Object-Oriented Design - Java Software Solutions: Foundations of Program Design - page 316

LISTING 7.4 continued

}

//-----------------------------------------------------------------
// Computes and returns the greatest common divisor of the two
// positive parameters. Uses Euclid's algorithm.
//-----------------------------------------------------------------
private int gcd ( int num1, int num2)
{
while (num1 != num2)
if (num1 > num2)
num1 = num1 - num2;
else
num2 = num2 - num1;

return num1;
}
}

The methods of the RationalNumber class, such as add , subtract , multiply ,
and divide , use the RationalNumber object that is executing the method as the
first (left) operand and the RationalNumber object passed as a parameter as the
second (right) operand.
The isLike method of the RationalNumber class is used to determine if two
rational numbers are essentially equal. It's tempting, therefore, to call that method
equals , similar to the method used to compare String objects (discussed in
Chapter 5). However, in Chapter 9 we will discuss how the equals method is
somewhat special due to inheritance, and that it should be implemented in a par-
ticular way. So to avoid confusion we call this method isLike for now.
Note that some of the methods in the RationalNumber class, including reduce
and gcd , are declared with private visibility. These methods are private because
we don't want them executed directly from outside a RationalNumber object.
They exist only to support the other services of the object.

Aggregation
Some objects are made up of other objects. A car, for instance, is made up of its
engine, its chassis, its wheels, and several other parts. Each of these
other parts could be considered a separate object. Therefore we can
say that a car is an aggregation —it is composed, at least in part, of
other objects. Aggregation is sometimes described as a has-a rela-
tionship. For instance, a car has a chassis.

KEY CONCEPT
An aggregate object is composed
of other objects, forming a has-a
relationship.