Lab: Designing your own classes

Wednesday, 6 September 2023
In this laboratory, you will extend your knowledge of numeric values in Java. You’ll also explore the creation of classes in Java.

Useful documenation:


a. Create a directory for this lab, such as ~/CSC207/Labs/intro-classes/ (aka `/home/USERNAME/CSC207/Labs/intro-classes).

mkdir -p ~/CSC207/Labs/intro-classes

b. Make copies of three files

c. Read through the code to make sure that you have a sense as to what it does and what parts there are. You don’t need to know or understand all of the details, but a big-picture view will be helpful.

d. In your Web browser, load the documentation for the three standard classes we’ll be using (links above).


Exercise 1: Multiplication

a. Extend the Fraction class so that it permits multiplication of two fractions. That is, you should add an appropriate multiply method to the class.

b. Write an experiment that allows you to explore the behavior of the new method. (The experiment is simply code in that shows what happens when you multiply two numbers.)

Exercise 2: Fractional portions

As you may know, we can represent every non-negative rational number as a whole number plus a fractional value no smaller than 0 and strictly less than 1.

a. Write a method of the Fraction class, fractional, that identifies and returns this fractional value as a Fraction. Your procedure need only work for positive numbers.

Here are some examples that illustrate what it’s supposed to do.

  Fraction f;

  f = new Fraction(11,3);
  pen.println(f.fractional());  // 2/3

  f = new Fraction(1,2);
  pen.println(f.fractional());  // 1/2

  f = new Fraction(5,2);
  pen.println(f.fractional());  // 1/2

  f = new Fraction(4,2);
  pen.println(f.fractional());  // 0/2 or 0

b. Check/test your procedure and correct any errors.

Exercise 3: A command-line tool

There is nothing to turn in for this exercise. It simply serves to get you thinking about the code.

The FractionSum class is intended to provide a simple command-line tool for adding fractions. It also provides a nice way to test the not-yet-implemented “build a fraction from a string” constructor. Read through the code and take notes to yourself as to what main does.

a. Compile

b. What do you expect to get as output if we type the following on the command line?

java FractionSum 2/7

c. Check your answer experimentally.

d. What do you expect to get as output if we type the following on the command line?

java FractionSum 1/2 1/3 1/7

e. Check you answer experimentally.

f. What do you expect to get as output if we type the following on the command line?

java FractionSum

g. Check your answer experimentally.

Exercise 4: From string to fraction

Write and check/test a third constructor for the Fraction class. This constructor should accept a string as a parameter, parse that string, and generate the appropriate fraction. For example,

  Fraction f;
  f = new Fraction("1/4");
  pen.println(f.numerator());   // 1
  pen.println(f.denominator()); // 4
  f = new Fraction("11/5");
  pen.println(f.numerator());   // 11
  pen.println(f.denominator()); // 5
  f = new Fraction("120/3");
  pen.println(f.doubleValue()); // Approximately 40

You can expect that the string will have two positive integers separated by a slash. You may find it useful to reflect on the indexOf method of the java.lang.String class and on various static methods of the java.lang.Integer class.

Exercise 5: A counter class

Write and experiment with a class, Counter, that generates objects that can count. Objects in class Counter should provide two methods: increment, which adds 1 to the counter, and get, which gets the current value of the counter. Your class needs one zeroary constructor which initializes the counter to 0.

Make sure to verify that if you create two separate Counter objects, you can change the two objects separately.

Exercise 6: Printing counters

a. If you’ve included a toString method in Counter, comment it out.

b. What do you expect to happen if we print out a Counter using instructions like the following?

  PrintWriter pen = new PrintWriter(System.out, true);
  Counter c1 = new Counter();
  Counter c2 = new Counter();
  pen.println("c1: " + c1);
  pen.println("c2: " + c2);

c. Check your answer experimentally.

d. As you should have discovered, when a class lacks a toString method, Java chooses a fairly naive representation for printing objects in that class. Add an appropriate toString method (e.g., one that returns the counter surrounded by angle brackets). Then verify that the lines above work as you expect.

Exercise 7: Enhancing counters

a. Update your Counter class to include a second constructor that allows the user to specify a starting value.

b. Update your Counter class to include a reset method that reset the counter to the starting value.

c. Test (or experiment with) both updates to ensure that they work appropriately.

For those with extra time

Extra 1: Simplifying fractions

Update the fraction class so that we simplify each fraction when we create it. In case you’ve forgotten, the simplified version of a fraction has a numerator and denominator with a greatest common divisor of 1; you can create the simplified version by finding the gcd and dividing both numerator and denominator by that gcd.

Extra 2: Further enhancing counters

Identify other methods that would be useful to include in the Counter class and add them.

Extra 3: Further enhancing fractions

Identify other methods that would be useful to include in the Fraction class and add them.

Submitting your work

Upload your updated and new .java files to GradeScope.