Grouping objects - Objects First with Java: A Practical Introduction Using BlueJ - page 100

detail is a benefit that abstraction gives us; it means that we can utilize ArrayList to write any
number of different classes that require storage of an arbitrary number of objects.

The second feature—the ArrayList object keeping its own count of inserted objects—has
important consequences for the way in which we implement the MusicOrganizer class.
Although an organizer has a getNumberOfFiles method, we have not actually defined a
specific field for recording this information. Instead, an organizer delegates the responsibility
for keeping track of the number of items to its ArrayList object. This means that an organizer
does not duplicate information that is available to it from elsewhere. If a user requests from the
organizer information about the number of file names in it, the organizer will pass the question
on to the files object and then return whatever answer it gets from there.

Duplication of information or behavior is something that we often work hard to avoid.
Duplication can represent wasted effort and can lead to inconsistencies where two things that
should be identical turn out not to be, through error. We will have a lot more to say about dupli-
cation of functionality in later chapters.

4.6 Generic classes

The new notation using the angle brackets deserves a little more discussion. The type of our
files field was declared as

ArrayList<String>

The class we are using here is called simply ArrayList , but it requires a second type to be
specified as a parameter when it is used to declare fields or other variables. Classes that re-
quire such a type parameter are called generic classes. Generic classes, in contrast to other
classes we have seen so far, do not define a single type in Java, but potentially many types. The
ArrayList class, for example, can be used to specify an ArrayList of String , an ArrayList of
Person , an ArrayList of Rectangle , or an ArrayList of any other class that we have available.
Each particular ArrayList is a separate type that can be used in declarations of fields, param-
eters, and return values. We could, for example, define the following two fields:

private ArrayList<Person> members;
private ArrayList<TicketMachine> machines;

These definitions state that members refers to an ArrayList that can store Person objects,
while machines can refer to an ArrayList to store TicketMachine objects. Note that
ArrayList<Person> and ArrayList<TicketMachine> are different types. The fields cannot
be assigned to each other even though their types were derived from the same ArrayList class.

Exercise 4.4 Write a declaration of a private field named library that can hold an
ArrayList . The elements of the ArrayList are of type Book .

Exercise 4.5 Write a declaration of a local variable called cs101 that can hold an
ArrayList of Student .