[SAMZA-1253] MessageStream.merge operator broken for nested types

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 0.13.0
• Assignee: Prateek Maheshwari
• Reporter: Prateek Maheshwari
• Created: 02/May/17 18:21
• Updated: 04/May/17 20:22
• Resolved: 04/May/17 20:22

Description

From cpettitt-linkedin:

I picked up the latest code and found that MessageStream.merge is now broken for nested types. For example, the following:
    final MessageStream<BeamMessage<InT>> inputStream = ...;
    final Collection<MessageStream<BeamMessage<InT>>> sideInputStreams = ...;
    final MessageStream<BeamMessage<OutT>> outputStream =
        inputStream
            .merge(sideInputStreams);
Now fails to compile:
incompatible types: java.util.Collection<org.apache.samza.operators.MessageStream<com.linkedin.beam.runners.samza.runtime.BeamMessage<InT>>> cannot be converted to java.util.Collection<org.apache.samza.operators.MessageStream<? extends com.linkedin.beam.runners.samza.runtime.BeamMessage<InT>>>
This forces the user to go to raw types, which is not intuitive and certainly not type safe.
Per http://www.angelikalanger.com/GenericsFAQ/FAQSections/TypeArguments.html#FAQ104, I believe we want the parameter type to be
Collection<? extends MessageStream<? extends M>>
, which does compile.