Question

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

public class twobuttons
{
    int x=70;
    int y=70;

    public static void main(String args[])
    {
        twobuttons gui =new twobuttons();
        gui.go();
    }

    public void go()
    {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        mydraw drawpanel = new mydraw();

        frame.getContentPane().add(drawpanel);
        frame.setSize(300,300);
        frame.setVisible(true);

        for(int i=0;i<130;i++)
        {
            x++;
            y++;

            drawpanel.repaint();
            try
            {
                Thread.sleep(50);
            }
            catch(Exception ex)
            {

            }
        }

    }
    class mydraw extends JPanel
    {

        public void paintconponent(Graphics g)
        {
            g.setColor(Color.green);
            g.fillOval(x, y, 40, 40);
        }
    }
}

this is a code from the Head First Java,chapter 12,p384(I am reading the Chinese edition ,maybe it’s not in the same page in the English edition.)If successful,there should be a dot running from the left corner to the right corner,however,I can’t see anything in the window.

Answer 1

You have a typo in the painting class (paintconponent => paintComponent). So you were not overriding the behavior of the JComponent super class. Try this:

class mydraw extends JPanel
{

    @Override
    public void paintComponent(Graphics g)
    {
        g.setColor(Color.green);
        g.fillOval(x, y, 40, 40);
    }
}