@Override
    public Object deserializeKey(String sspString, DeserializationContext ctxt) throws IOException {
      int idx = sspString.indexOf('.');
      int lastIdx = sspString.lastIndexOf('.');
      if (idx < 0 || lastIdx < 0) {
        throw new IllegalArgumentException("System stream partition expected in format 'system.stream.partition");
      }
      return new SystemStreamPartition(
          new SystemStream(sspString.substring(0, idx), sspString.substring(idx + 1, lastIdx)),
          new Partition(Integer.parseInt(sspString.substring(lastIdx + 1))));
    }
Builds {@link ObjectNode} which matches the {@link JobModel} built in setup.
samza-core/src/test/java/org/apache/samza/serializers/model/TestSamzaObjectMapper.java