Composing Objects - Java Concurrency in Practice - page 71

fit if there are internal consistency requirements on the location set, in which case returning
a consistent snapshot is critical, or a drawback if callers require up-to-date information for
each vehicle and therefore need to refresh their snapshot more often.

4.3. Delegating Thread Safety

All but the most trivial objects are composite objects. The Java monitor pattern is useful when
building classes from scratch or composing classes out of objects that are not thread-safe. But
what if the components of our class are already thread-safe? Do we need to add an additional
layer of thread safety? The answer is . . . “it depends”. In some cases a composite made of
thread-safe components is thread-safe ( Listings 4.7 and 4.9 ) , and in others it is merely a good
start ( 4.10 ).

In CountingFactorizer on page 23, we added an AtomicLong to an otherwise
stateless object, and the resulting composite object was still thread-safe. Since the state of
CountingFactorizer is the state of the thread-safe AtomicLong , and since Count-
ingFactorizer imposes no additional validity constraints on the state of the counter,
it is easy to see that CountingFactorizer is thread-safe. We could say that Count-
ingFactorizer delegates its

thread safety responsibilities to the AtomicLong :

CountingFactorizer is thread-safe because AtomicLong is. [5]