Fix Promise#allOf(Promise<?>...)

[natemort]

AllOfFuture completes with the list of values from each promise passed into its constructor. The varargs variant of Promise#allOf however has a return type of Void, so the intention seems to be to discard the results of the promises. Because a list of results is disjoint from Void any invocation of this method will throw an exception when the promises complete as we attempt to cast ArrayList to Void.

To resolve this we explicitly discard the result of the promises by mapping the result to null, the only possible value of Void.

What changed?
Change the behavior of Promise#allOf(Promise<?>...) to no longer throw an exception.

Why?
Allows for using Promise#allOf without wrapping the arguments in a collection first.

How did you test it?
Unit tests.

Potential risks
None

Release notes

Documentation Changes

[coveralls]

Pull Request Test Coverage Report for Build 2258

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.01%) to 60.297%

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Files with Coverage Reduction	New Missed Lines	%
src/main/java/com/uber/cadence/internal/sync/WorkflowThreadContext.java	1	82.46%

Totals
Change from base Build 2251:	0.01%
Covered Lines:	11457
Relevant Lines:	19001

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💛 - Coveralls