Get the system identifier that was set with setSystemId.

  Get the system identifier that was set with setSystemId.
Returns: The system identifier that was set with setSystemId, or null if setSystemId was not called.

  Set the system identifier for this Source.

  Set the system identifier for this Source.

The system identifier is optional if the source does not get its data from a URL, but it may still be useful to provide one. The application can use a system identifier, for example, to resolve relative URIs and to include in error messages and warnings.

Parameters:
- systemId - The system identifier as a URL string.