Re: inverse sort on key?

Daniele Futtorovic <>
Tue, 05 Aug 2008 22:49:45 +0200
On 05/08/2008 20:22, allegedly wrote:

I have an object (say, Car). Car contains some values, one of which
is speed. I want to store lots of cars in some sort of object that
will keep them sorted by speed. Some cars may have the same speed.
The catch is that I want to sort by highest speed, not lowest speed.
If I insert lots of cars into a TreeSet, with the key being the speed,
I get them in increasing order. If there are n cars with the same
speed, I get only one, because the keys are identical. I've been
unable to find an object that would work. I've tried implementing a
compare method in my object, and that doesn't help either. Any ideas?

To summarize, the container object must:
1) Support sorting in inverse order (or another order of my choice)
2) Keep the objects sorted.

Specify a proper Comparator. A TreeSet should do just fine. See example
code below.

3) Support duplicate objects.

I quite certain that's not a problem of order (Comparable/Comparator),
but rather one of _equality_. Do you override equals() in that class?
Does the overridden method specify that two Cars are equal if their
speeds are equal? If yes, there's your problem. If not, I think it's
somewhere in the vicinity.

<code what_about_the_javadoc="shrug">
public class InverseComparator<E>
implements Comparator<E>
     private Comparator<E>

     public InverseComparator(){

     public InverseComparator(Comparator<E> normalorder){
         backingComparator = normalorder;

     public int compare(E o1, E o2) {
         if( backingComparator != null ){
             return -, o2);
             if( ! (o1 instanceof Comparable) ){
                 throw new IllegalArgumentException("No normal order
Comparator specified -- Objects must implement Comparable");

             return - ((Comparable)o1).compareTo(o2);

     public static void main(String[] ss){
         List<Car> cars = Arrays.asList( new Car[]{ new Car(2),new
Car(3),new Car(5),new Car(7),new Car(11) } );

         List<Car> cars1 = new ArrayList<Car>(cars);
         List<Car> cars2 = new ArrayList<Car>(cars);
         List<Car> cars3 = new ArrayList<Car>(cars);
         List<Car> cars4 = new ArrayList<Car>(cars);

         Collections.sort(cars2, new InverseComparator<Car>());
         Collections.sort(cars3, new NormalOrderCarComparator());
         Collections.sort(cars4, new InverseComparator<Car>(new


     private static class Car
     implements Comparable<Car>
         int speed;

         public Car(int speed){
             this.speed = speed;

         public int compareTo(Car o) {
             return speed - o.speed;

         public String toString(){
             return "Car[speed=" + speed + "]";

     private static class NormalOrderCarComparator
     implements Comparator<Car>
         public int compare(Car o1, Car o2) {
             return o1.speed - o2.speed;



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