// "Create method 'nextFn'" "true-preview"
import java.util.function.Function;

class X {
  void x() {
    Function<Double, Character> fn1 = d -> String.valueOf(d)
      .charAt(0);
    Function<Double, Integer> fn = fn1.andThen(nextFn());
  }

    private Function<? super Character, Integer> nextFn() {
        return null;
    }
}