Ten Second Sketch

This is an example of a Java solution to Exercise[20].

import java.awt.BorderLayout;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;

import javax.swing.JFrame;
import javax.swing.JPanel;

/**
 * The class Timer and its constructor may be
 * used to produces instances of objects of 
 * type Timer. An object of type Timer has
 * methods start() and stop() that start
 * and stop a timer, a method called reset
 * that resets a timer, and a method called
 * elaspedTimeInMs that returns an amount of
 * elapsed time. If a timer has been started
 * but not stopped, then elapsedTimeInMs
 * returns the amount of time (in milliseconds)
 * that has elapsed since the timer was started.
 * If a timer has been started and stopped,
 * then elapsedTimeInMs returns the amount of
 * time that elapsed between the time the
 * timer was started and stopped.
 */
class Timer {
  long timeStarted;
  long timeStopped;
  Timer() {
    this.reset();
  }
  void reset() {
    this.timeStarted = 0;
    this.timeStopped = 0;
  }
  void start() {
    this.timeStarted = System.currentTimeMillis();
  }
  void stop() {
    this.timeStopped = System.currentTimeMillis();
  }
  long elapsedTimeInMs() {
    if (this.timeStarted != 0) {
      if (this.timeStopped != 0) {
        return this.timeStopped - this.timeStarted;
      }
      else {
        return System.currentTimeMillis() - this.timeStarted;
      }
    }
    else {
      return -1;
    }
  }
}

/**
 * ProgramController is a class that may be
 * used to control the execution of a Java
 * program. ProgramController objects have
 * the following methods:
 *
 * pause is a method defined in terms
 *   of a parameter called timeInMs that
 *   specifies the number of milliseconds
 *   that the Java program should pause
 *   and do nothing before resuming. The
 *   method does not return a value.
 *
 * quit is a method that, when called, causes
 *   the Java program to terminate. The
 *   method is not defined in terms of any
 *   parameters and does not return a value.
 */
class ProgramController {
  /*
   * pause does nothing for timeInMs millisends
   * before returning control to the code that
   * called pause. Internally, pause uses a
   * Timer object to "spin" in a trivial while
   * loop the specified amount of time before
   * completing.
   */
  void pause(long timeInMs) {
    // Previously missing code starts here:
    Timer timer = new Timer();
    timer.start();
    // Previously missing code ends here.
    while (timer.elapsedTimeInMs() < timeInMs);
  }
  /*
   * Calling quit makes the Java program
   * terminate.
   */
  void quit() {
    System.exit(0);
  }
}

/**
 * This is an extremely scaled-down sketching canvas; with it you
 * can only scribble thin black lines.  For simplicity the window
 * contents are never refreshed when they are uncovered.
 *
 * This implementation of TrivialSketcher and the corresponding
 * code in the static method QuickSketch.main below are based on
 * code found at:
 *
 *     http://cs.lmu.edu/~ray/notes/javagraphics/
 */
class TrivialSketcher extends JPanel {

  /**
   * Keeps track of the last point to draw the next line from.
   */
  private Point lastPoint;

  /**
   * Constructs a panel, registering listeners for the mouse.
   */
  public TrivialSketcher() {
    // When the mouse button goes down, set the current point
    // to the location at which the mouse was pressed.
    addMouseListener(new MouseAdapter() {
      public void mousePressed(MouseEvent e) {
        lastPoint = new Point(e.getX(), e.getY());
      }
    });

    // When the mouse is dragged, draw a line from the old point
    // to the new point and update the value of lastPoint to hold
    // the new current point.
    addMouseMotionListener(new MouseMotionAdapter() {
      public void mouseDragged(MouseEvent e) {
        Graphics g = getGraphics();
        g.drawLine(lastPoint.x, lastPoint.y, e.getX(), e.getY());
        lastPoint = new Point(e.getX(), e.getY());
        g.dispose();
      }
    });
  }
}

public class QuickSketch {
  public static void main(String[] args) {

    // Display the sketcher
    JFrame frame = new JFrame("Ten Second Sketch");
    frame.getContentPane().add(
      new TrivialSketcher(), BorderLayout.CENTER);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(400, 300);
    frame.setVisible(true);

    // The final countdown...
    ProgramController controller = new ProgramController();
    controller.pause(10000);
    controller.quit(); // The end!
  }
}