Lab 8: A first try at classes

The goal of this lab is to gain familiarity with writing simple classes.

I. Setup

Change into the appropriate directory, and then clone the repository for this week's lab:

hg clone /cslab/class/csci235/lab8

II. Complex numbers

Complex numbers are numbers that have a real part and an imaginary part (factor of squart root of -1 or i). Here are some examples of complex numbers:

Some programming languages, such as Fortran, have a primitive complex number type. Java does not. We will write a class for complex numbers (including some operations) in this lab.

Change into the complex directory in your working copy. You can look at the file ComplexDriver.java, which reads a couple of complex numbers from the keyboard and then prints the result of several arithmetic operations performed on them. You will not need to modify this file.

Now look at Complex.java, This provides you with the skeleton for a complex number class: the instance variables and constructor have been provided, but you will need to write the rest of the methods.

Glance down ComplexDriver and get a basic feel for what it does. We input two complex numbers from the user, print them out, and calculate their sum, product, and quotient. There are methods to do these operations, but not all of them are finished.

A. First, implement the method toString() to make a string representation of a complex number. Currently it returns an "not implemented". If your number has a 3.1 real part and 5.7 imaginary part, it should produce a string like 3.1+5.7i.

You should now be able to test your program, verifying that it prints correctly the numbers that were input. Take some care to do reasonable things when either (or both!) of the real or imaginary part is zero.

(You may want to commit this to you repository when you are satisfied that it works correctly.)

B. Now take a look at the plus() method, and find the place in ComplexDriver.java where it is invoked:

answer = comp1.plus(comp2);

This call involves two instances of class Complex. The method called belongs to the one on the left of the dot; we often call this one the receiver of the call. You can think of this call as sending a message to comp1 asking it, "give me the result of adding comp2 to yourself".

You need to fill in the body of plus(). Note that you refer to the instance variables of the receiver with unqualified names, while you refer to the instance variables of the parameter other using qualified names, such as other.real.

C.In similar fashion, complete the methods minus(), times() and dividedBy(). Do you need a hint for multiplication? for division?

Compile and test. (And, probably, commit.)

III. A game clock

1. Introduction and set up

The second example in the lab is a very different kind of object, one that models a game clock, as for a sporting event. This will give you a feel for the range of things that objects can be used to model or simulate.

Change to its directory by moving up one directory, and then down:

cd ..
cd gameclock

First try compiling and running the game clock program.

javac GameClock.java
java -cp .:gc.jar GC &

(You don't need to worry right now about what the -cp .:gc.jar stuff means.)

A window will pop up (yes, this is your first program with a graphical user interface, or GUI) representing the clock. You'll notice that the clock has a current time left, buttons for stopping and starting the clock, for adding seconds, ten seconds, and minutes, and for setting the clock to a specific time.

Try the buttons out. You'll see that although you can add time to the clock, the button to start it running doesn't work, and neither does the button to set the clock to a specific time. You will implement these.

2. The GameClock class

Open the file GameClock.java. You will notice the class has three instance variables: The time left on the clock (timeLeft), the time-of-day when we updated the time left (timeMark), and whether or not the clock is currently running (running). Internally we keep time by milliseconds (even though the clock will display only in seconds). We also use the long type instead of int, since that's what the Java method System.currentTimeMillis() uses.

Look also at the instance methods. You'll notice that the methods correspond to the buttons on the window. I've written the GUI component to this program so that the appropriate methods will be invoked on the object whenever the user clicks on the buttons.

The methods addSecond(), addTenSeconds(), and addMinute() are written for you. Figure out how they work. The method getTime() looks like it's written, but it's actually incomplete.

3. Your task

Here is how the clock should work:

When the clock is started, we record the time-of-day (we call System.currentTimeMillis()) and set the clock's state to be "running." Whenever we are asked for the current time on the clock (and we will be asked continually, several times a second), then we find the current time and subtract the recorded time from it. This gives us how much time has elapsed since the last time we checked the time-of-day, and how much we should decrement the time left on the game clock.

In light of this,

• Implement startStop() so that it toggles the running boolean variable and, when appropriate, records the time.
• Fix getTime() so that, if the clock is currently running, it will update the time left on the clock before returning the string representation.
• When the "Set" button is pressed, the text in the field next to it is send as a parameter to the setTime() method. Write setTime() so that it interprets this string and sets the time left on the clock appropriately. Assume this text is an integer and interpret it as the number of minutes (extra credit: assume nothing; interpret it as minutes if it is just an integer, but if it is an integer, colon, another integer (for example, "5:15"), interpret them as minutes and seconds; if it is neither of these, ignore the request).

Compile GameClock.java, and run the program as indicated above.

IV. Turn in

From your lab8 directory, make a typescript as you change into the complex directory and run the program. (Don't forget to end the script.) It doesn't make sense to make a script of the gameclock program.

From the lab8 directory, print your typescript and source files with the command (all on one line):

a2ps --file-align=virtual typescript 
        complex/Complex.java gameclock/GameClock.java

Turn in the same three files electronically as lab8.