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Note that since our methods addPlayer() and removePlayer() only use methods on a Set ,
we could have declared our private players variable simply as a Set rather than as a HashSet .
This would show that we depend only on the interface for a Set rather than on a particular im-
plementation of the Set . However, we still need to create a concrete instance of the Set some-
where. We could have some kind of factory method, perhaps controlled by some property, that
would determine at runtime what particular instantiation of the Set interface we would use.
But for the purposes of this example, we will tie the implementation of our Player interface
to an implementation of the Set interface, and create the HashSet directly in our PlayerImpl
Also note that what is returned from the method getRoster() is not a reference to the private
players field, but a new HashSet that is initialized with the contents of that private field. This
ensures that no one can manipulate the contents of the TeamImpl object's copy of the roster.
This approach, which tries to return copies of private data rather than references to that private
data, is generally a good idea. Since Java has garbage collection, we don't even have to worry
about this creating a memory leak. Although there are some performance implications, they
are generally small and worth the added guarantees of correctness.
To use the objects in a collection like a Set , we can get an Iterator . An Iterator is a special
object that has a very simple interface. In its basic form, an Iterator contains only the meth-
ods hasNext() , next() , and remove() . But since most of what we want to do is go through
the objects in a collection (and know when we are done with that), these suffice.
For example, suppose we wanted to add a method to our Formatter object that would print
out the roster for a team. We would like to print out the name of the Team , and then print out
the names of the players on that team below, one per line, properly indented. To do this, we
would add the method:
public static void FormatRoster(Team toFormat){
Player p;
Iterator e = toFormat.getRoster().iterator();
while (e.hasNext()){
p = (Player);
System.out.println("\t" + p.getName());
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