Threads

Exercises

1. We start, as usual, by running our lablet. The good news is that the operation of the TickTock program is very realistic in that it operates much like a modern digital clock or watch might. The bad news is that as a result of this realism our program is every bit as confusing to use (at first glance, anyhow) as is any new digital clock. Your first job, then, is to run the program and to use it enough so that you understand how it works. We'll offer the following hint to get you going: The clock has two modes, time mode and alarm mode.
   
   
   1. Compile and run TickTock now. Notice which buttons have been enabled and which have been disabled as you run it. Then, click on whatever buttons are enabled, and see what happens. Try setting the alarm. Once you see how to do this, set the alarm for some time in the very near future (making sure that you take the AM/PM indicator into account), and return to clock mode. Keep track of what happens every time you click on something.
      
      
   2. Just as with real clocks, there are two ways to figure out how TickTock operates. One is to start playing with it, as in part (a). The other is to RTFM ("read the fabulous manual"). We have provided you with a simple user's manual in the text [pp. 491-493 ]. Read through that now, to make sure you didn't miss any of TickTock's features.
      
      
   3. Now, compare your users' manual with the portions of TickTock's code to see how your descriptions are reflected in the lablet.
      
      
2. Let's experiment more directly with the lablet's code to see how it uses threads to accomplish its processing. You know the drill: Make each of these changes to the original version of TickTock, and record the impact of the change. Note in each case what type of error occurred (if one occurred at all) and/or what change in program behavior resulted from your modifications. Be forewarned that some of these changes produce very subtle differences in behavior. In some cases, you may have to click some buttons to detect the differences.
   
   
   1. Remove (or comment out) the phrase from the header of class Timer that reads:
      implements Runnable
      Result:
      
      
      
   2. Remove (or comment out) the entire header and body of Timer's start() method.
      Result:
      
      
      
   3. Change the body of Timer's start() method to read:
      clockThread = new Thread(this);
clockThread.start();
      (that is, eliminate the surrounding if statement).
      Result:
      
      
      
   4. Change the statement in Timer's start() method that reads:
      clockThread = new Thread(this);
      to read
      clockThread = new Thread();
      Result:
      
      
      
   5. Remove the phrase from Timer's run() method that reads:
      while(clockThread != null)
{
      and the next-to-last closing brace ("}") in that method.
      Result:
      
      
      
   6. Change the body of method updateTime() to read:
      hour = h;
minute = m;
second = s;
repaint();
      (that is, eliminate the surrounding if statement).
      Result:
      
      
      
   7. Change the line in method run() that reads:
      Thread.sleep(250);
      to read:
      clockThread.sleep(250);
      Result:
      
      
      
   8. Change the line in Timer's run() method that reads:
      Thread.sleep(250);
      to read:
      Thread.sleep(1);
      Result:
      
      
      
   9. Remove (or comment out) the try and catch clauses in Timer's run() method surrounding the line that reads:
      Thread.sleep(250);
      Result:
      
      
      
   10. Remove (or comment out) the entire try and catch clauses--including the Thread.sleep statement--from Timer's run() method.
       Result:
       
       
       
3. Return now to the original version of the lablet so that we can extend it to perform even more realistically. While each of the following extensions is independent of the others, their ordering reflects their relative difficulty. So, we recommend that you do them in the order presented.
   
   
   1. Write incrementAlarmMinute() so that it doesn't use an if statement.
      
      
   2. When the original lablet is in "alarm" mode, it displays the time in the same format it uses when in "clock" mode--that is, in "hour:minute:second" format. Normally, when setting an alarm, one isn't too concerned about the seconds. Fix the program so that when TickTock is in alarm mode, it only displays the hours and minutes, and not the seconds.
      
      
   3. Add a chronometer to our clock that can be started, and continues to display the elapsed time, even when in clock or alarm modes, until it is explicitly stopped (in other words, add a stopwatch feature to the applet). There are many ways to accomplish this, and we leave it to you to decide how you want to handle the problem. Any approach that we can think of entails some re-design of TickTock, since it presently only recognizes two modes of operation, and doesn't provide an easy way for getting, say, from alarm mode to chronometer mode.