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• No user-facing ClassManifests: Java API methods should never contain ClassManifest in their signatures, including type bounds.
• Methods added to JavaRDD should also be added to Java(Double|Pair)RDD: All of JavaRDD's methods obey the "same-result-type" principle (if they didn't, they should have been implemented in JavaRDDLike), so they should also be implemented in JavaDoubleRDD and JavaPairRDD.
• Handling default parameters: Java methods don't support optional parameters with default values. For Scala methods with optional arguments, like def textFile(path: String, minSplits: Int = defaultMinSplits), you can either define two separate methods (1- and 2-parameter versions) or make the optional argument into a required one.
• Unit tests: for non-trivial additions that might be prone to compile-time or run-time problems if untested, please add new unit tests to JavaAPISuite.

References and Resources

• Twitter's Scala School "Java + Scala" guide provides a nice overview of Java interoperability issues and explores some of the details of how Scala constructs are exposed in Java.