Any chance you could consider changing the signature of the aggregate
helper method to a single vararg argument, or adding an overloaded
signature that takes List<DBObject>? I assume the reason that the
first op was modelled as a separate argument was to make it clear to
users that at least one operation was required. I actually think
that's adequately clear as just a vararg argument, and having the
first argument separate makes it more complex when you already have
the pipeline as a list or array.

That is, from:

public AggregationOutput aggregate( DBObject firstOp, DBObject ... additionalOps)

to:

public AggregationOutput aggregate( DBObject ... ops)

or add:

public AggregationOutput aggregate( List<DBObject> ops)

The particular case I'm looking at is the run method in the class
below. I'm currently using a workaround that splits the first op and
the remainder before passing them to the helper method.

https://bitbucket.org/davidmc24/mongo-java-aggregate/src/6e0e93bfe2a4/src/main/java/org/bitbucket/davidm24/mongodb/aggregate/AggregateBuilder.java