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Design

Today we're going to look at an extended example of defining a class, to get a better feel for how the design process works. We'll define a class to represent a fraction. This class is actually included in the csbsju.cs160 library, almost exactly as presented here.

We begin our task by asking ourselves the question: What are the properties of a fraction? That is, what does an individual fraction need to ``remember''? In the case of a fraction, we reason, there are two properties of which we need to keep track: the numerator and the denominator. We're going to make the decision right now that we'll always store a fraction in reduced form - 2/4 will be remembered as 1/2, for example.

One decision that we'll make right now is that a fraction will be immutable - once created, its properties won't change. If we want a different fraction, then we'll have to create a brand-new one anew.

We also ask ourselves: What must a fraction ``do''? That is, what methods do we need to implement? There are a several things we might want to do to a fraction.

• Create a new fraction. When we create a new fraction, we'll want to specify its numerator and its denominator; thus, these will be parameters into the constructor method.

  Fraction(int numerator, int denominator)

• Convert the fraction into a double.

  double doubleValue()

• Add/subtract/multiply/divide it with another fraction. This will translate into the following four instance methods.

  Fraction add(Fraction other)

  Fraction subtract(Fraction other)

  Fraction multiply(Fraction other)

  Fraction divide(Fraction other)

• Compare the fraction to another. That is, we might want to ask the fraction if it is less than, equal to, or more than another fraction.

  int compareTo(Fraction other)

  Returns -1 if the receiving object is less than other, 0 if they are equal, and 1 if the receiving object is more than other.

• Convert the fraction into a string.

  String toString()

• Convert a string into a fraction. This won't really be an instance method, since it's not something that we'd instruct a particular fraction to do. Instead, it'll be a class method: We'll instruct the Fraction class to create a new fraction based on a string.

  static Fraction parseFraction(String str)

It's also useful to define two Fraction constants ZERO and ONE: Our reason for making these constants is similar to the red and black constants in the Color class: We will tend to want to refer to them often, and it would be nice if we don't have to create a new instance to do this.

Next: Coding. Up: Fraction class example. Previous: None.