Run the code, watch the output.
No matter what is clicked on the scrollbar, only one Adjustment
Event type is generated - TRACK. There is never UNIT_* event
nor BLOCK_* generated.

import java.awt.*;
import java.awt.event.*;
 
public class MyFrame extends Frame
{
        ScrollPane pane;
        class comp extends Component {
 
                public comp(){}
                public Dimension getPreferredSize() {
                        return new Dimension(400,400);
                }
        }
        class AdjustmentAdapter implements AdjustmentListener {
                public void adjustmentValueChanged(AdjustmentEvent e){}
        }
 
    AdjustmentAdapter adjustmentListener = new AdjustmentAdapter () {
            /**
             * Scrolls the column or row header depending on which scrollbar
             * in the scrollPane was clicked
             */
            public void adjustmentValueChanged(AdjustmentEvent e) {
            if (e.getAdjustmentType() == AdjustmentEvent.BLOCK_DECREMENT)
                System.out.println("BLOCK_DECREMENT");
            else if (e.getAdjustmentType() == AdjustmentEvent.BLOCK_INCREMENT)
                System.out.println("BLOCK_INCREMENT");
            else if (e.getAdjustmentType() == AdjustmentEvent.UNIT_DECREMENT)
                System.out.println("UNIT_DECREMENT");
            else if (e.getAdjustmentType() == AdjustmentEvent.UNIT_INCREMENT)
                System.out.println("UNIT_INCREMENT");
            else if (e.getAdjustmentType() == AdjustmentEvent.TRACK)
                System.out.println("TRACK");
            }
        };
 
 
 
 
        public  MyFrame() {
                super();
                add(pane = new ScrollPane(ScrollPane.SCROLLBARS_AS_NEEDED));
                pane.add(new comp());
        pane.getHAdjustable().addAdjustmentListener(adjustmentListener);
        pane.getVAdjustable().addAdjustmentListener(adjustmentListener);
        }
 
        public static void main(String [] args) {
                MyFrame f = new MyFrame();
                f.setSize(200,200);
                f.setVisible(true);
        }
}


======================================================================

NONE
======================================================================

Problem reproducible with JDK1.1.5.

  xxxxx@xxxxx   1997-11-23
=======================================================================
The bug is still reproducible with JDK 1.3RA under Sun4 Solaris 2.7.

  xxxxx@xxxxx   1999-12-15




When user scroll ScrollPane, we receive notifications in native code. Then we propogate
these notifications to Java code by issuing PeerEvent which calls setValue() with
new scrolling position on appropriate adjustable. ScrollPaneAdjustable.setValue() 
fires AdjustmentEvent for this change. The problem is that there is no way to pass 
scroll-type to setValue()(we can use only public api from peers, and setValue() only
accepts 'int position') so it always fires AdjustmentEvent.TRACK.

This happend for both Solaris and Windows as the code is absolutely the same.

Note that for Scrollbar we fire AdjustmentEvent from the peer so we not the 
scroll-type. Unfortunately, we can't setValue() for ScrollPane because it will
be very incompatible change.
  xxxxx@xxxxx   2002-07-25

======================================================================

1.2 still has this problem. Can't be *that* hard to fix.

Sounds like the same problem as in 4139140. I'm glad 
someone has finally admitted that it is a bug!

I would really like to know what to do about this because I 
would really like to be able to use the UNIT_* Events. Is 
there no other way to listen to those buttons on the 
jscrollbar or should I just create my own buttons?  How can 
there be a bug and no one tries to solve it?

The bug is still reproducible with JDK 1.4

this bug still exists in jdk1.4

I found the same bug and sent a mail to someone else. Lots of people 
observe that this is a bug, and I experienced the same bug with a similar 
code. This is a bug to be fixed!

This bug is not fixed yet. I have it in 1.6.0