Java 8: The Missing Parts, Chapter 1: Optionals.java

Optionals.java

import java.util.Arrays;
import java.util.List;
import java.util.Optional;
import java.util.Set;
import java.util.stream.Stream;

import static java.util.stream.Collectors.toList;
import static java.util.stream.Collectors.toSet;

public class Optionals {
    private Optionals() {
    }

    public static void main(String[] args) {
        Optional<String> opt1 = Optional.of("1");
        Optional<String> opt2 = Optional.of("2");
        Optional<String> opt3 = Optional.empty();

        // Transform an optional into something else via a stream:
        Set<String> res1 = stream(opt1).collect(toSet());

        // Flatten a collection of optionals:
        List<String> res2 = Stream.of(opt1, opt2, opt3)
                .flatMap(Optionals::stream)
                .collect(toList());

        // Combining multiple optionals, keeping the first present value:
        Optional<String> res3 = or(opt1, opt2, opt3);
    }

    /*
     * This will be included in Java 9, but until then...
     * (https://bugs.openjdk.java.net/browse/JDK-8050820)
     */
    public static <T> Stream<T> stream(Optional<T> self) {
        return self.map(Stream::of).orElseGet(Stream::empty);
    }

    public static <T> Optional<T> or(Optional<T>... xs) {
        return Arrays.stream(xs).flatMap(Optionals::stream).findFirst();
    }
}