Main.java

import java.util.function.Function;

class Program {
  public sealed interface Option<T> permits Some, None { } 
  public record Some<T>(T value) implements Option<T> { }
  public record None<T>() implements Option<T> { }

  public static <T, U> Option<U> map(Option<T> o, Function<T, U> f) {
    return o instanceof Some<T> s 
      ? new Some<>(f.apply(s.value()))      
      : new None<>();
  }

  public static void main(String[] args) {
    // prints Some[value=hi!]
    System.out.println(map(new Some<>("hi"), s -> s + "!"));                          
  }
}