Discuss user-defined and predicate-defined subclasses, and identify the differences between the two.

User-defined subclasses are those created by the user specifically by inheriting a base class. The properties of the base class are inherited by the user-defined subclass, although more features can be added or overridden by the user.

Predicate-defined subclasses are not explicitly defined by the programmer but are based on certain conditions or predicates. The objects of these subclasses satisfy conditions given in the form of a predicate.

The key differences between user-defined and predicate-defined subclasses could be outlined as follows: \n1. User-defined subclasses are created explicitly by the user, while predicate-defined subclasses are based on conditions. \n2. In the case of user-defined subclasses, it is the user who decides what properties to inherit, add, or override from the base class. Whereas, in predicate-defined subclasses the membership of an object is determined by whether it satisfies the given predicate. \n3. User-defined subclassing supports the concept of inheritance and method overriding. On the contrary, predicate-defined subclasses do not necessarily support these concepts because their classification is based on predicates.