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

forEach() — executes an action on each element (side-effect). collect() — accumulates elements into a result container. Think of forEach() as a security guard checking each person as they pass, while collect() as a bouncer gathering people into a room.

forEach():

• Terminal operation.
• Executes a Consumer on each element.
• Returns void.
• For side-effects: printing, logging, sending notifications.

1list.stream()
2    .filter(x -> x > 10)
3    .forEach(System.out::println);

collect():

• Terminal operation.
• Accumulates into a container (List, Set, Map, String).
• Returns the result.
• For building collections.

1List<Integer> result = list.stream()
2    .filter(x -> x > 10)
3    .collect(Collectors.toList());

Performance considerations:

• forEach() is slightly faster for pure side-effects (no result container).
• collect() is optimized for building collections internally.
• forEach() does not preserve encounter order in parallel streams.
• collect() preserves encounter order by default.

When to use:

• forEach(): side-effects, no result needed (printing, logging, notifications).
• collect(): building a result collection for later use.

1// forEach: side-effects
2orders.stream()
3    .filter(o -> o.isOverdue())
4    .forEach(notificationService::sendReminder);
5
6// collect: build result
7List<Order> overdueOrders = orders.stream()
8    .filter(Order::isOverdue)
9    .collect(Collectors.toList());

Common mistake:

1// WRONG: using forEach to build a list
2List<String> result = new ArrayList<>();
3list.stream().filter(s -> s.length() > 3).forEach(result::add);
4// This works but is not idiomatic and may have issues in parallel streams
5
6// CORRECT: use collect
7List<String> result = list.stream()
8    .filter(s -> s.length() > 3)
9    .collect(Collectors.toList());