In reference to deprecation of j.u.Observer/Observable, Doug Lea says,

"These days, anyone encountering these is probably hitting them
by mistake while using RxJava or other reactive-stream frameworks.
In which case, users will normally want to instead use the jdk9
java.util.concurrent.Flow APIs that all reactive-streams frameworks
should be compatible/interoperable with in their planned upcoming
jdk9-compatible versions."

http://mail.openjdk.java.net/pipermail/core-libs-dev/2016-April/040473.html

A note should be added to j.u.Observer/Observable mentioning this and pointing to the Flow APIs.