Learning Java - Chapter 6 : Java

An instance of the Circle class holds the radius and position (randomly chosen) of a circle. The paintComponent() method for DrawingPanel loops through an array of Circle objects and uses the fillOval() method to draw each circle.

The init() method in SimpleDraw2Applet selects a random number of circles and creates the array of Circle objects, which it passes to the DrawingPanel constructor. The DrawingPanel object is added to the applet's panel.

SimpleDraw2Applet.java

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

/** Drawing example. **/
public class SimpleDraw2Applet extends JApplet
{
  public void init () {
    Container content_pane = getContentPane ();

    Math.random ();

    // Get the drawing area of the applet
    int dY = getSize ().height;
    int dX = getSize ().width;

    // Create a random number of circles up to 3
    int numCircles =  (int) (2.0 * Math.random () + 1.0);

    // Create a circle array
    Circle [] circs = new Circle[numCircles];

    // Now create the circle instances
    for (int i=0; i < numCircles; i++) {
        circs[i] = new Circle (dX,dY);
    }

    // Create an instance of DrawingPanel
    DrawingPanel drawing_panel = new DrawingPanel (circs);

    // Add the DrawingPanel to the contentPane.
    content_pane.add (drawing_panel);

  } // init

} // class SimpleDraw2Applet

class DrawingPanel extends JPanel
{
  Circle [] fCircles;
  int fNumCircles;

  DrawingPanel  (Circle [] circles) {
    fCircles = circles;
    fNumCircles = circles.length;
  } // ctor

  public void paintComponent (Graphics g) {
    // First paint background
    super.paintComponent (g);

    // Add your drawing instructions here
    g.setColor (Color.red);

    // Draw the circles
    for ( int i=0; i < fNumCircles; i++){
      g.fillOval (fCircles[i].fX,fCircles[i].fY,
                  fCircles[i].fDx,fCircles[i].fDy);
    }

  } // paintComponent

} // class DrawingPanel

/** Circle class definition. **/
class Circle
{
  // These attributes provide the position &
  // dimensions of the circle and a box that
  // bounds it.
  int fCenterX,fCenterY;
  int fRadius;
  int fX,fY;
  int fDx,fDy;

  // Use constructor to
  Circle (int x_axis_width, int y_axis_height) {
    // Fix circles at 20% of horizontal axis
    fRadius = (int) (0.1 * x_axis_width);

    // choose random center location
    fCenterX = (int) ((x_axis_width) * Math.random ());
    fCenterY = (int) ((y_axis_height) * Math.random ());

    // Now calculate the bounding rectangle
    fX = fCenterX - fRadius;
    fY = fCenterY - fRadius;

    fDx = 2 * fRadius;
    fDy = 2 * fRadius;

    // Move away from edges.
    if (fX < 0) {
        fX = -fX;
        fCenterX += 2 * fX;
    } else if ((fX+fDx) > x_axis_width) {
        fX -= 2 * fDx;
        fCenterX -= 2 * fDx;
    } if (fY < 0) {
        fY = -fY;
        fCenterY += 2 * fY;
    } else if ((fY+fDy) > y_axis_height) {
        fY -= 2 * fDy;
        fCenterY -= 2 * fDy;
    }
  } // ctor
} // class Circle