Java: What is the difference between Stream.count() and Stream.reduce()?

count() — counts elements. reduce() — combines elements into a single value using a custom function. Think of count() as counting apples in a basket, while reduce() as combining apples into cider — each step merges two things into one.

count():

• Terminal operation.
• Returns long.
• Simple element count.
• Internally uses reduce(0L, Long::sum, Long::sum).

1long count = list.stream()
2    .filter(x -> x > 10)
3    .count();

reduce():

• Terminal operation.
• Returns Optional<T> or T.
• Custom combination logic.

1Optional<Integer> sum = list.stream()
2    .reduce(Integer::sum);
3Optional<Integer> max = list.stream()
4    .reduce(Integer::max);
5String concat = list.stream()
6    .reduce("", String::concat);

Key difference:

• count() — count elements, returns long.
• reduce() — combine elements, returns Optional or value.