Lazy optional stream

StreamOfOptional.java

package optional;

import java.util.Optional;
import java.util.function.Supplier;
import java.util.stream.Stream;

public class StreamOfOptional {

    public static void main(String[] args) {
        System.out.println("--Before eager--");
        process(eager());
        System.out.println("--After eager--");

        System.out.println("--Before lazy--");
        process(lazy());
        System.out.println("--After lazy--");

    };

    private static void process(Stream<Optional<String>> stream) {
        stream
            .filter(Optional::isPresent).map(Optional::get)  // if present, unpack
            .peek(s -> System.out.println("processed: " + s)) // print, what is currently in the pipeline
            .findFirst()
            .ifPresent(System.out::println);
    }

    private static Stream<Optional<String>> lazy() {
        // https://stackoverflow.com/a/28833677/3824515
        return Stream.<Supplier<Optional<String>>>of(
            () -> firstOpt(),
            () -> secondOpt()
        )
        .map(Supplier::get);
    }

    private static Stream<Optional<String>> eager() {
        return Stream.of(
            firstOpt(),
            secondOpt()
        );
    }

    private static Optional<String> firstOpt() {
        return Optional.ofNullable("first")
            .map(s-> {
                System.out.println("Cheap mapping on '"+s+"' done.");
                return s;
            });
    }

    private static Optional<String> secondOpt() {
        return Optional.ofNullable("second")
            .map(s-> {
                System.out.println("Really expensive mapping on '"+s+"' done (e.g. DB access)!");
                return s;
            });
    }
}