https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/concurrent/ConcurrentHashMap.KeySetView.html
ConcurrentHashMap.KeySetView (Java SE 11 & JDK 11 )
java seconcurrenthashmapjdk
https://developer.android.com/reference/java/util/concurrent/ConcurrentHashMap.KeySetView
ConcurrentHashMap.KeySetView | API reference | Android Developers
api referenceconcurrenthashmapandroiddevelopers