Applicative Functor in Guava

ApplicativeFunctor.java

import com.google.common.base.Function;
import com.google.common.base.Optional;
import org.junit.Test;

import javax.annotation.Nullable;

import static org.fest.assertions.Assertions.assertThat;

public class OptionalTest {

    @Test
    public void should_() {
        Optional<String> string = getSomething(true);

        Optional<Optional<String>> result = Optional.of(string).transform(ID_FUNCTION).transform(FAIL_FUNCTION).transform(addString(" world!"));

        assertThat(flatten(result).or("Fail")).isEqualTo("Fail");
    }

    private static <T> Optional<T> flatten(Optional<Optional<T>> optional) {
        if (optional.isPresent()) {
            return optional.get();
        }
        return Optional.absent();
    }

    private Optional<String> getSomething(boolean b) {
        return b ? Optional.of("Hello") : Optional.<String>absent();
    }

    public static final Function<Optional<String>,Optional<String>> ID_FUNCTION = new Function<Optional<String>, Optional<String>>() {
        @Override
        public Optional<String> apply(@Nullable Optional<String> input) {
            return input;
        }
    };

    public static final Function<Optional<String>,Optional<String>> FAIL_FUNCTION = new Function<Optional<String>, Optional<String>>() {
        @Override
        public Optional<String> apply(@Nullable Optional<String> input) {
            return Optional.absent();
        }
    };

    private Function<Optional<String>, Optional<String>> addString(final String value) {
        return new Function<Optional<String>, Optional<String>>() {
            @Override
            public Optional<String> apply(@Nullable Optional<String> input) {
                if (!input.isPresent()) {
                    return input;
                } else {
                    return Optional.of(input.get() + value);
                }
            }
        };
    }

}