Aha, can you help me with a hint? I didn't realize that an `RDD[Double]` isn't magically an `RDD<Double>` in Java. The Java signature of the `predict()` method already ends up at `RDD<Object>`, and Java casting doesn't help. I don't believe I want to change the method. (Which incidentally just does the same operation I 'inlined' into this example, which works. Hm.)