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Lab: Designing your own classes

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


a. Fork and clone the repository.

b. Import your clone into Eclipse.

  • Select File > Import…
  • Click on General and then Existing Projects into Workspace
  • Click Next
  • Under “Select root directory”, enter the path to your clone of the repository on the local filesystem.
  • Click Finish

c. Read through the code to make sure that you understand what it does. You don’t need to know all the details, but a big-picture view is helpful.

d. Bring up 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: 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.toReal());      // 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 4: 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 5: 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 6: 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.