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JavaAdvanced#streams
JavaIntermediate#streams
What is the difference between Stream.reduce() with and without an identity value?
reduce() with an identity always returns a value (the identity if the stream is empty). reduce() without an identity returns an Optional, since there's no default to fall back on for an empty stream.
Example
int sum = Stream.of(1,2,3).reduce(0, Integer::sum); // 6, has identity Optional<Integer> max = Stream.of(1,2,3).reduce(Integer::max); // Optional[3]