One or more variables that together maintain an initially zero
sum. When updates (method
) are contended across threads, the set of variables may grow dynamically to reduce contention. Method
) returns the current total combined across the variables maintaining the sum.
This class is usually preferable to
AtomicLong when multiple threads update a common sum that is used for purposes such as collecting statistics, not for fine-grained synchronization control. Under low update contention, the two classes have similar characteristics. But under high contention, expected throughput of this class is significantly higher, at the expense of higher space consumption.
LongAdders can be used with a
ConcurrentHashMap to maintain a scalable frequency map (a form of histogram or multiset). For example, to add a count to a
ConcurrentHashMap<String,LongAdder> freqs , initializing if not already present, you can use
freqs.computeIfAbsent(key, k -> new LongAdder()).increment();
This class extends
Number, but does not define methods such as
compareTo because instances are expected to be mutated, and so are not useful as collection keys.