Laboratory: Designing Your Own Classes

Fork and clone the repository.

Read through the code to make sure that you understand what it does.

a. Extend the Fraction class so that it permits multiplication of two fractions.

b. Write an experiment that allows you to explore the behavior of the new method.

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 tests that illustrate what it's supposed to do.

@Test
public void testFractional() 
{
  Fraction f = new Fraction(11,3);
  assertEquals("2/3", f.fractional().toString());

  f = new Fraction(1,2);
  assertEquals("1/2",  f.fractional().toString());

  f = new Fraction (5,2);
  assertEquals("1/2", f.fractional().toString());

  f = new Fraction(4,2);
  assertEquals("0/2", f.fractional().toString());
} // testFractional()

b. Test your procedure and correct any errors.

Write and 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,

public static void 
stringConstructorTest() 
{
  Fraction f = new Fraction("1/4");
  assertEquals(1, f.numerator());
  assertEquals(4, f.denominator());
  f = new Fraction("11/5");
  assertEquals(11, f.numerator());
  assertEquals(5, f.denominator());
  f = new Fraction("120/3");
  assertEquals(40.0, f.toReal(), 0.0001);
} // stringConstructorTest()

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.

Write and test 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 objects in class Counter, you can change the two objects separately.

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();
  c1.increment();
  c1.increment();
  c2.increment();
  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.

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 both updates to ensure that they work appropriately.

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

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