They forked Java by cherry picking features and APIs, and are making it increasingly difficult to write portable code across standard Java and Android Java.

Any Java library that depends on Java 7 or 8 standard libraries not available on Android, newly introduced JVM bytecodes, some of the Java 8 language features, annotation processors, ... isn't usable on Android.

Good luck getting Java 9 modules, or eventually Java 10 value types and updated generics when they become available.