FamilyTree 3/3

import java.util.*;
import java.util.stream.Collectors;

public class FamilyTree {

    private final List<Person> persons = new ArrayList<>();
    private final Map<String, List<String>> parentsAndChildren = new LinkedHashMap<>();

    public void addPerson(String name, String birthDate){
        persons.add(new Person(name, birthDate));
    }

    public void addIDs(String parentID, String childID){
        parentsAndChildren.putIfAbsent(parentID, new ArrayList<>());
        parentsAndChildren.get(parentID).add(childID);
    }

    public void buildRelations(){
        parentsAndChildren.forEach((parentID, children) -> {
            Person parent = getPerson(parentID);
            children.stream().map(this::getPerson).forEach(parent::addChild);
        });
    }

    public Person getPerson(String personID){
        return persons.stream().filter(person -> person.getName().equals(personID) || person.getBirthDate().equals(personID)).findFirst().orElseThrow();
    }

    public String getFamilyTreeFor(Person person){
        return new StringBuilder().append(person.toString()).append(System.lineSeparator())
                .append("Parents:").append(System.lineSeparator())
                .append(person.getParents().stream().map(Person::toString).collect(Collectors.joining(System.lineSeparator())))
                .append(person.getParents().isEmpty() ? "" : System.lineSeparator())
                .append("Children:").append(System.lineSeparator())
                .append(person.getChildren().stream().map(Person::toString).collect(Collectors.joining(System.lineSeparator())))
                .toString();
    }
}