Java AbstractSet is an abstract class that implements the Java Set interface, providing skeletal implementations of most of the methods in the Set interface. It is typically extended by other classes that provide concrete implementations for specific types of sets, such as Java HashSet or Java TreeSet. Introduced in Java 1997 with Java 1.2, AbstractSet serves as a convenient base class for developers who need to implement custom set behaviors while inheriting basic set functionality.

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