.collect(Collectors.toList()) and Streams on Java Method

Question

I have a collection (as hashmap) of Doctors, into a generic Hospital class.

Map<Integer, Doctor> doctors = new HashMap<Integer, Doctor>();

For each doctor I have some information such as in the class code (focus on the patients):

public class Doctor extends Person {
    private int id;
    private String specialization;
    private List<Person> patients = new LinkedList<Person>();

My purpose is to write this function which return busy doctors: doctors that has a number of patients larger than the average.

/**
 * returns the collection of doctors that has a number of patients larger than the average.
 */
Collection<Doctor> busyDoctors(){

    Collection<Doctor> doctorsWithManyPatients = 
            doctors.values().stream()
            .map( doctor -> doctor.getPatients() )
            .filter( patientsList -> { return patientsList.size() >= AvgPatientsPerDoctor; })
            .collect(Collectors.toList());

    return null;
}

I want to use the streams as above to perform this operation. The problem is in collect method because at that point of usage doctorsWithManyPatients is of type List<Collection<Person>> and not Collection<Doctor>. How could I do that?

Assume that AvgPatientsPerDoctor is already defined somewhere.

Answer 1

You needn't use map (Doctor -> List<Person>), it will be used in the filter:

doctors
    .values()
    .stream()
    .filter( d -> d.getPatients().size() >= AvgPatientsPerDoctor)
    .collect(Collectors.toList());

For your case, map( doctor -> doctor.getPatients() ) returns Stream<List<Person>> and you should convert it to Stream<Doctor> again after filtering and before calling the collect method.

---

There is a different way that isn't the best one. Keep in mind that it changes the origin collection.

doctors.values().removeIf(d -> d.getPatients().size() < AvgPatientsPerDoctor);