public SystemStream streamIdToSystemStream(String streamId) {
    return new SystemStream(getSystem(streamId), getPhysicalName(streamId));
  }
A streamId is translated to a SystemStream by looking up its System and physicalName. It
will use the streamId as the stream name if the physicalName doesn't exist.
samza-core/src/main/java/org/apache/samza/config/StreamConfig.java