Software Engineering for Students A Programming Approach

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}
public void setY(int newY) {
y = newY;
}
6.10 Object-oriented programming 83
Here the private and public elements are clearly distinguished. A third description,
protected, means that the item is not accessible to clients but is accessible to subclasses,
as we shall see shortly. Not shown in this example are private methods that are used by
a class as necessary to carry out its work.
It is of course possible to misuse objects, by grouping ingredients that are not related.
However it is the purpose of a good design approach to ensure that this does not arise
(see Chapter 11).
Object-oriented programming (OOP) completely eliminates global data; all data is
encapsulated within objects.
The open-closed principle
If you need to modify a class (or object), there is no need to make a separate edited
copy. Instead you can use the inheritance mechanism of OOP. So the original copy
of the class remains intact, but is reused with additional or changed methods. This
is called the open-closed principle. Using the example above, we can create a new
class called MovingBall with additional methods that cause the ball to move left and
right:
class MovingBall extends Ball {
}
public void moveLeft(int distance) )
x = x - distance;
}
public void moveRight(int distance) {
x = x + distance;
}
The new class MovingBall has all the features of the class Ball, but as the keyword
extends denotes, the new class has additional methods. The variables x and y in the
superclass are accessible in this subclass because they were declared as protected.
MovingBall makes use of Ball without altering it. Thus the modularity and integrity
of the original component remain intact.
There is a snag: inheritance creates an additional type of coupling between a class
and its superclasses. Thus if a subclass is changed, the programmer needs to re-examine
all the superclasses.
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