajdoo]

i'm trying to make a widget to work like JScrollPane.
my basic idea is this:
the widget has two scroll bars, and a panel;
the widget accepts another widget (the content) as the area to be scrolled.
the content is now added as a component into the panel;
when the bars are dragged, i change the location of the content.

i'm rather close, but its not working properly; im having trouble describing the problem because i'm not quite sure i understand what's actually happening incorrectly;

the code is very short, so i hope my error in logic will be easy to locate :

import java.awt.BorderLayout;
import java.awt.Component;
import java.awt.TextArea;
import java.awt.event.AdjustmentEvent;
import java.awt.event.AdjustmentListener;
import java.awt.event.ComponentEvent;
import java.awt.event.ComponentListener;

import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollBar;

public class MyScrollPane extends JComponent implements ComponentListener, AdjustmentListener
{
	private static final long serialVersionUID = -2808049373354755295L;
	
	private JScrollBar 	horizontalScroll 	= new JScrollBar( JScrollBar.HORIZONTAL );
	private JScrollBar 	verticalScroll		= new JScrollBar( JScrollBar.VERTICAL );
	private JPanel		panel 				= new JPanel();
	private Component 	internal;

	public static void main( String[] args )
	{
		JFrame frame = new JFrame();
		
		frame = new JFrame();
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.setSize(1000,1000);
		TextArea text = new TextArea("ssssssssssss");
		text.setSize(1000, 500);
		MyScrollPane p = new MyScrollPane(text);
		frame.add(p);
		
		frame.pack();
		frame.setSize(500,500);
		frame.setVisible(true);
	}
	
	public MyScrollPane( Component content )
	{
		addComponentListener(this);
		
		horizontalScroll.addAdjustmentListener(this);
		verticalScroll.addAdjustmentListener(this);
		
		setLayout( new BorderLayout() );

		add( panel, 			BorderLayout.CENTER	);
		add( horizontalScroll, 	BorderLayout.SOUTH	);
		add( verticalScroll, 	BorderLayout.WEST  	);	
		
		internal = content;
		
		panel.setLayout(null);
		panel.add(internal);
	}

	@Override
	public void componentHidden(ComponentEvent e)
	{	
	}

	@Override
	public void componentMoved(ComponentEvent e) 
	{	
	}

	@Override
	public void componentResized(ComponentEvent e) 
	{
		horizontalScroll.setValues( 0, panel.getWidth(),  0, internal.getWidth()  );
		verticalScroll	.setValues( 0, panel.getHeight(), 0, internal.getHeight() );
	}

	@Override
	public void componentShown(ComponentEvent e) 
	{
	}

	@Override
	public void adjustmentValueChanged(AdjustmentEvent e) 
	{
		boolean vertical = e.getSource().equals(verticalScroll);
		
		int x =  vertical ? internal.getX() : -e.getValue();
		int y = !vertical ? internal.getY() : -e.getValue();
		
		internal.setLocation(x,y);
	}
}

[Fubarable]

i'm trying to make a widget to work like JScrollPane.

I'm confused -- why not just use a JScrollPane?

[pbrockway2]

its not working properly; im having trouble describing the problem because i'm not quite sure i understand what's actually happening incorrectly

Frankly, it is necessary for you to describe the problem in order to make progress. Otherwise everybody else is left guessing. Its not that you are expected to say what the cause is, or what lines of code are problematic, but you must be seeing *something* on your screen that you regard as a problem or you wouldn't have posted. Say what that is.

Does the code compile? If not and you can't understand the compiler's messages, post them.

Does the code compile but do something unintended or unwanted at runtime (including throwing a runtime exception)? If so describe what the actual behaviour is (including stacktraces and other output and visual appearance) as well as saying what the intended or desired behaviour is.

[jajdoo]

I'm confused -- why not just use a JScrollPane?

because this is self exercise, not practical code.

Frankly, it is necessary for you to describe the problem in order to make progress. Otherwise everybody else is left guessing. Its not that you are expected to say what the cause is, or what lines of code are problematic, but you must be seeing *something* on your screen that you regard as a problem or you wouldn't have posted. Say what that is.

Does the code compile? If not and you can't understand the compiler's messages, post them.

Does the code compile but do something unintended or unwanted at runtime (including throwing a runtime exception)? If so describe what the actual behaviour is (including stacktraces and other output and visual appearance) as well as saying what the intended or desired behaviour is.

when i scroll, the content covers the scroll bar, instead of sliding "beneath" it, and instead of seeing the rest of the content, i see the background. i suspect that while re-positioning "internal" beyond the bounds of "panel" it moves both, therefor, not revealing the rest of internal.

[pbrockway2]

i suspect that while re-positioning "internal" beyond the bounds of "panel" it moves both, therefor, not revealing the rest of internal.

So test that.

@Override
public void adjustmentValueChanged(AdjustmentEvent e) 
{
    boolean vertical = e.getSource().equals(verticalScroll);

    int x =  vertical ? internal.getX() : -e.getValue();
    int y = !vertical ? internal.getY() : -e.getValue();

    internal.setLocation(x,y);

    System.out.println("internal: " + internal.getBounds());
    System.out.println("panel: " + panel.getBounds());
}

(internal's bounds are relative to panel, and panel's relative to the content pane of the frame.)

As far as seeing (the panel's) background is concerned it will make a difference whether text is opaque or not.

TextArea text = new TextArea("ssssssssssss");
System.out.println("text will paint itself fully: " + text.isOpaque());
text.setSize(1000, 500);
MyScrollPane p = new MyScrollPane(text);
frame.add(p);

-----

My take on it...

You're mixing Swing and pre-Swing components. See, eg, Mixing heavy and light components where the authors say "When a lightweight component overlaps a heavyweight component, the heavyweight component is always on top, regardless of the relative z-order of the two components". The TextArea is heavy while the JScrollBar and JPanel are both light.

Even if you are consistent and use Swing components throughout you might expect the background to show through since when the MyScrollPane instance is resized the (J)TextArea instance it contains is not. I think correcting this is part of making the contents of scrollpanes "scroll savvy".

[jajdoo]

thank you,
Java's gui is still very foreign to me. coming from Qt background, it takes some getting used to.
change to constructor to accepts a JComponent, to keep it swing.