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

peek() — intermediate operation for debugging, returns same stream. forEach() — terminal operation for side-effects, consumes the stream. Think of peek() as a camera recording passersby without stopping them, while forEach() as a checkpoint that processes everyone who passes.

peek():

• Intermediate operation.
• Returns same Stream — doesn't consume.
• For debugging/logging.
• Must be followed by a terminal operation.

1List<String> result = list.stream()
2    .peek(s -> System.out.println("Processing: " + s))
3    .filter(s -> s.length() > 3)
4    .collect(Collectors.toList());

forEach():

• Terminal operation.
• Returns void — consumes the stream.
• For side-effects.
• Cannot be followed by more operations.

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

Key difference:

• peek() — intermediate, non-consuming, for debugging.
• forEach() — terminal, consuming, for side-effects.