Synchronized JScrollPanes
R. Kevin ColeApril 17, 2004

Two JScrollPanes can be synchronized so that both panes will respond to the action of a single JScrollBar. The trick is to synchronize two JScrollBar data models. This example should be adaptable to JSliders and JProgressBars.

import java.awt.*;
import java.util.*;

import javax.swing.*;
import javax.swing.event.*;

/**
 *  Simple panel that illustrates the synchronization of two JScrollPanes. 
 *  The number of elements in the two JList components do not have to be
 *  the same. The ChangeListener will synchronize their respective 
 *  BoundedRangeModels and scale as necessary.
 *  <p> Note: Since it is the right JScrollBar's model-ChangeListener 
 *  that listens for changes from the left JScrollBar, it is the right JList 
 *  that controls the total number of elements shown in the JScrollPanes. </p>
 * 
 *  @author  R. Kevin Cole
 */
public class ScrollBarSynchronizer extends JPanel
{
  /**
   *   Two JLists are created and initialized from Vectors and 
   *   then added to JScrollPanes to test the synchronized JScrollBars.
   */
  public ScrollBarSynchronizer()
  {
    super( new BorderLayout() );

    //  create something to scroll
    Vector<String> leftData = new Vector<String>();
    Vector<String> rightData = new Vector<String>();
    for( int i = 0; i < 71; i++ )
    {
      rightData.add( "Right " + i );

      //  the modulo operator is used to create a different number 
      //  of elements within each scrolling region.
      if( (i % 2) == 0 )
        leftData.add( "Left " + i );
    }
    JScrollPane leftScroller = new JScrollPane( new JList( leftData ) );
    leftScroller.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_NEVER);

    JScrollPane rightScroller = new JScrollPane( new JList( rightData ) );
        
    JScrollBar  leftBar = leftScroller.getVerticalScrollBar();
    JScrollBar  rightBar = rightScroller.getVerticalScrollBar();
        
    MyChangeListener listener = new MyChangeListener(leftBar.getModel());
    rightBar.getModel().addChangeListener( listener );
        
    JSplitPane split = new JSplitPane(JSplitPane.HORIZONTAL_SPLIT
            , leftScroller 
            , rightScroller );
    add( split, BorderLayout.CENTER );
  }
    

  /**
   *   Test routine.
   *   @param arg    ignored.
   */
  public static void main( String[] arg )
  {
    JFrame  frame = new JFrame("Test ScrollBarSynchronizer");
    frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );

    frame.setContentPane( new ScrollBarSynchronizer() );
    frame.setSize( 640, 480 );
    frame.setLocation( 100, 100 );
    frame.setVisible(true);
  }
}
 
/**
 *  Synchronize the data models of any two JComponents that use a
 *  BoundedRangeModel ( such as a JScrollBar, JSlider or ProgressBar).
 *
 *  @see javax.swing.BoundedRangeModel
 *  @see javax.swing.event.ChangeListener
 *  @Author  R. Kevin Cole	
 */
class MyChangeListener implements  ChangeListener
{
  private BoundedRangeModel myModel;

  /**
   *  @param   model  This model is forced to move in synchronization
   *  to this ChangeListener's event-source.
   */
  public MyChangeListener( BoundedRangeModel model )
  {
    myModel = model;
  }
 
// - begin  implementation of ChangeListener
//
  /**    Envoked when the target of the listener has changed its state.
   */
  public void stateChanged( ChangeEvent e )
  {
    BoundedRangeModel sourceModel = (BoundedRangeModel) e.getSource();
        
    int sourceDiff  = sourceModel.getMaximum() - sourceModel.getMinimum();
    int destDiff  = myModel.getMaximum() - myModel.getMinimum();
    int destValue = sourceModel.getValue();

    if( sourceDiff != destDiff )
      destValue   = (destDiff * sourceModel.getValue())/sourceDiff;
        
    myModel.setValue( myModel.getMinimum() + destValue );
  }
//
// - end  implementation of ChangeListener
}