java8 - 映射列表的列表 : check if nested list is null

Question

考虑这个对象:

final List<ApplicationUser> output = Arrays.asList(

    new ApplicationUser() {

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


    }
);

public interface Role {
    String getId();
}

我想返回 Role::id 列表。

如果getAssociationRoles不为空，下面的代码片段将起作用

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

如果getAssociationRoles为空，则抛出NullPointerException。

如何防止这种情况发生？

Answer 1

只需添加一个过滤器即可过滤掉空关联角色:

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