code: private String buildCacheKey(final String[] objectIds, final String namespace) {
          if (objectIds.length == 1) {
              checkKeyPart(objectIds[0]);
              return namespace + SEPARATOR + objectIds[0];
          }
  
          StringBuilder cacheKey = new StringBuilder(namespace);
          cacheKey.append(SEPARATOR);
          for (String id : objectIds) {
              checkKeyPart(id);
              cacheKey.append(id);
              cacheKey.append(ID_SEPARATOR);
          }
  
          cacheKey.deleteCharAt(cacheKey.length() - 1);
          return cacheKey.toString();
      }
docs: Build cache key.
  
  @param objectIds
  @param namespace
  @return