Resizing a JPanel

Question

Heres the problem i'm having:
I'm trying to implement my own tooltips for a bar graph class that i have created. I need tooltips to appear when i move the mouse over the top of each of the bars.
The graph is built from an array of data which is an instance variable of the class. The data in this array is updated periodically (say every 10 secs). The heights of the bars in the graph depend on the height of the panel so are set in the paintComponent method.
Currently in the paintComponent method i also update the instance variable array with the rectangle created for each bar in the graph so that the mouseMotionListener (a subclass of this class) can check if the mouse is within the rectangle.
This works fine until the data in the instance varible array is updated. Here null pointers are set for each of the rectangles until a repaint is called and the rectangles are updated. This means that before a repaint takes place, the mouseListener can try and access the rectangles in the array and get a nullPointerException
Is there a better way of doing this? I dont think that i should be updating the data array from the paintComponent method but cant think of another way to do it as the bars depend on the height...
Any help is greatly appreciated.

Answer 1

Why not make the bars jcomponents and use the native tool tip?
import javax.swing.*;
import javax.swing.event.*;
import javax.swing.table.*;
import java.util.*;
import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeEvent;
import java.awt.*;
import java.awt.event.*;
public class Demo extends JFrame
    public Demo()
        super( "Demo");
        setDefaultCloseOperation( EXIT_ON_CLOSE );
        setContentPane( new GraphPanel() );
        setSize( 500 , 500 );
        show();
    public static void main(String[] args)
        try
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        catch( Exception e )
        new Demo();
    public class GraphPanel extends JPanel implements ComponentListener
        ArrayList bars = new ArrayList();
        Random rand = new Random();
        public GraphPanel()
            setLayout( null );
            addBar( Color.blue, 50 );
            addBar( Color.red,  50 );
            addBar( Color.yellow, 50 );
            addBar( Color.green,  50 );
            addComponentListener( this );
            javax.swing.Timer timer = new javax.swing.Timer( 2000, new ActionListener(){
                     * Invoked when an action occurs.
                    public void actionPerformed(ActionEvent e)
                        randomAdjust();
            timer.start();
        private void addBar( Color color, int value )
            bars.add( new BarPanel( color, value ) );
            layoutBars();
        private void layoutBars()
            removeAll();
            int max = 0;
            for( int i = 0; i < bars.size(); i ++ )
                int value = ((BarPanel) bars.get( i )).getValue();
                max = value > max?value:max;
            Dimension d = getSize();
            int spacer = d.width/100 +1;
            int width = (d.width-spacer)/bars.size();
            for( int i = 0; i < bars.size(); i ++ )
                BarPanel bp =(BarPanel) bars.get( i );
                double bpWeight =  1 - (max-bp.getValue())/(double)max;
                int bpHeight =  (int)(bpWeight * d.height);
                bp.setBounds((i+1)*spacer + i*width,
                             d.height-bpHeight,
                             width,
                             bpHeight);
                add( bp );
            revalidate();
            repaint();
        private void randomAdjust()
            for( int i = 0; i < bars.size(); i ++ )
                int adjust = rand.nextInt( 100 )-50;
                BarPanel bp = (BarPanel) bars.get( i );
                bp.setValue( bp.getValue()+adjust );
                if( bp.getValue() < 0 )
                         bp.setValue( 0 );
            layoutBars();
         * Invoked when the component's size changes.
        public void componentResized(ComponentEvent e)
            layoutBars();
         * Invoked when the component's position changes.
        public void componentMoved(ComponentEvent e)
         * Invoked when the component has been made visible.
        public void componentShown(ComponentEvent e)
            layoutBars();
         * Invoked when the component has been made invisible.
        public void componentHidden(ComponentEvent e)
    public class BarPanel extends JPanel
        private Color color;
        private int value;
        public BarPanel( Color color, int value )
            this.color = color;
            setValue( value );
            setBackground( color );
        public int getValue()
            return( value );
        public void setValue( int value )
             setToolTipText( "Value = "+value );
             this.value = value;
}By the way... That was fun to write and is the fist time I have ever found a use for a null layout.
Hope this helps,
Todd