Sorting a list with parent/child object relations

Question

Hi,
I have an ArrayList of Genre objects that I want to sort:
class Genre{
int CategoryID;
int ParentCategoryID;
String name;
String level;
Example objects:
Pop (CategoryID=10, ParentCategoryID=root, "popular music", 1);
Jazz (CategoryID=11, ParentCategoryID=root, "jazz music", 1);
Indie pop (CategoryID=12, ParentCategoryID=10, "indie pop", 2);
American Indie pop (CategoryID=13, ParentCategoryID=12, "american indie pop", 3);
Soul (CategoryID=15, ParentCategoryID=root, "soul music", 1);
Commercial pop (CategoryID=14, ParentCategoryID=10, "commercial pop", 2);
I hope that you see the point:
I want that the order will be like this in my arraylist:
jazz music
popular music
commercial pop
indie pop music
american indie pop
soul music
I'll implement the comparator-class in Genre and then add the compareTo-method in my genre-class.
But my brain is too tired to figure out what to write in the compareTo-method?
Have somebody done a similar method??
Best regards,
Thomas

Answer 1

Unfortunately is this solution not plausible since I
don't can have the parent genre in the genre class.
... Hashmap?Hashmap works. Here you go, using your structure, minus the level, I'm sure you can work around that though. Don't use -1 as a category id, it's reserved for your root. This code is not thread safe, I leave that to you. Don't forget that as implemented, parent Genres must be constructed prior to any of their children. You could get around this with a two pass initialization process.
// Not thread safe.
import java.util.*;
public class Genre implements Comparable {
     private static Map<Integer, Genre> genreForId = new HashMap<Integer, Genre>();
     private int id;
     private int level;
     private int parentId;
     private String name;
     public Genre(int id, int parentId, String name) {
          this.id = id;
          this.parentId = parentId;
          this.name = name;
          this.level = level(this);
          Genre.genreForId.put(id, this);
     private static int level(Genre genre) {
          int i = 0;
          while (genre != null) {
               genre = genre.getParent();
               i++;
          return i;
     public static void main(String[] args) {
          Genre pop = new Genre(10, -1, "popular music");
          Genre jazz = new Genre(11, -1, "jazz music");
          Genre indiePop = new Genre(12, 10, "indie pop");
          Genre americanIndiePop = new Genre(13, 12, "american indie pop");
          Genre soul = new Genre(15, -1, "soul music");
          Genre commercialPop = new Genre(14, 10, "commercial pop");
          Genre[] genres = { pop, jazz, indiePop, americanIndiePop, soul,
                    commercialPop };
          Arrays.sort(genres);
          System.out.println(Arrays.toString(genres));
     public int compareTo(Object o) {
          Genre lhs = this;
          Genre rhs = (Genre) o;
          switch (Integer.signum(lhs.getLevel() - rhs.getLevel())) {
          case -1:
               rhs = rhs.getParent();
               if (lhs == rhs)
                    return -1;
               break;
          case 0:
               if (lhs == rhs)
                    return 0;
               else {
                    if (lhs.getParent() == rhs.getParent())
                         return lhs.getName().compareTo(rhs.getName());
                    else {
                         lhs = lhs.getParent();
                         rhs = rhs.getParent();
               break;
          case 1:
               lhs = lhs.getParent();
               if (lhs == rhs)
                    return 1;
          return lhs.compareTo(rhs);
     public int getLevel() {
          return level;
     public String getName() {
          return name;
     public Genre getParent() {
          return Genre.genreForId.get(getParentId());
     public int getParentId() {
          return parentId;
     public String toString() {
          return name;
}-S