Software Engineering for Students A Programming Approach

212 Chapter 15 ■ Object-oriented programming
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Each time that the for statement repeats, it obtains the next element from the array
list game.
15.6 ● Single versus multiple inheritance
As we have seen, Java supports single inheritance – a class can inherit from only one
immediate superclass. Seen as a class diagram, the relationships between classes appear
as a tree (a computer science tree, with the root at the top). Smalltalk, Ada, C# and
Visual Basic.Net also provide single inheritance.
However, the widely used language C++ provides multiple inheritance, as does
Eiffel. In such a language, a class can inherit from not just one but several superclasses.
In life we are not just a person, we also belong to other categories, such as brothers,
daughters, soccer lovers, carnivores. So a class representing a person is a subclass of all
these superclasses, inheriting variables and methods from them all.
There is no doubt that multiple inheritance is more complicated – both to provide
in the language and to use. C++ was widely seen as an overcomplicated language and
subsequent languages, such as Java and C#, have seen simplifications in many areas,
including abandoning multiple inheritance in favor of single. In some languages,
including Java and C#, one role of multiple inheritance has been replaced by the interface
facility described in Chapter 16 on programming in the large.
15.7 ● Generics
The strong typing philosophy of programming languages like Java and Ada can have a
detrimental effect on programming efficiency. For example, suppose we defined a stack of
strings class with the normal stack operations of push and pop, as posed in the self-test
question above. If we subsequently needed another stack type but one in which the elements
were Booleans rather than strings then clearly the specification and implementation
would be identical apart from the different stack element types. In some languages, our only
recourse would be to duplicate the stack code, but with minor differences. A more powerful
stack abstraction is required which allows the stack element type to be parameterized.
We will use the Java cyberspace invaders game discussed above to see how generics
can be used. An array list named game contains objects representing various items
(alien, bomb, laser) at various positions within a panel. To display all the shapes, we execute
a loop:
for (int s = 0, s < game.size(); s++) {
sprite sprite = (Sprite) game.get(s);
sprite.display(paper);
}
Notice that the objects retrieved from the array list need to be casted into Sprite
objects using a casting operator, (Sprite) in this case. This is because an array list
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