Object interaction

Aim

To understand how objects cooperate to perform a common task.

Clock display exercise

• a). Open the clock-display project (in chapter 3). Do the exercises 3.6 to 3.14, 3.22 to 3.24, 3.28 and 3.29.
  • Download the clock display project.

Additional clock display exercises

• a). Implement the setAlarm method which takes three parameters, the hours, minutes and a message specifying the time of the alarm and the message to be displayed. When the clock reaches that time the message is printed on the display
• b). Display seconds. Add a new number display to display the seconds. This modification requires the following changes to the code. Add a new method similar to setTime in the ClockDisplay class but which takes seconds as well as hours and minutes into account (setTime(int hour, int minute, int second). Also modify the methods timeTick to work for seconds rather than minutes and updateDisplay to take seconds into account
• c). Implement the forward method which takes one integer parameter. This method brings forward the clock by the number of seconds specified in the parameter.

Debugging exercises

• a). Open the rectangle project. This simulates the behaviour of rectangles and points which are situated at specific coordinates. Create a Rectangle object with parameters 6, 10, 16 and 4 (in that order). This means that this rectangle has its top left corner at (6,10) and its bottom right one at (16,4). Then call the inOrOut method with 12 and 6. This means a point which is located at (12,6). This method returns a message saying the point is outside the rectangle but it really is inside. Your task is to use the Bluej debugger to try and spot where the error is. Make sure you set some breakpoints, single-step, step-into and inspect some of the instance and local variables. Do this both for creating a rectangle as well as for invoking the inOrOut method. The point of this exercise is to make sure you can use the Bluej debugger, rather than to actually spot the error.
  • Download this program
• b). Open the clock-display-alarm project. This project simulates the behaviour of a clock display similar to the one in chapter 3 of the Bluej book but with added functionality for setting an alarm. Create a new object of ClockDisplay type. Invoke the setTime method with parameters 6 and 29 (the clock is set to 06:29 am). Inspect the object and verify that the displayString field reflects this change. Now invoke the setAlarm method with parameters 6 and 30 (alarm is set at 06:30 am). You will notice that the alarmSet field has changed from false to true as a result of this. When invoking the timeTick method, this program should print the message `It's 06:30 already!' to the terminal. However if you invoke this method you will find that this doesn't happen. There is an error in this program. Your task is to use the Bluej debugger to try and spot where the error is. Make sure you set some breakpoints, single-step, step-into and inspect some of the instance and local variables. Do this when invoking both the setTime and timeTick methods. The point of this exercise is to make sure you can use the Bluej debugger, rather than to actually spot the error.
  • Download this program

Last updated at 8:57pm, Thursday September 22nd 2011 Dr Natalia Beloff (N.Beloff@sussex.ac.uk)