TreeSet and Comparator problem

Question

Hi all ,
I am using jdk1.3 , jbuilder4.0 , Apache Tomcat 1.3.14 and Window 98 and Access Database.
I have comparator code below which give me some problem
public int compare(Object x, Object y) {
CompanyJB a = (CompanyJB)x , b = (CompanyJB)y;
double aMinratio = a.getMinratio();
double bMinratio = b.getMinratio();
int aCompid = a.getCompanyid();
int bCompid = b.getCompanyid();
String aRankstr = a.getRank();
String bRankstr = b.getRank();
int tmp;
tmp = new Integer(aCompid).compareTo(new Integer(bCompid));
if (tmp == 0) {
return tmp;
tmp = a.getRank().compareTo(b.getRank());
if (tmp !=0) {
return tmp;
tmp = new Double(a.getMinratio()).compareTo(new Double(b.getMinratio()));
if (tmp !=0) {
return tmp;
return new Double(a.getMinratio()).compareTo(new Double(b.getMinratio()));
Client source code below :
TreeSet list2 = new TreeSet(new test());
//list2.add(new test(2,1.9,4.2,"NA")); // 1
list2.add(new test(11,8.5,8.5,"2")); // 3
list2.add(new test(1,0.6,0.6,"NA")); // 2
list2.add(new test(11,2.5,2.3,"NA")); // 4
list2.add(new test(12,0.3,0.3,"1")); // 5
list2.add(new test(2,1.9,1.2,"NA"));
list2.add(new test(12,0.9,0.9,"NA"));
Iterator iterator2 = list2.iterator();
while(iterator2.hasNext()){
System.out.println(((test)iterator2.next()).getCompanyid());
My problem is that the print out display duplicate company id "12" in the set.
It should not display the duplicate company id in the set.
What is the problem here ?? Anybody have ideas ??

Answer 1

Actually, a Set, as defined in java.util.Set, should not contain duplicate members. With SortedSets, however, there must also be consistency between equals() and the compareTo() (or Comparator.compare()). In other words, equal objects must sort to the same or an adjacent place in order for the Set semantics to work.
In your example, "equal" objects can appear in completely different positions in the sort order, throwing off this consistency. When adding, it's traversing the tree in Rank,MinRatio order. If by chance it runs across a node that has the same company id, it stops and leaves the trees alone (as it does with id=11). If you try graphing the tree out by hand using your compare() as left/right criteria (negative left, positive right, 0 don't change), it'll become immediately clear what's happening.
Also of interest: you'll find that you can't add test(34,1.9,1.2,"NA") to the list: because it's Rank and MinOrder is the same as the id=2 entry, it'll never get added.