AddPersonPresenterImpl.java

public class AddPersonPresenterImpl implements IAddPersonPresenter{

    List<Person> mPersons;
    private IAddPersonView mView;

    public AddPersonPresenterImpl(IAddPersonView view) {
        this.mView = view;
        this.mPersons = new ArrayList<>();
        updateNumberOfAddedPersons();
    }

    @Override
    public void onAddPersonButtonClicked(String nameOfPerson) {
        mPersons.add(new Person(nameOfPerson));
        updateNumberOfAddedPersons();
    }

    private void updateNumberOfAddedPersons() {
        mView.showNumberOfPersons(mPersons.size());
    }

}