How does java.util.EnumSet<E> work?

Question

The EnumSet<E> class is defined as:

public abstract class EnumSet<E extends Enum<E>>
extends AbstractSet<E>
implements Cloneable, Serializable

in JCF. Also, most of the methods that the class itself implements are static. Lastly, the class does not seem to implement the add(),iterator(),remove(),size(),contains() or isEmpty() methods and just inherits them from AbstractSet which doesn't implement them. I have two questions:

1. How exactly EnumSet objects are instantiated and used?
2. Why can i use the add() method for EnumSet objects?

Answer 1

Most of the static methods you see are factory methods (of(), complementOf(), allOf(), etc.).

These methods return an instance of EnumSet. The actual type of the EnumSet created and returned by these methods is a subclass of EnumSet (RegularEnumSet, JumboEnumSet), which are not part of the public API, but implement all the required methods. All you need to know is that they implement EnumSet.

Answer 2

EnumSet is an abstract class itself, so it doesn't have to implement any abstract methods it inherits. It passes that responsibility on to its non-abstract subclasses.

You can call the unimplemented methods because you're actually calling them on an instance of a subclass of EnumSet. (As EnumSet is abstract, it cannot be directly instantiated itself.)