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

you can extend LinkedHashSet adding your desired getIndex() method. It's 15 minutes to implement and test it. Just go through the set using iterator and counter, check the object for equality. If found, return the counter.

After creating Set just convert it to List and get by index from List:

Set<String> stringsSet = new HashSet<>();
stringsSet.add("string1");
stringsSet.add("string2");

List<String> stringsList = new ArrayList<>(stringsSet);
stringsList.get(0); // "string1";
stringsList.get(1); // "string2";

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

 public static <T> int getIndex(Set<T> set, T value) {
   int result = 0;
   for (T 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;
   }
 }

One solution (though not very pretty) is to use Apache common List/Set mutation

import org.apache.commons.collections.list.SetUniqueList;

final List<Long> vertexes=SetUniqueList.setUniqueList(new LinkedList<>());

it is a list without duplicates

https://commons.apache.org/proper/commons-collections/javadocs/api-3.2.2/index.html?org/apache/commons/collections/list/SetUniqueList.html