import java.util.Optional;
import java.util.function.Function;

public class Main {
  public static void main(String[] args) {
    Optional.of("aaa")
        .map(s -> new F<>(s)) // (1) no problem
        .map(t ->             // (2) <U> is Object. this is incorrect. JDK resolve <U> is Integer.
            t.map(f -> 200))  // (3) <Z> is Integer. no problem.
        .map(n -> n + 300)    // (4) n is Object. but should be Integer.
        .ifPresent(System.out::println);
  }

  static class F<T> {
    T value;

    F(T value) {
      this.value = value;
    }

    public <Z> Z map(Function<? super T, ? extends Z> fn) {
      return fn.apply(this.value);
    }
  }
}