code: public List<String> listInstanceIDs(final Set<String> instanceIDs) {
          Set<String> allInstanceIDs = allMockEc2Instances.keySet();
          if (null == instanceIDs || instanceIDs.size() == 0) {
              return new ArrayList<String>(allInstanceIDs);
          } else {
              List<String> filteredInstanceIDs = new ArrayList<String>();
              for (String id : allInstanceIDs) {
                  if (null != id && instanceIDs.contains(id)) {
                      filteredInstanceIDs.add(id);
                  }
              }
              return filteredInstanceIDs;
          }
      }
docs: List mock ec2 instance IDs in current aws-mock.
  
  @param instanceIDs
  a filter of specified instance IDs for the target instance to describe
  @return a collection of IDs with specified instance IDs, or all of the mock ec2 instanceIDs if not filtered