Implementing Monads in Java 8

FunctorsMonads.java

import java.util.function.Function;

public class FunctorsMonads {

  public static void main(String[] args) {
    System.out.println(tryParse("47"));
    System.out.println(tryParse("a"));

    FOptional<String> str = FOptional.of("47");
    System.out.println(str);
    FOptional<FOptional<Integer>> num1 = str.map(FunctorsMonads::tryParse);
    System.out.println(num1);
    FOptional<Integer> num2 = str.flatMap(FunctorsMonads::tryParse);
    System.out.println(num2);
  }

  static FOptional<Integer> tryParse(String s){
    try {
      final int i = Integer.parseInt(s);
      return FOptional.of(i);
    } catch (NumberFormatException e) {
      return FOptional.empty();
    }
  }
}

interface Functor<T, F extends Functor<?, ?>> {
  <R> F map(Function<T, R> f);
}

interface Monad<T, M extends Monad<?, ?>> extends Functor<T, M> {
  M flatMap(Function<T, M> f); // From original article
  //<R> M flatMap(Function<T, FOptional<R>> f); // Compiles but polutes the interface
  //<R> M<R> flatMap(Function<T, M<R>> f);
  /* You cannot implement a fully type-safe Monad interface in Java. 
  The correct signature for flatmap would be something like <R> M<R> flatMap(Function<T, M<R>> f), 
  but this is not expressible in Java. This M<R> expression is called a higher-kinded type
  (http://dev.stephendiehl.com/fun/001_basics.html#higher-kinded-types).*/
}

//class FOptional<T> implements Functor<T, FOptional<?>>
class FOptional<T> implements Monad<T, FOptional<?>> {
  private final T valueOrNull;

  private FOptional(T valueOrNull) {
    this.valueOrNull = valueOrNull;
  }

  public <R> FOptional<R> map(Function<T, R> f) {
    if (valueOrNull == null)
      return empty();
    else
      return of(f.apply(valueOrNull));
  }

  //public FOptional<?> flatMap(Function<T, FOptional<?>> f) { // wrong asnwer from Stackoverflow. It doesn't return FOptional<Integer>
  public <R> FOptional<R> flatMap(Function<T, FOptional<R>> f) {
    if (valueOrNull == null)
      return empty();
    else
      return f.apply(valueOrNull);
  }

  public static <T> FOptional<T> of(T a) {
    return new FOptional<T>(a);
  }

  public static <T> FOptional<T> empty() {
    return new FOptional<T>(null);
  }

  @Override
  public String toString() {
    return getClass().getName() + "<" + valueOrNull + ">";
  }
}