Convert flat table to tree

package tr2;

import java.util.ArrayList;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class Alta {

    static Map<Long, Linie> m = new ConcurrentHashMap<Long, Linie>();

    public static void main(String[] args) {
        Response response = new Response();
        populateMap();
        // afiseazaMap();
        Long actual = getFirstAncestor(m);
        response.setId(actual);
        response.setName(m.get(actual).getName());
        response.setChildren(takeTree(actual, m));
        System.out.println(response);

        Gson gs = new GsonBuilder().setPrettyPrinting().create();
        String jso = gs.toJson(response);
        System.out.println(jso);
    }

    private static ArrayList<Response> takeTree(Long actual,
            Map<Long, Linie> map) {
        System.out.println("[takeTree] actual = " + actual);
        ArrayList<Response> al = new ArrayList<Response>();
        for (Long key : map.keySet()) {
            Linie l = map.get(key);
            if (l != null && l.getParentId() != null && l.getParentId().equals(actual)) {
                Response r = new Response();
                r.setId(l.getId());
                r.setName(l.getName());
                r.setChildren(takeTree(l.getId(), map));
                al.add(r);
            }
        }
        return al;
    }

    private static Long getFirstAncestor(Map<Long, Linie> m) {
        for (Long i : m.keySet()) {
            return m.get(i).getUltimateId();
        }
        return null;
    }

    public static void afiseazaMap(Map<Long, Linie> m) {
        for (Long key : m.keySet()) {
            Linie l = m.get(key);
            System.out.println(l.getId() + " --- " + l.getParentId() + " --- "
                    + l.getUltimateId() + " --- " + l.getName());
        }
    }

    public static void populateMap() {
        add(1, 3, 5);
        add(2, 3, 5);
        add(3, 5, 5);
        add(4, 5, 5);
        m.put((long) 5, new Linie(new Long(5), null, new Long(5)));
    }

    public static void add(long i, long j, long k) {
        m.put((long) i, new Linie(i, j, k));
    }
}

class Linie {

    Long id;
    Long parentId;
    Long ultimateId;
    String name = "unNume";

    public Linie(long id, long parentId, long ultimateId) {
        this.id = id;
        this.parentId = parentId;
        this.ultimateId = ultimateId;
        this.name = this.name + id;
    }

    public Linie(Long id, Long parentId, Long ultimateId) {
        this.id = id;
        this.parentId = parentId;
        this.ultimateId = ultimateId;
        this.name = this.name + id;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public Long getParentId() {
        return parentId;
    }

    public void setParentId(Long parentId) {
        this.parentId = parentId;
    }

    public Long getUltimateId() {
        return ultimateId;
    }

    public void setUltimateId(Long ultimateId) {
        this.ultimateId = ultimateId;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

---------------------


package tr2;

import java.io.Serializable;
import java.util.ArrayList;

public class Response implements Serializable {
    /**
     *
     */
    private static final long serialVersionUID = 1L;

    Long id;
    String name;
    ArrayList<Response> children;

    @Override
    public String toString(){
        return "id = " + id + "; name = " + name + " children = \n " + children;
    }

    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public ArrayList<Response> getChildren() {
        return children;
    }
    public void setChildren(ArrayList<Response> children) {
        this.children = children;//new ArrayList(children);
    }

}