Grouping objects - Objects First with Java: A Practical Introduction Using BlueJ - page 94

It is typical that the number of items stored in a collection will vary from time to time. So far,
we have not met any features of Java that would allow us to group together arbitrary numbers of
items. We could, perhaps, define a class with a lot of individual fields to cover a fixed but very
large number of items, but programs typically have a need for a more general solution than this
provides. A proper solution would not require us either to know in advance how many items we
wish to group together, or to fix an upper limit to that number.

So we will start our exploration of the Java library by looking at a class that provides the
simplest possible way of grouping objects, an unsorted but ordered flexible-sized list:
ArrayList . In the next few sections, we shall use the example of keeping track of a personal
music collection to illustrate how we can group together an arbitrary number of objects in a
single container object.

4.3 An organizer for music files

We are going to write a class that can help us organize our music files stored on a computer. Our
class won't actually store the file details; instead, it will delegate that responsibility to the stand-
ard ArrayList library class, which will save us a lot of work. So, why do we need to write our
own class at all? An important point to bear in mind when dealing with library classes is that
they have not been written for any particular application scenario—they are general-purpose
classes. One ArrayList might store student-record objects, while another stores event remind-
ers. This means that it is the classes that we write for using the library classes that provide the
scenario-specific operations, such as the fact that we are dealing with music files or playing a
file that is stored in the collection.

For the sake of simplicity, the first version of this project will simply work with the file
names of individual music tracks. There will be no separate details of title, artist, playing
time, etc. That means we will just be asking the ArrayList to store String objects rep-
resenting the file names. Keeping things simple at this stage will help to avoid obscuring
the key concepts we are trying to illustrate, which are the creation and usage of a collection
object. Later in the chapter, we will add further sophistication to make a more viable music
organizer and player.

We will assume that each music file represents a single music track. The example files we have
provided with the project have both the artist's name and the track's title embedded in the file
name, and we will use this feature later. For the time being, here are the basic operations we
will have in the initial version of our organizer:

■ It allows tracks to be added to the collection.
■ It has no predetermined limit on the number of tracks it can store, aside from the memory
limit of the machine on which it is run.
■ It will tell us how many tracks are in the collection.

■ It will list all the tracks.