private MockId getUniqueId(short accountId, short containerId) {
    MockId id;
    do {
      id = new MockId(TestUtils.getRandomString(MOCK_ID_STRING_LENGTH), accountId, containerId);
    } while (generatedKeys.contains(id));
    generatedKeys.add(id);
    return id;
  }
Get a unique MockId with specified accountId and containerId.
@param accountId the accountId of the MockId.
@param containerId the containerId of the MockId.
@return a {@link MockId} that is unique and has not been generated before in this run.
ambry-store/src/test/java/com/github/ambry/store/BlobStoreTest.java