2.x: Java 9 compatibility fixes (March 3)

[akarnokd]

Java 9 has changed its overload resolution algorithm and things that resolved unambiguously in Java 8 no longer resolve:

Flowable<? extends T> source = ...

source.subscribe(new FlowableSubscriber<T>() { ... });

With the code above, javac 8, Eclipse and IntelliJ 2017 EAP picks Flowable.subscribe(FlowableSubscriber<? super T> s) as expected. However, javac 9 finds it ambiguous with Flowable.subscribe(Subscriber<? super T> s) despite IntelliJ 2017 EAP not indicating any error and still jumping to the right method via CTRL+Click.

The problem may come from the ? extends T part of the declaration. Having just Flowable<T> compiles properly with javac9.

Luckily, we don't need ? extends T and this PR changes the internal signatures of the affected components.

[codecov]

Codecov Report

Merging #5153 into 2.x will increase coverage by 0.13%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5153      +/-   ##
============================================
+ Coverage     95.85%   95.99%   +0.13%     
- Complexity     5603     5608       +5     
============================================
  Files           620      620              
  Lines         39856    39856              
  Branches       5604     5604              
============================================
+ Hits          38205    38258      +53     
+ Misses          660      633      -27     
+ Partials        991      965      -26

Impacted Files	Coverage Δ	Complexity Δ
.../internal/operators/flowable/FlowableRefCount.java	98.5% <ø> (ø)	7 <0> (ø)	❌
...vex/internal/operators/flowable/FlowableCache.java	94% <ø> (ø)	7 <0> (ø)	❌
...operators/flowable/BlockingFlowableMostRecent.java	100% <100%> (ø)	2 <1> (ø)	❌
...l/operators/flowable/BlockingFlowableIterable.java	98.57% <100%> (ø)	2 <1> (ø)	❌
...a/io/reactivex/internal/util/QueueDrainHelper.java	60.28% <0%> (-4.26%)	32% <0%> (-3%)
.../internal/operators/maybe/MaybeTakeUntilMaybe.java	95.91% <0%> (-4.09%)	2% <0%> (ø)
...internal/operators/completable/CompletableAmb.java	94.44% <0%> (-3.71%)	10% <0%> (-1%)
...al/operators/observable/ObservableSampleTimed.java	88.33% <0%> (-3.34%)	3% <0%> (ø)
...activex/internal/operators/single/SingleCache.java	97.05% <0%> (-2.95%)	23% <0%> (-1%)
...x/internal/operators/maybe/MaybeSwitchIfEmpty.java	97.22% <0%> (-2.78%)	2% <0%> (ø)
... and 40 more

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[JakeWharton]

Hopefully Travis adds early JDK 9 builds soon.

[akarnokd]

If you need only JDK 9, that's possible with some experimental setup. The bigger problem is that Gradle 3.x doesn't work on Jigsaw enabled Jdk 9 builds such as 154+.