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 lt String gt  set   MySet lt String gt     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

User · Answer

After creating Set just convert it to List and get by index from List   Set lt String gt  stringsSet   new HashSet lt  gt     stringsSet add  string1    stringsSet add  string2     List lt String gt  stringsList   new ArrayList lt  gt  stringsSet   stringsList get 0       string1   stringsList get 1       string2

User · Answer

How about add the strings to a hashtable where the value is an index     Hashtable lt String  Integer gt  itemIndex   new Hashtable lt  gt       itemIndex put  First String  1     itemIndex put  Second String  2     itemIndex put  Third String  3      int indexOfThirdString   itemIndex get  Third String

User · Answer

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

User · Answer

A small static custom method in a Util class would help    public static int getIndex Set lt   extends Object gt  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 lt T gt  implements Set      private Set lt T gt  set     public IndexAwareSet Set lt T gt  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

User · Answer

One solution  though not very pretty  is to use Apache common List Set mutation  import org apache commons collections list SetUniqueList   final List lt Long gt  vertexes SetUniqueList setUniqueList new LinkedList lt  gt        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

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

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