JPanel vs. JFrame

[Brandt Charles]

When displaying graphics, is it not possible to draw them directly to the container of JFrame? All the examples I've seen in my text involve using JPanel, but I was still curious. Does JPanel provide a separate container from that of JFrame?

I am quite the graphics novice, so forgive my naivete.

[Michael Dunn]

it is possible to draw directly to the frame, using frame's paint(..) method,
but better to use a JPanel because of its paintComponent(..)

although, in this recent thread (with example of using JFrame and paint())
http://www.coderanch.com/t/343029/GUI/java/repaint-method
it was suggested paintComponent(..) should be used for the JFrame.

I still can't find a paintComponent() method for a JFrame, so I'll leave it
for you to experiment

[Craig Wood]

Does JPanel provide a separate container from that of JFrame?
Yes. Confining graphics to a separate graphics component simplifies things.
You can draw graphics using a containers "paint" method but it is more complicated. The tutorial has an article (with links to others) that can help you get started:
Lesson: Performing Custom Painting
Since this is lower-level work there are a lot of things you have to figure out/put together on you own. This example tries to point out some of them.

import java.awt.*;
import java.awt.geom.*;
import javax.swing.*;
 
public class ContainerDrawing extends JFrame
{
    public ContainerDrawing()
    {
        // Default layout manager is BorderLayout.
        // Adding graphic component to center section will expand
        // it to fill the available space so it does not need to
        // provide any size hints for this parent container now.
        GraphicPanel graphicPanel = new GraphicPanel();
        //System.out.println("prefSize = " +
        //                    graphicPanel.getPreferredSize());
        add(graphicPanel);     // default center section
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setSize(400,400);
        setLocation(200,200);
        setVisible(true);
        //System.out.println("size = " + graphicPanel.getSize());
    }
 
    /**
     * Container method draws container and passes graphics context
     * on to components for them to draw themselves. You can draw
     * over everything in the content pane with this method...
     */
    public void paint(Graphics g)
    {
        // see what happens when you remove/move this next line
        super.paint(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        int width = getWidth();
        int height = getHeight();
        Insets insets = getInsets();
        double w = width - insets.left - insets.right;
        double h = height - insets.top - insets.bottom;
        double x = insets.left + w/3.0;
        double y = insets.top + h/3.0;
        g2.setPaint(Color.red);
        g2.fill(new Rectangle2D.Double(x, y, w/3, h/3));
    }
 
    public static void main(String[] args)
    {
        new ContainerDrawing();
    }
}
 
class GraphicPanel extends JPanel
{
    // A parent has no idea that this component has graphics on it
    // the layout manager (default, or one you set) will look for
    // child components and, finding none, will report either
    // a default size of (10, 10) or a size of (0, 0).
    // You can set the preferredSize or override getPreferredSize
    // to provide a size hint to parent containers, if desired.
 
    protected void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        int w = getWidth();
        int h = getHeight();
        g2.setPaint(Color.green.darker());
        g2.fill(new Ellipse2D.Double(w/16, h/16, w*7/8, h*7/8));
        g2.setPaint(Color.blue);
        g2.draw(new Line2D.Double(w, 0, 0, h));
    }
}

[memati bas]

I think it does not matter whether you draw your picture on Frame or Panel but you should be careful that while drawing on Frame use paint(Graphics g) method and while drawing on Panel use paintComponent(Graphics g)method.