Grouping objects - Objects First with Java: A Practical Introduction Using BlueJ - page 101

Exercise 4.6 Write a declaration of a private field called tracks for storing a collection of
MusicTrack objects.

Exercise 4.7 Write assignments to the library, cs101, and track variables (which
you defined in the previous three exercises) to create the appropriate ArrayList objects.
Write them once without using diamond notation and once with diamond notation if you are
using a Java 7 compiler.

Generic classes are used for a variety of purposes; we will encounter more of them later in the
book. For now, collections such as ArrayList , and some other collections that we shall en-
counter shortly, are the only generic classes we need to deal with.

4.7 Numbering within collections

When exploring the music-organizer-v1 project in the first few exercises, we noted that it was
necessary to use parameter values starting at 0 to list and remove file names in the collection.
The reason behind this requirement is that items stored in ArrayList collections have an im-
plicit numbering, or positioning, that starts from 0. The position of an object in a collection is
more commonly known as its index. The first item added to a collection is given index number
0, the second is given index number 1, and so on. Figure 4.3 illustrates the same situation as
above, with index numbers shown in the ArrayList object.

It is important to be aware that this means that the last item in a collection has the index size-1 .
For example, in a list of 20 items, the last one will be at index 19.

Figure 4.3
Index numbers of
elements in a
collection

P \0XVLF
0XVLF2UJDQL]HU

$U UD\/LVW6WULQJ!

ILOHV

6WULQJ 6WULQJ 6WULQJ

³0RUQLQJ%OXHVPS´ ³'RQW*RPS´ ³0DWFK%R[%OXHVPS´