import java.util.stream.Collectors;
import java.util.stream.Stream;

public class StreamPeekDemo
{
    public static void main(String[] args)
    {

        /*
         * peek(): This method exists mainly to support debugging,
         * where you want to see the elements as they flow past a
         * certain point in a pipeline
         */
        Stream.of("one", "two", "three", "four")
                .filter(e -> e.length() > 3)
                .peek(e -> System.out.println("Filtered value: " + e))
                .map(String::toUpperCase)
                .peek(e -> System.out.println("Mapped value: " + e))
                .collect(Collectors.toList());

    }
}

Filtered value: three
Mapped value: THREE
Filtered value: four
Mapped value: FOUR