java8 - map a list of a list: check if nested list is null

Question

Consider this object:

final List<ApplicationUser> output = Arrays.asList(

    new ApplicationUser() {

        @Override
        public List<Role> getAssociationRoles() {
            //return null;
            return Arrays.listOf(...);              
        }


    }
);

public interface Role {
    String getId();
}

I want to return a List of Role::id.

The snippet below works if getAssociationRoles is not null

final List<String> rolesId = applicationUsers
            .stream()
            .map(ApplicationUser::getAssociationRoles)
            .flatMap(List::stream)
            .map(Role::getId)
            .collect(Collectors.toList());

Hovever a NullPointerException is thrown if getAssociationRoles is null.

How can I prevent this ?

Just fix getAssociationRoles() to never return null. Use Collections.emptyList() as default value. — Holger
– Holger, Commented Sep 6, 2019 at 7:43
@Holger The first thing that came to my mind after reading getAssociationRoles(). That should be the actual answer to the question. — Naman
– Naman, Commented Sep 6, 2019 at 9:13
The rule that collections should not be null is a universal one. Violating it is bad coding style. You may send a bug report to whoever is responsible for the method. Besides that, you have an answer that shows how to deal with such bad methods. — Holger
– Holger, Commented Sep 6, 2019 at 9:32

Sweeper · Accepted Answer · 2019-09-06 07:29:00Z

1

Just add a filter to filter out the null association roles:

final List<String> rolesId = applicationUsers
    .stream()
    .map(ApplicationUser::getAssociationRoles)
    .filter(Objects::nonNull) <------- here!
    .flatMap(List::stream)
    .map(Role::getId)
    .collect(Collectors.toList());

answered Sep 6, 2019 at 7:29

Sweeper

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1 Comment

Naman Over a year ago

A better suggestion.

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