Trying to Sort in Java 8 with Nulls in the Data

Question

I am trying to sort a list that I get by the comparator given. I am passing the comparator through the NullsLast method to get a Null-Friendly version of my comparator like the JavaDoc states. However, when I run the code I am still getting a Null Pointer Exception when I pass a String comparator to the method.

private <T> void sortWithDirection(List<T> details, boolean isAscending, Comparator<T> comparator){

    if(isAscending){
        details.sort(Comparator.nullsLast(comparator));
    } else {
        details.sort(Comparator.nullsLast(comparator.reversed()));
    }
}

List<UserDTO> users = new ArrayList<>();
sortWithDirection(users, false, Comparator.comparing(UserDTO::getName));

Am I using this method wrong? Is there a different way that I can prevent NPEs without removing them from the result set?

Answer 1

nullsLast will handle the case of UserDTO being null. It will not save you from getName returning null. To handle null names, you would need to apply nullsLast before comparing:

details.sort(comparing(
    UserDTO::getName, 
    nullsLast(naturalOrder())
));

or, for reverse order:

details.sort(comparing(
    UserDTO::getName, 
    nullsLast(reverseOrder())
));