code: private Promise<PrivateKey> fetchPreKey(long keyId) {
          try {
              return Promise.success(ManagedList.of(ownKeys.getPreKeys())
                      .filter(PrivateKey.PRE_KEY_EQUALS_ID(keyId))
                      .first());
          } catch (Exception e) {
              Log.d(TAG, "Unable to find own pre key #" + keyId);
              throw e;
          }
      }
docs: Fetching own pre key by id
  
  @param keyId pre key id