code: public void setKeyAndKeyGroup(@Nonnull K key, @Nonnegative int keyGroupId) {
  		try {
  			serializeKeyGroupAndKey(key, keyGroupId);
  		} catch (IOException shouldNeverHappen) {
  			throw new FlinkRuntimeException(shouldNeverHappen);
  		}
  	}
docs: Sets the key and key-group as prefix. This will serialize them into the buffer and the will be used to create
  composite keys with provided namespaces.
  
  @param key        the key.
  @param keyGroupId the key-group id for the key.