How to implement the empty set - ∅? [closed]

Question

Assuming that you want to implement set theory concepts such as element, set, collection and relation in Java: How would you represent the empty set ∅?

Do I mislead myself, if I think of the NULL concept as it is used by the three-valued logic of databases?

Answer 1

Use Collections.emptySet():

Returns the empty set (immutable). This set is serializable. Unlike the like-named field, this method is parameterized. This example illustrates the type-safe way to obtain an empty set:

 Set<String> s = Collections.emptySet();   

Implementation note: Implementations of this method need not create a separate Set object for each call. Using this method is likely to have comparable cost to using the like-named field. (Unlike this method, the field does not provide type safety.)