Getting clipped graphics when painting in response to mouse event

Question

Can anyone tell me how to get a graphics object with the clipping region set appropriately when painting in response to a mouse action
rather than in paintComponent where the system provides the Graphics object?
The issue is that a graphical component I have written needs to repaint parts of itself in response to mouse actions. I have been getting the Graphics object by calling getGraphics on the component (based on JPanel) and using that to do the required painting. This works fine as long as there are no lightweight Swing objects in front of my component, but as I recently discovered when I added this component to a couple of JInternalFrames, mouse-triggered painting can paint on the other JInternalFrame.
The example code below illustrates the problem.
Thanks in advance
import javax.swing.*;
import java.awt.*;
import java.awt.geom.Ellipse2D;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionListener;
* This class illustrates a problem I am having with a graphical component I have
* written that works fine when placed in a JFrame, but when placed inside a JInternalFrame,
* can draw outside the bounds of the JInternalFrame.
* The real component contains many sub-objects, and users can select an individual sub-object,
* and trigger actions specific to that sub-object. To aid in sub-object selection,
* when the mouse pointer is over a sub-object, the color of the object is changed to
* a highlight color. This is where the problem of drawing outside the JInternalFrame occurs.
* I am pretty sure that the reason the drawing can occur outside the bounds of the
* JInternalFrame is that the graphics context used for the highlighting action does not
* have the clipping region set. Unlike in paintComponent where the system provides the
* Graphics object with the clipping region set appropriately, painting of individual
* sub-objects in response to a MouseEvent is done using a call to getGraphics on the
* component. This has the clipping region set to the dimensions of the JPanel.
* This example creates two overlapping JInternalFrames. Inside each is a simple component
* based on a JPanel. The component draws a single filled circle which corresponds to
* one of the sub-objects in the real component. When the mouse pointer is over a circle,
* the color changes from gray to red. Where the circle overlaps the other JInternalFrame,
* the circle will paint over that frame.
public class GraphicsProblemDemo extends JFrame {
    private static final int APP_FRAME_SIZE = 400;
    public GraphicsProblemDemo()
        JDesktopPane desktop = new JDesktopPane();
        setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        getContentPane().setLayout( new BorderLayout() );
        getContentPane().add( desktop, BorderLayout.CENTER );
        setSize( APP_FRAME_SIZE, APP_FRAME_SIZE );
        setVisible( true );
        // Create two JInternalFrames, each with a single component that
        // draws a filled circle.
        JInternalFrame frame = createInternalFrame( 0, 0 );
        frame.setVisible( true );
        desktop.add( frame );
        frame = createInternalFrame( 200, 200 );
        frame.setVisible( true );
        desktop.add( frame );
    public JInternalFrame createInternalFrame( int left, int top )
        JInternalFrame frame = new JInternalFrame( "", true, true, true, true );
        int INTERNAL_FRAME_SIZE = 300;
        frame.setBounds( left, top, INTERNAL_FRAME_SIZE, INTERNAL_FRAME_SIZE );
        myComponent shape = new myComponent();
        frame.add( shape );
        return frame;
     * This component simulates the more complex component that I am trying to debug.
     * The real component contains many sub-objects and is expensive to render.
     * Individual sub-objects are selectable within the component, and to indicate to the
     * user that the mouse is positioned over a sub-object, the sub-object color is changed.
     * Because it is expensive to render the component completely, the MouseMotionListener
     * does not call repaint on the component. Instead, it gets the graphics context,
     * sets the color, and renders the component under the mouse pointer.
    class myComponent extends JPanel implements MouseMotionListener {
        private Ellipse2D.Double subObject = new Ellipse2D.Double( 10, 10, 200, 200 );
        private Color unselectedColor = Color.LIGHT_GRAY;
        private Color selectedColor = Color.RED;
        private Color renderColor = unselectedColor;
        public myComponent()
            addMouseMotionListener(this);
        // Painting is fine when this method is called by the system.
        public void paintComponent( Graphics g )
            super.paintComponent( g );
            // In the real component, many objects would be rendered here, but
            // this example component has only a single sub-object.
            renderMySingleSubObject( g );
        // But when this method is called by my mouse listener using the
        // graphics context from the containing JPanel, the circle is rendered on top
        // of the other JInternalFrame.
        private void renderMySingleSubObject( Graphics g )
            Graphics2D g2d = (Graphics2D) g;
            g2d.setColor( renderColor );
            g2d.fill( subObject );
         * If the mouse pointer is over my single sub-object, then change
         * the color from gray to red and render  the sub-object.
         * @param e The MouseEvent
        public void mouseMoved( MouseEvent e )
            if( subObject.contains( e.getX(), e.getY() ) )
                renderColor = selectedColor;
            else
                renderColor = unselectedColor;
            Graphics g = myComponent.this.getGraphics();
            // This graphics context does not have the clipping region set
            renderMySingleSubObject( g );
        public void mouseDragged( MouseEvent e )
    public static void main( String[] args )
        new GraphicsProblemDemo();
}

