Grouping objects - Objects First with Java: A Practical Introduction Using BlueJ - page 93

become familiar with the contents of the library and how to use the most common classes. The
power of abstraction is that we don't usually need to know much (if anything, indeed!) about
what the class looks like inside to be able to use it effectively.

If we use a library class, it follows that we will be writing code that creates instances of those
classes, and then our objects will be interacting with the library objects. Object interaction will
also figure highly in this chapter, therefore.

You will find that the chapters in this topic continually revisit and build on themes that have
been introduced in previous chapters. We refer to this in the preface as an “iterative approach.”
One particular advantage of the approach is that it will help you to gradually deepen your un-
derstanding of topics as you work your way through the topic.

In this chapter, we also extend our understanding of abstraction to see that it does not just mean
hiding detail but also means seeing the common features and patterns that recur again and again
in programs. Recognizing these patterns means that we can often reuse in a new situation part
or all of a method or class we have previously written. This particular applies when looking at
collections and iteration.

4.2 The collection abstraction

One of the abstractions we wish to explore in this chapter is the idea of a collection —the no-
tion of grouping things so that we can refer to them and manage them all together. A collec-
tion might be: large (all the students in a university); small (the courses one of the students is
taking); or empty even (the paintings by Picasso that I own!).

If we own a collection of stamps, autographs, concert posters, ornaments, music, or whatever,
then there are some common things we will want to do to the collection from time to time,
regardless of what it is we collect. For instance, we will likely want to add to the collection,
but we also might want to cut it down —say if we have duplicates or want to raise money for
additional purchases. We also might want to arrange it in some way—by date of acquisition or
value, perhaps. What we are describing here are typical operations on a collection.

In a programming context, the collection abstraction becomes a class of some sort, and the operations
would be methods of that class. A particular collection (my music collection) would be an instance of
the class. Furthermore, the items stored in a collection instance would, themselves, be objects.

Concept:

Collection
A collection object
can store an
arbitrary number of
other objects.

Here are some further collection examples that are more obviously related to a programming context:

■ Electronic calendars store event notes about appointments, meetings, birthdays, and so on.
New notes are added as future events are arranged, and old notes are deleted as details of past
events are no longer needed.
■ Libraries record details about the topics and journals they own. The catalog changes as new
topics are bought and old ones are put into storage or discarded.

■ Universities maintain records of students. Each academic year adds new records to the col-
lection, while the records of those who have left are moved to an archive collection. Listing
subsets of the collection will be common: all the students taking first-year CS or all the
students due to graduate this year, for instance.