Is there any examples for java emitter to begin with? Moreover, I find DatahubGraph in python to fetch metadata. I wonder is there class with similar function in java?

We have a couple starter examples here and here depdending on if you prefer the MetadataChangeProposalWrapper interface or the OpenAPI interface. Looks like the DatahubGraph class is just a thin wrapper around the RESTEmitter, we don't have a class like that on the Java side, but all of the utility methods that are useful in there are straightforward to construct with the Java side request objects.

Thanks, I will try it out!