Java HashMap.keySet() is a method in the Java HashMap class that returns a set view of the keys contained in the map. This method allows iteration over the keys in the map without modifying the map itself. Introduced in Java 1995 with Java 1.2, keySet() is useful for retrieving and manipulating the keys in a map, such as when checking for the presence of specific keys or when extracting them for further processing.

https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html