Github user srowen commented on the issue:
https://github.com/apache/spark/pull/22383
Oh, hm:
```
Serialization stack:
- object not serializable (class: java.util.Optional, value:
Optional[x])
- field (class: scala.Tuple2, name: _2, type: class java.lang.Object)
- object (class scala.Tuple2, (1,Optional[x]))
- field (class: scala.Tuple2, name: _2, type: class java.lang.Object)
- object (class scala.Tuple2, (1,(1,Optional[x])))
- element of array (index: 0)
- array (class [Lscala.Tuple2;, size 5)
```
So `java.util.Optional` isn't `Serializable`. Well, that may scuttle this
whole idea. I think we're going to find a number of instances where Spark or
user apps need to `collect()` `Optional` objects.
Unless someone has a bright idea I think we can't do this. Same reason I
was unable to change Spark to use `java.util.function` interfaces -- lambdas
aren't otherwise `Serializable` in Java!
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