isAccumulator

Whether fun is a valid accumulator function for values of type ValueType.

For this to be true, fun must be a callable matching one of the following argument lists:

fun(ValueType[] values);
fun(ValueType[] values, Exception[] exceptions);

The second version is passed the collected array exceptions from all the clients in the pool.

The return value of the accumulator function is passed to the client (via the result future). If it throws an exception, the operation is marked as failed with the given exception instead.