How to apply in Function<T, R> argument from List (not List)

Question

I have a method which returns company as key and list of employeer as values <T> Map<String, List<T>> getUserPerCompany(final Function<User, T> converter). The method accepts the converter parameter which in the tests returns the String (name + lastname of the employee). It should returns: Map<String, List<String>>. I created this implementation:

return getUserStream().collect(toMap(Company::getName, c -> converter.apply(c.getUsers())));

Error is: apply (domain.User) in Function cannot be applied to (java.util.List<domain.User>)

My problem is that I do not know how to pass the employee to the 'apply' list instead of the list in full.

My other attempts:

• return getUserStream().collect(toMap(Company::getName, c -> converter.apply((User) c.getUsers().listIterator())));
• return getUserStream().collect(toMap(Company::getName, c -> converter.apply((User) c.getUsers().subList(0, c.getUsers().size()))));
• return getUserStream().collect(toMap(Company::getName, c -> converter.apply((User) c.getUsers().iterator())));

Answer 1

I suppose this is what you're looking for

<T> Map<String, List<T>> getUserPerCompany(final Function<User, T> converter) {
    return getUserStream().collect(Collectors.toMap(
            c -> c.getName(),
            c -> c.getUsers()
                  .stream()
                  .map(converter)
                  .collect(Collectors.toList())
    ));
}

Usage example is

final Map<String, List<String>> users = getUserPerCompany(user -> user.getName() + " " + user.getSurname());

Basically you need to map each User, applying the input Function.