code: public static IChemObject getMappedChemObject(IReaction reaction, IChemObject chemObject) {
          for (IMapping mapping : reaction.mappings()) {
              if (mapping.getChemObject(0).equals(chemObject)) {
                  return mapping.getChemObject(1);
              } else if (mapping.getChemObject(1).equals(chemObject)) return mapping.getChemObject(0);
          }
          return null;
      }
docs: get the IAtom which is mapped
  
  @param reaction   The IReaction which contains the mapping
  @param chemObject The IChemObject which will be searched its mapped IChemObject
  @return           The mapped IChemObject