Github user leventov commented on the issue:
https://github.com/apache/zookeeper/pull/490
Yes, Java 8 is required for the build, but the produced bytecode could
target Java 6 or 7. Here is a citation from
http://errorprone.info/docs/installation:
> Our goal is to make it simple to add Error Prone checks to your existing
Java compilation. Please note that Error Prone must be run on JDK 8 or newer.
It can be used to build Java 6 or 7 code by setting the appropriate -source /
-target / -bootclasspath flags.
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