code: @Override
      public boolean exists(String key) {
          switch (keyMode) {
          case MONOPOLISTIC:
          case NAMESPACE:
              final String KEY = keyMode == KeyMode.MONOPOLISTIC ? key : calcCacheKeyNamespace(key);
              try {
                  return getMemcachedClient().get(KEY) != null;
              } catch (Exception e) {
                  throw e instanceof CacheException ? (CacheException) e : new CacheException(e);
              }
          case XNAMESPACE:
              try {
                  return getMemcachedClient().withNamespace(getName(),
                          new MemcachedClientCallable<Boolean>() {
                              @Override
                              public Boolean call(MemcachedClient client) throws InterruptedException,
                                      MemcachedException, TimeoutException {
                                  return client.get(key) != null;
                              }
  
                          });
              } catch (Exception e) {
                  throw e instanceof CacheException ? (CacheException) e : new CacheException(e);
              }
          default:
              throw new IllegalStateException("Invalid key mode: " + keyMode);
          }
      }
docs: {@inheritDoc}