Positioning Slider of vertical scrollbar not working

[christopher Mg]

I have written a drum machine using Java. Into said drum machine I list all 47 available MIDI drums. The instrument list is placed into a JPanel, along with a gridArray of checkboxes. The JPanel is then in turn added to a scrollpane. I have listed the MIDI drums in order of 'ground-up'. For example, the bass drum is generally at the bottom of most drum kits, so it is at the bottom of my MIDI drums list. Next up would be the snare, followed by the cymbals, and then by all the specialty percussives. Since most beats are started from the 'ground-up', laying to foundation, so to speak, I want the scroll bar of the scrollpane which houses the JPanel, which in turn houses the instrument list and the beat checkboxes, to be positioned, upon opening, at the bottom of the scroll pane. Nothing I have tried works. It always positions the slider somewhere in the middle of the scroll track. Below is a listing of just the buildGUI portion of the code, all the MIDI functionality has been removed to keep the size of the post to this forum down.

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

public class BeatBoxG 
{
	JFrame overallOuterFrame;
    	JScrollPane namesAndSquares;
	JPanel gridPanel;
        JCheckBox c;
	ArrayList<JCheckBox> checkboxList;
        boolean[] checkboxState = new boolean[1504];
      
// Midi required items
        String[] GUIinstruments = { 
          "Mute cuica","Open cuica","Claves","Short guiro","Long guiro","Short whistle",
          "Long whistle","Maracas", "Cabasa","High agogo","Low agogo", "Vibraslap",
          "Tambourine", "Hand clap",  "Ride bell","Cowbell","Mute triangle",
          "Open triangle", "High timbale","Low timbale","Hi wood block", 
          "Low wood block", "Hi bongo","Low bongo","Mute hi conga","Open hi conga",
          "Low conga", "High tom","Hi-mid tom", "Low-mid tom",  
          "Low tom","High floor tom","Low floor tom","Ride cymbal 1","Ride cymbal 2", 
          "Crash cymbal 1","Crash cymbal 2",
          "Splash cymbal","Chinese cymbal","Open hi-hat","Closed hi-hat","Pedal hi-hat",
          "Side stick","Electric snare", "Acoustic snare","Bass drum 1", "Acoustic bass drum"};
//end of GUIinstruments
	int[] instruments = {78,79,75,73,74,71,72,70,69,67,68,58,54,39,53,56,
		80,81,65,66,76,77,60,61,62,63,64,50,
		48,47,45,43,41,51,59,49,57,55,52,46,
		42,44,37,40,38,36,35};

        public static void main (String[] args)//here is the required main()
        {
		new BeatBoxG().buildGUI();
	}//end of main

	public void buildGUI()
	{
		overallOuterFrame = new JFrame("BeatBox version G");
		overallOuterFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

		BorderLayout layout = new BorderLayout();
		JPanel bckgnd4OuterFrame = new JPanel(layout);
		bckgnd4OuterFrame.setBorder(BorderFactory.createEmptyBorder(10,10,10,10));

		checkboxList = new ArrayList<JCheckBox>();

          	Box instrumentNamesBox = new Box(BoxLayout.Y_AXIS);
          	for (int i = 0; i < 47; i++) 
          	{
              		instrumentNamesBox.add(new Label(GUIinstruments[i]));
          	}
// Put the instrumentNamesBox into a JPanel
          JPanel instrumentNames = new JPanel();
          instrumentNames.add(instrumentNamesBox);
          
          GridLayout grid = new GridLayout(47,32);//originally (16,16);
          grid.setVgap(1);
          grid.setHgap(2);
          gridPanel = new JPanel(grid);
          for (int i = 0; i < 1504; i++) //orig. < 265
          {                    
                c = new JCheckBox();
        	c.setName(""+i);
                c.setSelected(false);
        	gridPanel.add(c);            
		checkboxList.add(c);
          } // end for

//create a new grid layout to hold two items: 1) the instruments names and 2) checkboxes
          BorderLayout namesAndCBsLayout = new BorderLayout();
          JPanel namesAndCBsPanel = new JPanel(namesAndCBsLayout);
   	  namesAndCBsPanel.add(BorderLayout.CENTER, gridPanel);
          namesAndCBsPanel.add(BorderLayout.WEST, instrumentNames);

          namesAndSquares = new JScrollPane();
          namesAndSquares.getViewport().add(namesAndCBsPanel);
          JScrollBar vertical = namesAndSquares.getVerticalScrollBar();
        	     vertical.setValue(vertical.getMaximum() );
/* these dont work either
      	     vertical.setValue(vertical.getMaximum() + 500 );
      	     vertical.setValue(vertical.getMaximum() + 500 );
*/
          bckgnd4OuterFrame.add(BorderLayout.CENTER, namesAndSquares);

          overallOuterFrame.getContentPane().add(bckgnd4OuterFrame);
          overallOuterFrame.setBounds(5,5,1000,720);
          overallOuterFrame.setResizable(false);
          overallOuterFrame.setVisible(true);
} // close BuildGUI method

} // close class

[Rob Camick]

Your can try using:

    list.ensureIndexIsVisible(....);

Not that you will probably need to invoke this method AFTER the frame is visible to make sure all the components have been painted on the frame and the proper size can be calculated.

[Campbell Ritchie]

Welcome to the Ranch
I added code tags to your post. Always use them: see how much better it looks now

[christopher Mg]

Thanks for the code tags! I was wondering how other posts accomplished it.

[christopher Mg]

@Rob Camick - Thanks for the quick response. The only 'list' in my code is the ArrayList, which is imported from java.util.*. The 'ensureIndexIsVisible(int index)' method appears to not be valid for an ArrayList. Ist there an equivalent for a JPanel? I've not found one going through the ...oracle/java/docs... route.

[Rob Camick]

Sorry about that. For some reason I thought your were using a JList.

Anyway, the basic comment about not be able to set the vertical scrollbar to its maximum until the GUI is visible still applies.