Google’s Android operating system currently supports Java as its first-class language, and sources say Swift is not meant to replace Java, at least initially. While the ongoing litigation with Oracle is likely cause for concern, sources say Google considers Swift to have a broader “upside” than Java.
If Google were to support Swift for Android, it would need to create a runtime for the language. Additionally, its whole library would need to be made Swift ready. Some low level Android APIs are C++ which Swift can't bridge to. They would have to be rewritten. Also, some higher level APIs in Java would need to be rewritten.
A change to support Swift for Android isn't impossible. Apple did it for iOS. It's said that Google is also considering Kotlin as a language but "Google’s current mindset is that Kotlin is a bit too slow when compiling."
Facebook and Uber's interest in Swift is said to do with the language's ability to function client and server side.
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