One or more variables that together maintain a running long value updated using a supplied function. When updates (method accumulate(long)) are contended across threads, the set of variables may grow dynamically to reduce contention. Method get() (or, equivalently, longValue()) returns the current value across the variables maintaining updates.

This class is usually preferable to AtomicLong when multiple threads update a common value 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.

The order of accumulation within or across threads is not guaranteed and cannot be depended upon, so this class is only applicable to functions for which the order of accumulation does not matter. The supplied accumulator function should be side-effect-free, since it may be re-applied when attempted updates fail due to contention among threads. For predictable results, the accumulator function should be associative and commutative. The function is applied with an existing value (or identity) as one argument, and a given update as the other argument. For example, to maintain a running maximum value, you could supply Long::max along with Long.MIN_VALUE as the identity.

Class LongAdder provides analogs of the functionality of this class for the common special case of maintaining counts and sums. The call new LongAdder() is equivalent to new LongAccumulator((x, y) -> x + y, 0L).

This class extends Number, but does not define methods such as equals, hashCode and compareTo because instances are expected to be mutated, and so are not useful as collection keys.