An example of Java 15 sealed classes, pattern matching instanceof and records

Option.java

import java.util.function.Function;
import java.util.function.Supplier;

public class Main {
  public static void main(String args[]) {
    var some = new Some<>("hello");

    var result = some.map(String::toUpperCase);

    System.out.println(result.orElse(""));
  }
}

sealed interface Option<T> permits Some, None {

  default <R> Option<R> map(Function<T, R> mapper) {
    return fold(None::new, mapper.andThen(Some::new));
  }

  default T orElse(T value) {
    return fold(() -> value, Function.identity());
  }

  default <R> R fold(Supplier<R> noneCase, Function<T, R> someCase) {
    if (this instanceof Some<T> some) {
      return someCase.apply(some.value());
    } else {
      return noneCase.get();
    }
  }
}

record Some<T>(T value) implements Option<T> {}

record None<T>() implements Option<T> {}