Lesson: Performing Custom Painting (The Java Tutorials)

[Mohammed Azeem]

Hello Everyone,

I'm studying the Lesson on Performing Custom Painting in the Java Tutorials. The lesson is in three steps.

In step 2, a JFrame named f is created. A JPanel object called MyPanel is also created and then added to the frame f.

A method named getPreferredSize is defined to set MyPanel's size but the problem I have with understanding why this method is never invoked anywhere in the code. So how does MyPanel and ultimately the JFrame to which it is attached get their sizes.

I am always impressed by the insights offered in this forum, thank you for you help.

The code listed below:

package painting;

import javax.swing.SwingUtilities;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.BorderFactory;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;

public class SwingPaintDemo2 {
   
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI(); 
            }
        });
    }

    private static void createAndShowGUI() {
        System.out.println("Created GUI on EDT? "+
        SwingUtilities.isEventDispatchThread());
        JFrame f = new JFrame("Swing Paint Demo");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.add(new MyPanel());
        f.pack();
        f.setVisible(true);
    }
}

class MyPanel extends JPanel {

    public MyPanel() {
        setBorder(BorderFactory.createLineBorder(Color.black));
    }

    public Dimension getPreferredSize() {
        return new Dimension(250,200);
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);       

        // Draw Text
        g.drawString("This is my custom Panel!",10,20);
    }  
}

[Darryl Burke]

Mohammed Azeem wrote:A method named getPreferredSize is defined to set MyPanel's size but the problem I have with understanding why this method is never invoked anywhere in the code. So how does MyPanel and ultimately the JFrame to which it is attached get their sizes.

This recent thread may be of interest: When is the preferredSize() method of the Container class called?

[Mohammed Azeem]

Thank you Darryl.

Just to clarify my further reading around this issue: am I correct in thinking that pack() calls the getPreferredSize method automatically in this example?

Thanks.

[Darryl Burke]

Nothing automatic about it. If you really want to know what goes on under the hood, study the JDK source code which you can find in src.zip in your JDK install directory.