title: Setting Custom Coders & Handling Parameterized types
body: <p>I have two questions related to coder issues I am facing with my Dataflow pipeline.</p>
  
  <ul>
  <li>How do I go about setting a coder for my custom data types? The class consists of just three items - two doubles and another parameterized property. I tried annotating the type with SerializableCoder but I still end up with the error "com.google.cloud.dataflow.sdk.coders.CannotProvideCoderException: Cannot provide coder based on value with class interface java.util.Set: No CoderFactory has been registered for the class." The Set actually contains the parameterized custom data-type - so I am assuming that the custom datatype is the problem. I could not find enough documentation/examples on the right way to do this. Please point me to the right place if its available.</li>
  <li>Even without the custom datatype, whenever I try switching to a parameterized version of Transform functions, it results in coder errors. Specifically, inside a complex transform which is parameterized, a ParDo works with parameterized types but when I apply a Combine.PerKey on the resulting PCollection after the ParDo, it results in the CoderNotFoundException.</li>
  </ul>
  
  <p>Any help regarding these two items would be helpful as I am kind of stuck on this for sometime now.</p>