public abstract double reduce (double identity, DoubleBinaryOperator op)

Performs a reduction on the elements of this stream, using the provided identity value and an associative accumulation function, and returns the reduced value. This is equivalent to:

     double result = identity;
     for (double element : this stream)
         result = accumulator.applyAsDouble(result, element)
     return result;
 

but is not constrained to execute sequentially.

The identity value must be an identity for the accumulator function. This means that for all x, accumulator.apply(identity, x) is equal to x. The accumulator function must be an associative function.

This is a terminal operation.

Parameters:

identity		the identity value for the accumulating function
op		an associative, non-interfering, stateless function for combining two values

Returns:  the result of the reduction

See also:
sum(), min(), max(), average()

@apiNote Sum, min, max, and average are all special cases of reduction. Summing a stream of numbers can be expressed as:

     double sum = numbers.reduce(0, (a, b) -> a+b);
 

or more compactly:

     double sum = numbers.reduce(0, Double::sum);
 

While this may seem a more roundabout way to perform an aggregation compared to simply mutating a running total in a loop, reduction operations parallelize more gracefully, without needing additional synchronization and with greatly reduced risk of data races.