Java HashSet.add() is a method in the Java HashSet class that adds a specified element to the set. If the element is already present, the set remains unchanged. Introduced in Java 1995 with Java 1.2, add() is a crucial method for adding unique elements to a set, ensuring that no duplicates are present.

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