Creating a Bull's eye with for loops

Question

So, i was reading my book and making exercises, it all went perfect until i had to do the first graphical exercise, i'll post the description of the exercise:

Draw a “bull's eye”—a set of concentric rings in alternating black and white colors. Your program should be composed of classes BullsEye, BullsEyeComponent, BullsEyeViewer.

This is my BullsEye class: http://pastebin.com/4K7J2BM1

It's a tad long so i posted it on pastebin.

in case you need it, those are the BullsEyeComponent class and the BullsEyeViewer class:

    package GraphicalExecises;

    import java.awt.Graphics;
    import java.awt.Graphics2D;

    import javax.swing.JComponent;

    public class BullsEyeComponent extends JComponent {
        public void paintComponent(Graphics g) {
            Graphics2D g2 = (Graphics2D) g;
            BullsEye bullsEye = new BullsEye(20, 10);

            bullsEye.draw(g2);
        }
    }

And

    package GraphicalExecises;

    import javax.swing.JFrame;


    public class BullsEyeViewer {
        public static void main(String[] args) {
            JFrame frame = new JFrame();
            BullsEyeComponent comp = new BullsEyeComponent();

            frame.setSize(200, 200);
            frame.setResizable(false);
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.setVisible(true);
            frame.add(comp);
        } 
   } 

Edit: So, it seems that i've forgotten to add the question\problem:

The problem is that it just draws the first circle and not the others. I see that someone has responded with the recursive method, but i think it's a little too advanced for me.

Answer 1

If loops aren't a requirement, you could look into using a recursive approach. Something like this method would work. Every time a circle is drawn, you subtract 5 from the radius until the radius is less than 5.

private void displayOvals(Graphics g, int radius, int centerx, int centery) {
    if (radius >= 5) {
        g.drawOval(centerx - radius, centery - radius, 2 * radius, 2 * radius);

        displayOvals(g, radius - 5, centerx, centery);
    }
}

Here is an example

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

public class RecursiveCirclesPanel extends JPanel {

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

        int radius = (int) (Math.min(getWidth(), getHeight()) * 0.9 / 2);
        int centerx = getWidth() / 2;
        int centery = getHeight() / 2;

        displayOvals(g, radius, centerx, centery);
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(300, 300);
    }

    private void displayOvals(Graphics g, int radius, int centerx, int centery) {
        if (radius >= 5) {
            g.drawOval(centerx - radius, centery - radius, 2 * radius, 2 * radius);

            displayOvals(g, radius - 5, centerx, centery);
        }
    }

    public static void main(String[] args) {
        JOptionPane.showMessageDialog(null, new RecursiveCirclesPanel());
    }
}

---

EDIT

What you want to do is have a List of BullsEye objects in your BullsEyeComponent class. Then just loop through them in the paintComponent method. What I would do, instead of just taking x and y arguments to the BullEye object, is instead to take the a radius argument. You can use that raidus in your draw method, doing some calculations on it to get the x, y, width, and height.

In your BullsEyeComponent you can add the BullsEye objects to the List<BullsEye>. Use a while loop, saying something like

while (radius >= 5 ) { 
    // add to the list a BullsEye (passing it the radius) and decrease the radius
}

This is pretty much the same as using the recursive method, but instead you make the recursion an iteration (loop)

NOTE: This is more of an OOP approach. You could do the same just using one BullsEye object, and while looping, decrease the radius and draw another circle in the draw method. So you have two options.

HINT #1

class BullsEye {
    public void draw(Graphics2D g2, int radius, int xCenter, int yCenter) {
        while (radius >= 5) {
            g2.drawOval(xCenter - radius, yCenter - radius, 2 * radius, 2 * radius);
            radius -= 15;
        }
    }
}

HINT #2

class BullsEyeComponent extends JComponent {
    BullsEye bullsEye = new BullsEye();

    public void paintComponent(Graphics g) {
        Graphics2D g2 = (Graphics2D) g;
        int radius = (int) (Math.min(getWidth(), getHeight()) * 0.9 / 2);
        int xCenter = getWidth() / 2;
        int yCenter = getHeight() / 2;
        bullsEye.draw(g2, radius, xCenter, yCenter);
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(300, 300);
    }
}