lambda - Java stream "forEach" but not consuming stream

Question

Sometimes it would be handy do "something" (e.g. print) with every element in a stream in between steps of processing the stream, e.g. for debugging.

A simple example could look like this, unfortunately this does not work as forEach consumes the stream:

List<String> list = new ArrayList<>();
list.add("one");
list.add("two");
list.add("three");
list.add("four");

List<String> filteredList = 
        list.stream()
        .filter(s -> s.startsWith("t"))
        .forEach(System.out::println)
        .collect(Collectors.toList());

How can this be achieved?

Answer 1

You are looking for the peek operation:

This method exists mainly to support debugging, where you want to see the elements as they flow past a certain point in a pipeline

This method will execute the given action on all elements of the Stream pipeline as they are consumed. As such, it allows to take a peek of the elements.

List<String> filteredList = 
    list.stream()
        .filter(s -> s.startsWith("t"))
        .peek(System.out::println)
        .collect(Collectors.toList());