java unchecked cast

Assuming that you're using this comparator to sort a TreeMap, then this isn't going to work. TreeMap comparators are for comparing only the map keys, not the key->value entries. If your comparator needs access to the values, then it will have to look them up in the map itself, e.g.

final Map<Integer, Double> map = ....

public class ScoreComp implements Comparator<Integer>  {
   public int compare(Integer key1, Integer key2) {
    Double x = map.getValue();
    Double y = map.getValue();
    if (x < y)
        return -1;
    else if (x == y)
        return 0;
    else
        return 1; 
   }
}

edit: From your comments, I think your best option is to create a class that encapsulates the ID and the value, put those values into a List, and sort that.

public class Item implements Comparable<Item> {
   int id;
   double value;

   public int compareTo(Item other) {
      return this.value - other.value;
   }
}

and then

List<Item> list = new ArrayList<Item>();
// ... add items here
Collections.sort(list);

Since Item is itself Comparable, you don't need an external Comparator (unless you want one).

What's about rewriting as

public class ScoreComp implements Comparator<Map.Entry<Integer, Double>> {

    public int compare(Map.Entry<Integer, Double> o1, Map.Entry<Integer, Double> o2) {
        if ( o1.getValue()  < o2.getValue()  ) return -1;
        else if ( o1.getValue() == o2.getValue()  ) return 0;
        return 1;
    }
}