Java: How to use Comparator with arraylist as values of HashMap

Question

I have this HashMap:

 HashMap< itemDetails, Pair<ArrayList<itemDetails>, AssociatedInfo>> AssociatedItemCasuaList = new HashMap<>();

where its key is class, and its values consist of pair (a= arraylist of class itemDetails , b= class AssociatedInfo ):

class itemDetails {
    public ArrayList<Integer> itemId; 
    public float expectedSupport = 0;
    // etc
}

and

  class Pair<T, U> {

        T a;
        U b;

        Pair(T a, U b) {
            this.a = a;
            this.b = b;
        }

        T getA() {
            return a;
        }

        U getB() {
            return b;
        }
    }

and

class AssociatedInfo { 
    public int noOfKCasual = 0;
    public int AssociateListStart = 0;
    public int AssociateListEnd = 0;
}

I want to sort the first pair of the the values of HashMap which is the ArrayList<Integer> itemId in the class itemDetails

I used this Comparator

 public class ItemComparator implements Comparator<ArrayList<Integer> >{
     @Override
    public int compare(final ArrayList<Integer> entry1, final ArrayList<Integer> entry2){
        if (entry1 == null && entry2 == null)
            return 0;
        if (entry1 == null)
            return 1;
        if (entry2 == null)
            return -1;
        if (entry1.isEmpty() && entry2.isEmpty())
            return 0;
        if (entry1.isEmpty())
            return 1;
        if (entry2.isEmpty())
            return -1;
        return entry1.get(0).compareTo(entry2.get(0));
    }
  }

I don't know how to write Collections.sort

if (AssociatedItemCasuaList.containsKey(LHS)) {
    AssociatedItemCasuaList.get(LHS).a.add(RHS2);
    AssociatedItemCasuaList.get(LHS).b.AssociateListStart = 0;
    AssociatedItemCasuaList.get(LHS).b.AssociateListEnd += 1;
    AssociatedItemCasuaList.get(LHS).b.noOfKCasual += 1;
} else {
    ArrayList<itemDetails> ArrayListRHS = new ArrayList<itemDetails>();
    ArrayListRHS.add(RHS2);
    AssociatedInfo AttribAssociatedInfo1 = new AssociatedInfo();
    AttribAssociatedInfo1.noOfKCasual = 1;
    AttribAssociatedInfo1.AssociateListStart = 0;
    AttribAssociatedInfo1.AssociateListEnd = 0;
    AssociatedItemCasuaList.put(LHS, new Pair(ArrayListRHS, AttribAssociatedInfo1));
}
//      Collections.sort(AssociatedItemCasuaList.get(LHS), new ItemComparator());

Update: Example:

AssociatedItemCasuaList < **key**=LHS, **value**=Pair<a, b> >

Let key=LHS:

LHS.itemId=1
LHS.expectedSupport=87.5

and values = Pair < a, b >

Let focus here in a only in this example.

a= ArrayList<itemDetails> 

Let itemDetails RH2

and every time (in loop) I add RHS2 to key LHS as:

AssociatedItemCasuaList.get(LHS).a.add(RHS2)

here RHS2 takes different values every time

  RHS2.itemId
  RHS2.expectedSupport

until now, I don't have problem.

I want to sort the ArrayList in a (that I filled with RHS2 ) based on its itemId

Answer 1

Ignoring the fact I didn't understand a single word, what about this:

public class ItemComparator2 implements Comparator<itemDetails> {
  Comparator myComp = new ItemComparator();
  @Override
  public int compare(itemDetails a, itemDetails b){
    return myComp.compare(a.itemId, b.itemId);
  }
}

// ...

Collections.sort(AssociatedItemCasuaList.get(LHS).getA(), new ItemComparator2());