I am sorry. I solved it in the following way. 

 IAttributeHandle attributeHandle = workItemServer.findAttribute(iProjectArea, attributeName, monitor);

  IAttribute artifactAttribute = (IAttribute) itemService.fetchItem( attributeHandle, IRepositoryItemService.COMPLETE );

  Object artifactObj = workItem.getValue(artifactAttribute);
 
  ArrayList list = (ArrayList<String>) artifactObj;
  
  for( int i=0;i < list.size() ; i++ ){
 
   IEnumeration enumeration = workItemCommon.resolveEnumeration( artifactAttribute, monitor );

   ILiteral literal = enumeration.findEnumerationLiteral( (Identifier<ILiteral>) list.get( i ) );

   targetName = literal.getName();

  }