How to get index of an item in java.util.Set

Question

I know the differences between Set and List(unique vs. duplications allowed, not ordered/ordered, etc). What I'm looking for is a set that keeps the elements ordered(that's easy), but I also need to be able to recover the index in which an element was inserted. So if I insert four elements, then I want to be able to know the order in which one of them was inserted.

MySet<String> set = MySet<String>();
set.add("one");
set.add("two");
set.add("three");
set.add("four");

int index = set.getIndex("two");

So at any given moment I can check if a String was already added, and get the index of the string in the set. Is there anything like this, or I need to implement it myself?

question from:https://stackoverflow.com/questions/4395871/how-to-get-index-of-an-item-in-java-util-set

Answer 1

A small static custom method in a Util class would help:

 public static int getIndex(Set<? extends Object> set, Object value) {
   int result = 0;
   for (Object entry:set) {
     if (entry.equals(value)) return result;
     result++;
   }
   return -1;
 }

If you need/want one class that is a Set and offers a getIndex() method, I strongly suggest to implement a new Set and use the decorator pattern:

 public class IndexAwareSet<T> implements Set {
   private Set<T> set;
   public IndexAwareSet(Set<T> set) {
     this.set = set;
   }

   // ... implement all methods from Set and delegate to the internal Set

   public int getIndex(T entry) {
     int result = 0;
     for (T entry:set) {
       if (entry.equals(value)) return result;
       result++;
     }
     return -1;
   }
 }