Algorithms and OOD (CSC 207 2014S) : Labs

Laboratory: Designing Your Own Classes


Summary: In this laboratory, you will extend your knowledge of numeric values in Java.

Primary Classes Used:

GitHub Repository: https://github.com/Grinnell-CSC207/lab-classes.

Exercises

Preparation

Fork and clone the repository.

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

Exercise 1: Multiplication

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.

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 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.

Exercise 3: From String to Fraction

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.

Exercise 4: A Counter 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.

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();
  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.

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 both updates to ensure that they work appropriately.

For Those With Extra Time

Extra 1: Further Enhancing Counters

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

Extra 2: Further Enhancing Fractions

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

Copyright (c) 2013-14 Samuel A. Rebelsky.

Creative Commons License

This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/ or send a letter to Creative Commons, 543 Howard Street, 5th Floor, San Francisco, California, 94105, USA.