Composing Objects - Java Concurrency in Practice - page 85

If extending a class to add another atomic operation is fragile because it distributes the lock-
ing code for a class over multiple classes in an object hierarchy, client-side locking is even
more fragile because it entails putting locking code for class C into classes that are totally
unrelated to C . Exercise care when using client-side locking on classes that do not commit to
their locking strategy.
Client-side locking has a lot in common with class extension—they both couple the behavior
of the derived class to the implementation of the base class. Just as extension violates encap-
sulation of implementation [EJ Item 14], client-side locking violates encapsulation of syn-
chronization policy.
4.4.2. Composition
There is a less fragile alternative for adding an atomic operation to an existing class: com-
position . ImprovedList in Listing 4.16 implements the List operations by delegating
them to an underlying List instance, and adds an atomic putIfAbsent method. (Like
Collections.synchronizedList and other collections wrappers, ImprovedList
assumes that once a list is passed to its constructor, the client will not use the underlying list
directly again, accessing it only through the ImprovedList .)
Listing 4.16. Implementing Put-if-absent Using Composition.