java 8: inferred type does not conform to upper bound(s) on Netbean IDE

Question

I copied this code below from book to prepare for Java Certificate exam. When compile on Netbeans IDE(jdk 1.8.0_144), I got the compiler error " inferred type does not conform to upper bound(s) on Netbean IDE"

public static void main(String[] args) {
        Stream<String> ohMy = Stream.of("lions", "tigers", "bears");
        Map<Integer, Optional<Character>> map = ohMy.collect(
                Collectors.groupingBy(
                        String::length,
                        Collectors.mapping(s -> s.charAt(0),
                        Collectors.minBy(Comparator.naturalOrder()))));

        System.out.println(map); 
}

It's still compile on Eclipse, please explain what is wrong here? I uploaded error detail here:

Answer 1

You may need to help the compiler here as it's a type inference issue:

You can solve the problem by explicitly specifying the type for the Comparator or any of the Collectors.

• Collectors.<Character>minBy()
• Comparator.<Character>naturalOrder()
• Collectors.mapping((String s) -> s.charAt(0)