[SPARK-25362][JavaAPI] Replace Spark Optional class with Java Optional

[mmolimar]

What changes were proposed in this pull request?

Previous Spark versions didn't require Java 8 and an Optional Spark Java API had to be implemented to support optional values.

Since Spark 2.4 uses Java 8, the Optional Spark Java API should be removed so that Spark uses the original Java API.

How was this patch tested?

OptionalSuite class was removed to test Spark Java API Optional class (this class as well).
Notice that the get method in the Spark Java API Optional class throws a NullPointerException when the value is not set whereas the native Java API java.util.Optional throws a NoSuchElementException.

[vanzin]

We can't do this until we decide whether the next version will be a major version or not. This breaks backwards compatibility. (See the bug I duplicated your bug to.)

[srowen]

Yeah this is the right kind of change but needs to wait for now. Can you update the title?

[mmolimar]

Done @srowen

[srowen]

@mmolimar we can move forward with this now. Can you put SPARK-25395 in the title instead? that was the original JIRA.

[srowen]

@mmolimar Could you revert these whitespace changes?

[mmolimar]

Updated @srowen
The PR title already contains SPARK-25395, is that what you're expecting or another PR?

[srowen]

Oops, I mean SPARK-25362. SPARK-25395 was a duplicate.

[mmolimar]

No problem. Done ;-)

[SparkQA]

Test build #4368 has finished for PR 22383 at commit 054c79a.

• This patch fails MiMa tests.
• This patch merges cleanly.
• This patch adds no public classes.

[srowen]

Yeah, you'll have to add this to the 3.0 excludes section of project/MimaExcludes:

ProblemFilters.exclude[MissingClassProblem]("org.apache.spark.api.java.Optional")

[SparkQA]

Test build #4376 has finished for PR 22383 at commit 6eaa3f1.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #4379 has finished for PR 22383 at commit f188243.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[srowen]

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!

[mmolimar]

I agree @srowen.
What do you think about reusing the current implementation we already have, for example, in the guava lib instead of having that class in Spark?

[vanzin]

Spark's class only reason to exist is so we do not use the Guava class, since everyone and their pet want to use a different version of Guava (and Spark shades it for that reason).

[srowen]

Yeah, I think we just can't do this unfortunately. It was worth looking into.