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Is there a path to upgrading Java on Android? Surely new Android releases could do easily?

No, because Dalvik (https://en.wikipedia.org/wiki/Dalvik_(software)) / ART (https://en.wikipedia.org/wiki/Android_Runtime) are forks of Java. New features need to be re-implemented due to licensing issues. And Google seems to be focussed more on Kotlin than on newer Java features.