Problems With Creating Group of Objects from a Collection

Question

Hi, please kindly help.
I have a Collection. Its name is "activities". It is a collection of the Activity object with "color" as one of the attributes. I am writing a method and takes "activities" as a parameter (see below). The method is supposed to group "activities" by "color" and return a Collection called "activitiesGroups". And I have created an "activitiesGroup" (note: singular not plural) class with the getters and setters of all its attributes.
I first sorted the "activities" by color. So far so good. Thereafter, I am iterating through this sorted list. If activity.getColor().trim() is tested different, I know that a new group, i.e., activitiesGroup, should be created and I should code activitiesGroups.add( activitiesGroup );Problems are:
1. If I am at the first record of the sorted collection while iterating, I do not what to add a new group to the returned List activitiesGroups. I want to greate a new group only.
Because "colorString" is initialized as a blank, when I code activitiesGroups.add( activitiesGroup );, I depend on that I am not at the first record of the sorted collection by testing if colorString.length(); is not a zero. I do not think it is the correct way to do it.
What is the proper way of knowing I am at the first record of the sorted collection while iterating?
2. I have to sum up two of the attributes of the ActivitiesGroup object: increaseToValue and decreaseToValue. While iterating, I repeat for each record within a group:
aGroup.setIncreaseToValue( increase );
aGroup.setDecreaseToValue( decrease ); How do I avoid the repetition?
     private Comparator colorComparator;
     private List activitiesGroups;
     private ActivitiesGroup aGroup;
     private String colorString = "";
     private String nextColorString = "";
     private BigDecimal increase;
     private BigDecimal decrease;               
     public List groupingByColor( List activities ) {
          if ( activities == null) {
               return new ArrayList(0);
          if ( !activities.isEmpty() ) {
               Collections.sort( activities, colorComparator);  // It works.
               activitiesGroups = new ArrayList();
               Iterator it = activities.iterator();
               while ( it.hasNext() ) {
                    Activity activity = ( Activity )it.next();
                    nextColorString = activity.getColor().trim();                    
                    if ( !nextColorString.equalsIgnoreCase( colorString.trim() ) ) {
                         int length = colorString.length();
                         if ( length != 0 ) { activitiesGroups.add( aGroup ); } // Problem # 1
                         aGroup = new ActivitiesGroup();
                                                          aGroup.setColor( activity.getColor() );
                         increase = increase.add( activity.getIncreaseToValue() );
                         decrease = decrease.add( activity.getDecreaseToValue() );
                         aGroup.setIncreaseToValue( increase ); // Problem #2: repeated for every record in a group
                         aGroup.setDecreaseToValue( decrease ); // Problem #2: repeated for every record in a group
                    } else {
                        increase = increase.add( activity.getIncreaseToValue() );
                        decrease = decrease.add( activity.getDecreaseToValue() );
                        aGroup.setIncreaseToValue( increase ); // Problem #2: repeated for every record in a group
                        aGroup.setDecreaseToValue( decrease );      // Problem #2: repeated for every record in a group                         
                                                                colorString = nextColorString;
          return activitiesGroups;
     }

Answer 1

I first sorted the "activities" by color. So far so good. Thereafter, I am iterating through this sorted list. If activity.getColor().trim() is So in fact you are not dealing with an arbitrary Collection (as you stated in the first place), you are dealing with a List.
You should know that there is a way to do this just with an iterator on an arbitrary collection - without needing to sort your list, and do it in O(n) time.