Sorting parallel ArrayList

Question

I have a model class that contains 3 ArrayList which are in order by parallel of the same size. <Object><Calendar><Long> I want to sort it by the <Long> Is this the most clean? is there a better way? This doesn't seem memory efficient.

 public class ResultModel{
 private ArrayList<Object> sets = new ArrayList<Object>();
 private ArrayList<Calendar> dates=new ArrayList<Calendar>();
 private ArrayList<Long> unixtimes=new ArrayList<Long>();
 private class WinningSet implements Comparable<WinningSet>{
 private long unixtime;
 private Object set;
 private Calendar date;
 WinningSet(Object set,Calendar date,long unixtime){
 this.unixtime=unixtime;
 this.set=set;
 this.date=date;
 }
 @Override
 public int compareTo(WinningSet another) {
 return (int) (this.unixtime-another.getUnixtime());
 }
 public long getUnixtime() {
 return unixtime;
 }
 public Object getSet() {
 return set;
 }
 public Calendar getDate() {
 return date;
 }
 }
 public void sortSelf(){
 ArrayList<WinningSet> list=new ArrayList<WinningSet>();
 for(int i=0;i<this.unixtimes.size();i++){
 list.add(new WinningSet(this.sets.get(i),dates.get(i),unixtimes.get(i)));
 }
 Collections.sort(list,Collections.reverseOrder());
 demap(list);
 }
 private void demap(ArrayList<WinningSet> list) {
 ArrayList<Object> tSets = new ArrayList<Object>();
 ArrayList<Calendar> tDates=new ArrayList<Calendar>();
 ArrayList<Long> tUnixtimes=new ArrayList<Long>();
 for(WinningSet temp:list){
 tDates.add(temp.getDate());
 tSets.add(temp.getSet());
 tUnixtimes.add(temp.getUnixtime());
 }
 this.sets=tSets;
 this.dates=tDates;
 this.unixtimes=tUnixtimes;
 }
}

Answer 1

1. Correctnes.

compareTo() method of the WinningSet class is not correct. As of now Integer.MAX_VALUE < System.currentTimeMillis(). Consider the following example:

 long time1 = System.currentTimeMillis();
 System.out.println(time1 > 3*Integer.MAX_VALUE);
 System.out.println(time1 - 3*Integer.MAX_VALUE);
 System.out.println((int)(time1 - 3*Integer.MAX_VALUE));

It could be the case that your events are generated around the same time and it will not result in any error but in general - e.g. in future, if you restore some serialized objects, it could lead to a possible problem. You can rewrite it int the following way:

 @Override
 public int compareTo(WinningSet another) {
 if (this.unixtime > another.getUnixtime()) return 1;
 if (this.unixtime < another.getUnixTime()) return -1;
 return 0;
 }

2. Performance

Copy of the 3 lists is indeed a bad thing. You can use set() method from ListIterator to traverse through the lists and replace values in place.

private void demap(ArrayList<WinningSet> list) { 
 Iterator li = list.iterator();
 Iterator si = sets.iterator();
 Iterator di = dates.iterator();
 Iterator ui = unixtimes.iterator();
 //you can add iterators for other lists here. 
 while (li.hasNext()) {// we assume number is the same
 WinningSet temp = li.next();
 si.next(); //need to advance to the right element; 
 di.next(); //need to advance to the right element; 
 ci.next(); //need to advance to the right element; 
 si.set(temp.getSet());
 di.set(temp.getDate());
 ui.set(temp.getUnixTime());
 }
 }

3. Design

Are you sure that instead of a 3 lists you can't consider TreeMap<Long, SomeClass> where SomeClass is pair or Calendar and Object ? In this case you will always have the right order of items to work with - there is no need to sort lists on demand and do an extra work.

• wtsang02: I agree with point 2 and 3, but I am not understanding 1. If the two unixtime is very close then it should give a small Integer (less than 10 secs) . So how does Integer.MAX_VALUE have anything to do with this. Thanks.
• Andrey Taptunov: @wtsang02, if you have 100% guarantee that they are close and will always be this close - it's not a problem. However, if by some accident you decide to compare two WinningSets by Unix time (month or so apart, if I recall correctly) you could have a problem. For example, if you decide in future to accumulate them somewhere for a long time then this way of comparison can result in a wrong behaviour. I just always prefer a usual way to write comparison just not to think about such cases.