Question

I am looking for a way to find out in run-time, whether a collection is ordered or not. Any way to do this?

EDIT: I am sorry for wrongly asked question. I meant whether there is some general way to say HashMap does not store order of elements being inserted, where LinkedHashMap does.

Answer 1

For elements that implement the Comparable interface, you can check to see if they are in their “natural” order.

public static <T extends Comparable<? super T>> boolean isOrdered(Iterable<T> list) {
  Iterator<T> i = list.iterator();
  if (i.hasNext()) {
    T previous = i.next();
    while (i.hasNext()) {
      T current = i.next();
      if (previous.compareTo(current) > 0)
        return false;
      previous = current;
    }
  }
  return true;
}

Otherwise, you’ll have to define a Comparator that can compare your objects according to your definition of order, and pass that to the test.

public static <T> boolean isOrdered(Iterable<T> list, Comparator<? super T> c) {
  Iterator<T> i = list.iterator();
  if (i.hasNext()) {
    T previous = i.next();
    while (i.hasNext()) {
      T current = i.next();
      if (c.compare(previous, current) > 0)
        return false;
      previous = current;
    }
  }
  return true;
}