Software Engineering for Students A Programming Approach

SELF-TEST QUESTION
15.4 ● Inheritance
15.4 Inheritance 207
15.4 Write a class Stack that implements a first-in, last-out structure. Use an
array list to hold items of type String. Provide public operations pop
and push. push adds an item to the top of the stack. push removes an
item from the top of the stack. The class could be used like this:
Stack stack = new Stack();
stack.push("Mary");
Real-world systems depend on our ability to classify and categorize. Elephants, tigers,
polar bears, horses and cows are all mammals; lead, silver and platinum are metals; savings,
current and term deposits are types of bank accounts, and so on. Through classification,
we are able to associate characteristics common to all members of a class. All
mammals are vertebrates (have backbones), are warm-blooded and have hair on their
bodies; all metals have atomic weights; and all bank accounts have balances.
We often think of objects as specializations of other objects. Precious metals are
specializations of metals, sports cars are specializations of cars, romance novels are specializations
of books, and so on. All precious metals are metals but not all metals are
precious metals. Similarly, all sports cars are cars and all romance novels are books, but
the reverse is not true. Similarly, quadrilaterals and triangles are polygons, and squares
and rectangles are special kinds of quadrilaterals. Furthermore, a square is a special
kind of rectangle. Extending this notion, we can view one class of objects as a subclass
of another. We can also talk about one class being a superclass of another.
What does it mean to say that one class is a subclass of another? Intuitively, we mean
that the subclass has all the characteristics of the more general class but extends it in
some way. Precious metals have all the characteristics of metals but, in addition, they can
be distinguished from some metals on the basis of monetary value. Similarly, quadrilaterals
are specializations of polygons with four sides. Polygons can have any number of
sides. Squares are specializations of quadrilaterals where all four sides have equal length,
and adjacent sides are perpendicular to one another. Applying these arguments in
reverse, we can describe the superclass of a class as being a generalization of the class.
One of the best ways to describe something new to someone else is to describe it in
terms of something that is similar, that is, by describing how it differs from something
known. An example is that a zebra is a horse with stripes! This concise definition conveys
a substantial amount of information to someone familiar with horses but not with zebras.
We now extend the cyberspace invaders program so that we can create and display
the other objects – the bomb, the laser and the user. We already have a class Alien that
describes aliens. We now consider writing a class Bomb to describe bombs. But we soon
realize that aliens and bombs have things in common, for example, their x, y coordinates
and their size, so although we could write completely separate classes, we can