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

collect() — mutable accumulation into a container. reduce() — immutable combination of values. Think of collect() as filling a box — you add items to the same box over and over — while reduce() as folding items together — each fold produces a new combined result.

collect():

• Uses a Collector (supplier, accumulator, combiner, finisher).
• Mutates the container (adds to a List, inserts into a Map).
• For building collections, grouping, partitioning, joining.
• Always returns a result — never Optional.

1List<Integer> list = stream.collect(Collectors.toList());
2Map<Boolean, List<Integer>> groups = stream
3    .collect(Collectors.partitioningBy(x -> x > 5));
4String joined = stream.collect(Collectors.joining(", "));

reduce():

• Uses a BinaryOperator to combine values.
• Creates a new value each step (immutable).
• For sum, max, min, product, custom aggregates.
• Returns Optional when no identity is provided.

1Optional<Integer> sum = stream.reduce(Integer::sum);
2Optional<Integer> max = stream.reduce(Integer::max);
3int product = stream.reduce(1, (a, b) -> a * b);

Key differences:

• collect() — mutable, flexible, uses Collector.
• reduce() — immutable, simpler, uses BinaryOperator.
• collect() always returns a value; reduce() may return Optional.
• collect() is better for building complex results; reduce() for simple aggregates.