How to log filtered values in Java Streams

Question

I have a requirement to log/sysout the filtered values in Java Streams. I am able to log/sysout the non-filtered value using peek() method

Answer 1

As there's no way to run terminal actions on elements matching opposite filters on the same stream, the best option may be just to use a condition in a peek's consumer.

That avoids traversing the stream/collection twice.

List persons = Arrays.asList(new Person("John"), new Person("Paul"));

//A consumer that only logs "John" persons
Consumer logger = p -> {
    if ("John".equals(p.getName())) //condition in consumer
        System.out.println("> " + p.getName());
};

Then we can pass that consumer to peek, only filtering for the ultimate action after that:

List nonJohns = persons.stream()
                  .peek(logger)
                  .filter(p -> !"John".equals(p.getName()))
                  .collect(Collectors.toList());