Answer 1

Checking the method description for the 'getGraphics' method in the Component api the first sentence says "Creates a graphics context for this component." Emphasize 'creates'. You can demonstrate the difference by printing the Graphics context g in each case. Checking for the clip in each returns null for the 'getGraphics' call.
            super.paintComponent( g );
            // In the real component, many objects would be rendered here, but
            // this example component has only a single sub-object.
            renderMySingleSubObject( g );
            System.out.println("paintComponent g = " + g);
            System.out.println("paintComponent g clip = " + g.getClip().toString());
            Graphics g = myComponent.this.getGraphics();
            System.out.println("getGraphics g = " + g);
            // NullPointer here...
            System.out.println("getGraphics g clip = " + g.getClip().toString());Suggests that 'getGraphics' might not be the way. You could, of course, check all the iframes and find out how much of the moused-over ellipse is visible and put together something to send along for the re-rendering...
Here's another possibility. The repaint code doesn't recover well for mouse exits of the lower iframe into the upper but it will give an idea about economy and control.
import javax.swing.*;
import java.awt.*;
import java.awt.geom.Ellipse2D;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionListener;
public class GPD extends JFrame {
    private static final int APP_FRAME_SIZE = 400;
    public GPD()
        JDesktopPane desktop = new JDesktopPane();
        setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        getContentPane().setLayout( new BorderLayout() );
        getContentPane().add( desktop, BorderLayout.CENTER );
        setSize( APP_FRAME_SIZE, APP_FRAME_SIZE );
        setVisible( true );
        // Create two JInternalFrames, each with a single component that
        // draws a filled circle.
        JInternalFrame frame = createInternalFrame( 0, 0 );
        frame.setVisible( true );
        desktop.add( frame );
        frame = createInternalFrame( 200, 200 );
        frame.setVisible( true );
        desktop.add( frame );
    public JInternalFrame createInternalFrame( int left, int top )
        JInternalFrame frame = new JInternalFrame( "", true, true, true, true );
        int INTERNAL_FRAME_SIZE = 300;
        frame.setBounds( left, top, INTERNAL_FRAME_SIZE, INTERNAL_FRAME_SIZE );
        myComponent shape = new myComponent();
        frame.add( shape );
        return frame;
    class myComponent extends JPanel implements MouseMotionListener {
        private Ellipse2D.Double subObject = new Ellipse2D.Double( 10, 10, 200, 200 );
        private Color unselectedColor = Color.LIGHT_GRAY;
        private Color selectedColor = Color.RED;
        private Color renderColor = unselectedColor;
        int count = 0;
        public myComponent()
            addMouseMotionListener(this);
        // Painting is fine when this method is called by the system.
        public void paintComponent( Graphics g )
            super.paintComponent( g );
            g.setColor( renderColor );
            ((Graphics2D)g).fill( subObject );
         * If the mouse pointer is over my single sub-object, then change
         * the color from gray to red and render  the sub-object.
         * @param e The MouseEvent
        public void mouseMoved( MouseEvent e )
            boolean colorHasChanged = false;
            if( subObject.contains( e.getX(), e.getY() ) )
                if( renderColor == unselectedColor )
                    renderColor = selectedColor;
                    colorHasChanged = true;
            else if( renderColor == selectedColor )
                renderColor = unselectedColor;
                colorHasChanged = true;
            if( colorHasChanged )
                System.out.println("repainting " + count++);
                repaint();
        public void mouseDragged(MouseEvent e) { }
    public static void main( String[] args )
        new GPD();
}