List of Lists

Task: to order input data according 2 integer keys.
Example: we have bunch of options. Options have indexes and legs. Legs have indexes and other useful information.
Question: how to extract one type of information from legs?
Answer:
...
private final List<List<FinDate>> expiryDateByOptionAndLeg;
expiryDateByOptionAndLeg = new ArrayList<List<FinDate>>(size){
...
@Override
public List<FinDate> get(int index) { 
 List<FinDate> rez = null;
  rez = super.get(index);
  if(rez == null) {
   rez = new ArrayList<FinDate>(2);
   add(index, rez);
  }
  return rez;
}
};
...
  expiryDateByOptionAndLeg.get(optionsIndex).set(legIndex, expiryDate);



Answer: To use approach suggested in ThreadLocal class javadocs. In this way we receive clear 2-indeger index without need to create list in business code. Similar approach could be applied to Maps.
It's simpler than creating  separate business object and still readable and expressive.