A filter is a special kind of transform which changes the set only by excluding some of its members. One common filter might be public, which can be applied to a set expression that returns classes, methods, or fields and keeps only those that are declared public. So again, the set expression public methods in class "java.util.ArrayList" will return just what it says. The language implements many filters, described in the reference.

Filters represent a boolean condition, and so you can combine them with boolean operators (not, and, or, xor) to create a more complex filter. ( public or protected ) methods in class "java.util.ArrayList" will give the list of all the methods in the class that are visible outside its package.

By definition, the transform represented by a filter is stateless.