Garbage Collection and Memory - The Java Programming Language - page 676

If lastData were a direct, strong reference to the last data returned, that
data would not be collected, even if the invoking program had long since
dropped all references to the array. Such a strong reference would keep
the object alive as long as the DataHandler object itself was reachable.
This could be quite unfortunate. Using a WeakReference allows the space
to be reclaimed, at the cost of occasionally re-reading the data from
disk.
Notice that invoking get on lastData makes the byte array strongly
reachable once again because its value is bound to an active local vari-
ablea strong reference. If get returns a non- null reference there is no
possibility of the byte array being reclaimed as long as readFile is ex-
ecuting, or the block of code to which readFile returns the reference is
executing. That invoking code can store the reference in a reachable
place or return it to another method, thereby ensuring the data's reach-
ability. An object can only be less than strongly reachable when none
of its references are strong. Storing a non- null reference fetched from
a reference object's get method creates a strong reference that will be
treated like any other strong reference.
Weak references typically store information about an object that might
be time consuming to compute but that need not outlive the object it-
self. For example, if you had some complicated information built up from
using reflection on an object, you might use the java.util.WeakHashMap
class, which is a hashtable for mapping a weakly-held object to your in-
formation. If the object becomes unreachable the WeakHashMap will clean
up the associated information, which presumably is no longer useful
(since the object will not be used in any future computation because of
its weak reachability). You will learn about WeakHashMap with the rest of
the collection classes in Chapter 21 , specifically in Section 21.9 on page
594 .
Exercise 17.2 : Modify DataHandler so that lastFile is also stored
weakly.