Circles.java

import java.awt.Graphics;
import java.awt.Color;
import java.lang.Math;
import javax.swing.JPanel;
import javax.swing.JFrame;
import javax.swing.JOptionPane;

public class Circles
{
  public static void main(String[] args)
  {
    JFrame frame = new JFrame("Circles Demo");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(800, 600);

    boolean loop = false;
    int rad, xc, yc;
    do
    {
      String tryRad = JOptionPane.showInputDialog(
        "Please enter a radius. (0-300)");
      rad = isValidNumber(tryRad);
      if (rad < 0)  loop = true;
    }while(!loop);

    loop = false;
    do
    {
      String tryCoord = JOptionPane.showInputDialog(
        "Please enter an x-coordinate. (" + rad + "-" + (800-rad) + ")");
      xc = isValidNumber(tryCoord);
      if (xc < rad || xc > (800-rad))  loop = true;
    }while(!loop);

    loop = false;
    do
    {
      String tryCoord = JOptionPane.showInputDialog(
        "Please enter a y-coordinate. (" + rad + "-" + (600-rad) + ")");
      yc = isValidNumber(tryCoord);
      if (yc < rad || yc > (600-rad))  loop = true;
    }while(!loop);

    CirclesPanel cp = new CirclesPanel(rad, xc, yc);
    frame.add(cp);
    frame.setVisible(true);

  }

  public static int isValidNumber(String s)
  {
    try
    {
      int result = Integer.parseInt(s);
      return result;
    }
    catch (NumberFormatException ex)
    {
      return -1;
    }
  }
}

class CirclesPanel extends JPanel
{
  int rad, xc, yc;

  public CirclesPanel(int r, int x, int y)
  {
    rad = r;
    xc = x;
    yc = y;
  }

  public void paintComponent(Graphics g)
  {
    super.paintComponent(g);
    this.setBackground(Color.WHITE);

    g.drawOval(xc, yc, 2*rad, 2*rad);
  }
}