Re: Custom JScrollPane - Double JScrollBars

"David A. Redick" <>
Mon, 28 Apr 2008 12:59:00 -0700 (PDT)
On Apr 26, 3:59 am, pek <> wrote:

OK.. I must admit, the code was awesome! I didn't imagine what it
would do! Thank you very much.

No prob.

So, since I don't know how (or even why) this code works, I'll just
post what exactly I want to do and hope that you'll also help me on

The how and why are fairly easy once you start thinking in the Java
way (or OO way).
Think of JScrollPane as just a container for 2 scrollbars and a view.
How does it know where to put things? Simple, the ScrollPaneLayout
tells it.

Every panel/pane type object uses a Layout object to place things.
Some are easily hijacked (like JScrollPane) some are harder (JTable
seems to be near impossible).

In the example I used earlier I just shuffled stuff around and resized
every time the JScrollPane wanted an update.


The model can be thought of as the actual data interface (JTextArea).
The view can be thought of as the thing that controls what you see
The controller can be thought of as the thing that handles events

Of course the boundaries are rather fuzzy.

If you zoom in on the scroll bars then what does what is a bit
Model: BoundedRangeModel (which is inside JScrollBar)
View: JScrollBar (I'm guessing via BoxLayout. Docs don't say)
Controller: JScrollBar


Now, I didn't know where to start. I actually thought I had to create
a new component and implement its' own look and feel. Which goes to
say what is the level of my knowledge in Swing.

You should be careful with your choice of words.
People used look and "feel to mean" a wide variety of concepts.
In the Java world look and feel just means graphical look (see PLAF).

I prefer to break the GUI into three parts.
Look -
         the graphical side (does a button have the 3d look or flat?
         Is there a drop shadow? Does a button have rounded edges or
hard corners.
         Is this a red or gray or blue button. Does it have hash

Behavior (what non-programmers usually mean by feel) -
         How does scroll bar act?
         User interaction with widgets.

Layout -
         Where stuff is placed on the screen, the size of that stuff.

Naturally there is some overlap.
And this is really just meant as an aside.
A FYI, so to speak. Take with liberal amounts of salt.

To put this in the same context as my reply above...

Look is controlled by the UIManager object and ScrollBarUI,
ScrollPaneUI, TextUI (all behind the scenes).
Behavior is controlled by the JScrollPane, JScrollBars and whatever
the JViewport is.
Layout is controlled by ScrollPaneLayout.

So, what do you think? Can you help? Or at least give me some pointers
and hints?

Well below is a basic template for DoubleScollPane.
The inner scrollbars are the standard ones. The outer ones will be
the wacky Picassa ones.
At the moment they just sit there and don't do a damn thing.

I'm still not 100% sure of how this Picassa scroll bars will behave so
difficult to say on how to implement them.

You'll probablity have to make a PicassaScrollBar object that extends

Override several of the methods (maximum, minimum, BlockIncrement,
UnitIncrement, value, IsAdjusting, VisibleAmount)
and fittle with BoundedRangeModel.

Make DoubleScrollPane an AdjustmentListener for the two
And adjust the view port in either the pane itself or the layout.
To be honest, I'm not sure which is the more proper. I can see it for
either one but
I'm really not sure. I'd guess the pane.

Here's my guess of the Picassa model:


When the knob is at B it has a scroll speed of 0
When the knob is at A it has a scroll speed of -x
When the knob is at C it has a scroll speed of x

So you'll have to figure out how to compute the speed based on the
position (fittle with BoundedRangeModel)

Heres the DoubleScrollPane template (its much cleaner than my original

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

class DoubleScrollPaneLayout extends ScrollPaneLayout
    protected JScrollBar m_pHSB2;
    protected JScrollBar m_pVSB2;

    DoubleScrollPaneLayout(JScrollBar pHSB2, JScrollBar pVSB2)

        m_pHSB2 = pHSB2;
        m_pVSB2 = pVSB2;

    public void layoutContainer(Container parent)

        Rectangle hRec = hsb.getBounds();
        Rectangle vRec = vsb.getBounds();
        Dimension size = viewport.getExtentSize();

        // make viewport size a bit smaller
        size.width -= vRec.width;
        size.height -= hRec.height;

        // scoot VSB1 over to the left
        vsb.setLocation(vRec.x - vRec.width, vRec.y);

        // scoot HSB1 up
        hsb.setLocation(hRec.x, hRec.y - hRec.height);

        // adjust size to make the corner
        hRec.width -= vRec.width;
        vRec.height -= hRec.height;
        hsb.setSize(hRec.width, hRec.height);
        vsb.setSize(vRec.width, vRec.height);

        // set our second set of scroll bars to be
        // the same size as its partner
        m_pHSB2.setSize(hRec.width, hRec.height);
        m_pVSB2.setSize(vRec.width, vRec.height);

        // set the location of our second set of scroll bars
        m_pHSB2.setLocation(hRec.x, hRec.y);
        m_pVSB2.setLocation(vRec.x, vRec.y);

// 2 horiz. bars and 2 vert. bars.
class DoubleScrollPane extends JScrollPane
    protected JScrollBar m_pHSB2;
    protected JScrollBar m_pVSB2;

    DoubleScrollPane(Component pView)



        m_pHSB2 = new JScrollBar(JScrollBar.HORIZONTAL);
        m_pVSB2 = new JScrollBar(JScrollBar.VERTICAL);



        setLayout(new DoubleScrollPaneLayout(m_pHSB2, m_pVSB2));

public class GUI extends JFrame
    // GUI objects
    JTextArea m_pText;
    DoubleScrollPane m_pScrollPane;

    public GUI()
        super("Test GUI");

        m_pText = new JTextArea("This is a test.");
        m_pText.setFont(new Font(Font.MONOSPACED, Font.PLAIN, 12));

        m_pScrollPane = new DoubleScrollPane(m_pText);


    public static void main(String[] args)
        GUI pApp = new GUI();


        catch(Exception ex)
            System.out.println("Java's PLAF is fubar");


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