Software Engineering for Students A Programming Approach

15.8 Dynamic data structures and pointers 213
holds only objects of the class Object. We can avoid this if we create an array list that
can only contain Sprite objects, as follows:
ArrayList shapes = new ArrayList();
The declaration, with the class Sprite enclosed in diamond brackets, says that this
new array list is to contain only Sprite objects. Remember that ArrayList is a Java
library class. We have qualified it by saying it must contain only Sprite objects. So now
we can avoid the casting operation, rewriting the above as follows:
for (int s = 0, s < game.size(); s++) {
Sprite sprite = game.get(s);
sprite.display(paper);
}
But there is much more to be gained than brevity. The compiler can check that only
objects of the class Sprite (or its subclasses) are added to the array list in statements
such as:
game.add(alien);
Thus errors can be caught at compile time, rather than at (more embarrassingly) run
time. The run-time error would be an InvalidCastException when an object copied
from the array list is casted.
In summary, generics allow more concise programming (by avoiding casting) and
better compile-time checking.
SELF-TEST QUESTIONS
15.6 Write a method that accepts as a parameter an array list of String objects.
Each string is an integer number. Return the sum of the numbers.
15.7 Suggest a drawback of generics.
Generics are provided in Ada, Java and C++ but are not provided in C.
15.8 ● Dynamic data structures and pointers
Many programs need to acquire temporary memory to carry out their task. Examples
are a graphics program that needs to acquire sufficient memory to represent an image
in memory, and a word processor that needs memory to hold the text of a document.
In the cyberspace invaders game, objects representing lasers and bombs are created and
destroyed.
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