Espresso: A Concentrated Introduction to Java

Laboratory: Standard Object Methods

Summary: In this laboratory, you will explore some of the standard methods by adding them to existing classes.

Primary Classes Used:

Source Files:

• Fraction.java
• TestFraction.java

Contents

• Exercises
  • Exercise 0: Preparation
  • Exercise 1: Converting Fractions to Strings
  • Exercise 2: Comparing Fractions for Equality
  • Exercise 3: Equality, Revisited
  • Exercise 4: Ordering Fractions
  • Exercise 5: Hash Codes
  • Exercise 6: Cloning Fractions

Exercises

Exercise 0: Preparation

In this lab, you will continue to use the Code project and the username.fractions package.

a. As always, start Eclipse.

b. If you've configured Fraction so that it automatically simplifies fractions when they are constructed, remove the code to do so.

Exercise 1: Converting Fractions to Strings

a. In your main class, create a Fraction of your choice and print it out. What happens?

b. The Fraction class comes with a toString method. Remove it.

c. Run your main class again. What happens? You've now seen the default toString method.

d. Reinsert the toString method.

Exercise 2: Comparing Fractions for Equality

a. In your main class, create the fractions fourThirds and oneThird with the natural value for each. For example,

  Fraction fourThirds = new Fraction(4,3);
  Fraction oneThird = new Fraction(1,3);

b. Determine whether or not the fractional portion of fourThirds is the same as oneThird. For example,

  if (fourThirds.fractional().equals(oneThird)) {
    pen.println("The fractional portion of " + fourThirds 
      + " equals " + oneThird);
  }
  else {
    pen.println("The fractional portion of " + fourThirds 
      + " does not equal " + oneThird);
  }

c. Write an equals method that indicates that two fractions are equal if their numerators are the same and their denominators are the same.

d. Verify that the code from step b now indicates that the two fractions are equal.

Exercise 3: Equality, Revisited

a. Extend your main class to create two fractions, oneHalf and one, with the obvious values.

b. Write code that determines whether oneHalf.add(oneHalf) is equal to one. (Hint: If you followed instructions above, it should not be.)

c. Fix the equals method so that it considers the two equal. Note that you may not use gcd or simplify in determining equality.

d. Test your change using the code from step b.

Exercise 4: Ordering Fractions

a. Write a compareTo(Fraction other) method for your Fraction class.

b. Test your compareTo method with each of the following pairs

• 1/3 and 1/2
• 1/2 and 1/3
• 1/3 and 11/34
• 11/34 and 1/3
• 1/3 and 3/9
• oneHalf.add(oneHalf) and one

c. Correct your method if you identified any errors above.

Exercise 5: Hash Codes

Write and test a hashCode method for the Fraction class.

Exercise 6: Cloning Fractions

Write and test the clone method for the Fraction class.

History

Wednesday, 22 February 2006 [Samuel A. Rebelsky]

• Created as a blank document.

Sunday, 26 February 2006 [Samuel A. Rebelsky]

• Designed.

Monday, 27 February 2006 [Samuel A. Rebelsky]

• Filled in the details.