A_few_java_FP_snippets_for_demo.java

import io.vavr.control.Either;
import io.vavr.control.Try;

import java.time.Clock;
import java.time.ZonedDateTime;
import java.util.List;
import java.util.Optional;
import java.util.function.Function;
class Snippets {
    public static Either<String, Integer> toInt(String text) {
        return Try.of(() -> Integer.parseInt(text)) //Try<Integer>
                .toEither() //Either<Throwable, Integer>
                .mapLeft(t -> "Expected an integer for days but found: " + text); //Either<Throwable, String>
    }

    public static Either<String, ZonedDateTime> addDaysToNow(int days, ZonedDateTime now) {
        return Try.of(() -> now.plusDays(days))
                .toEither()
                .mapLeft(t -> "Given " + days + " exceed the date range");
    }

    static class UserRequest {
        final String days;
        final ZonedDateTime now;

        UserRequest(String days, ZonedDateTime now) {
            this.days = days;
            this.now = now;
        }
    }

    static class ValidatedUserInput {
        final int days;
        final ZonedDateTime now;


        ValidatedUserInput(int days, ZonedDateTime now) {
            this.days = days;
            this.now = now;
        }
    }


    public static Either<String, ZonedDateTime> serveUser(UserRequest userRequest) {
        return toInt(userRequest.days) //Either<String, Integer>
                .map(
                    days -> new ValidatedUserInput(days, userRequest.now)
                ) //Either<String, ValidatedUserInput>
                .flatMap(
                    validatedInput -> addDaysToNow(validatedInput.days, validatedInput.now)
                ); //Either<String, ZonedDateTime>
    }

    public static Function<String, Function<Clock, Either<String, ZonedDateTime>>> serveUserFunction() {
        return days -> //String =>
                    clock -> // Clock =>
                            serveUser(
                                new UserRequest(days, ZonedDateTime.now(clock))
                            ); //Either<String, ZonedDateTime>
    }

    public static void main(String[] args) {
        Clock clock = Clock.systemUTC();
        String days = args[0];
        serveUserFunction().apply(days).apply(clock);
    }
}

handle null values in pojos

public class GetterLens {

   public static <T, G> Optional<G> safeGetter(T obj, Function<T, G> extractor) {
       return Optional.ofNullable(extractor.apply(obj));
   }

   public static <T, G> Optional<G> safeGetter(Optional<T> opt, Function<T, G> extractor) {
       return opt.flatMap(obj -> safeGetter(obj, extractor));
   }

   public static <A, B, C, O> Optional<O> safeGetter(
           Optional<A> opt,
           Function<A, B> fa,
           Function<B, C> fb,
           Function<C, O> fc) {
       Optional<B> b = safeGetter(opt, fa);
       Optional<C> c = safeGetter(b, fb);
       Optional<O> o = safeGetter(c, fc);
       return o;
   }

   public static <A, B, C, O> Optional<O> safeGetter(
           A a,
           Function<A, B> fa,
           Function<B, C> fb,
           Function<C, O> fc) {
       Optional<B> b = safeGetter(a, fa);
       Optional<C> c = safeGetter(b, fb);
       Optional<O> o = safeGetter(c, fc);
       return o;
   }

}

Example

Optional<O> o = safeGetter(a, A::getB, B::getC, C::getO)