Streams: peek is for observing, not for changing state

peek runs only for elements the pipeline actually pulls, and a stream with no terminal operation never runs at all - which is why it is a debugging tool, not a processing step.

Code

var log = new ArrayList<String>();

Stream.of("a", "b", "c").peek(log::add).filter(s -> s.equals("a")).findFirst();
System.out.println("short-circuited, peek saw: " + log);

log.clear();
Stream.of("a", "b", "c").peek(log::add);
System.out.println("no terminal operation, peek saw: " + log);

Output

short-circuited, peek saw: [a]
no terminal operation, peek saw: []

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Published 2026-07-30