Union design pattern: inheritance and polymorphism  (Page 3/4)

Our class structure should look like this ( Mammal2.java ):

Hoisting does not guarantee proper abstraction. Hoisting should be driven by a need for abstraction, not by coincidence.

Another key notion that the union pattern emphasizes is levels of abstraction . What we see is that the concept of a liquid is more abstract than milk. In turn, the general concept of milk is more abstract than "2% milk" or "skim milk" which would be subclasses of milk. In general we can see that a superclass is a distinctly higher level of abstraction than any of its subclasses. One of the key tools we use to help us design and build high quality object-oriented systems is careful attention to the abstraction level at any given moment.

Good OOP code always maintains a consistent level of abstraction.

Levels of abstraction illustrate another important aspect of an OO program. Since a superclass represents the union of the subclasses or conversely, that the superclass can be represented by any of its subclasses, we see that the superclass is an embodiment of all the invariant aspects of the subclasses. That is, the superclass's definition is all that is abstractly equivalent about the subclasses--all that does not change from subclass to subclass. Note that this does not imply that the values of common fields are necesarily the same, just that, perhaps, that the field exists. Not does it imply that what is common to all the subclasses is necessarily what is abstractly equivalent about them (see the note above). The differences between the subclasses is what creates the variations in how the program behaves when any given subclass is used in place of the superclass. We call this the variant aspects of the system.

The total behavior of a program is the combination of its variant and invariant behaviors.

Inheritance and polymorphism

Inheritance and polymorphism are really just two ways of looking at the same class relationship.

Inheritance is looking at the class hierarchy from the bottom up. A subclass inherits behaviors and attributes from its superclass. A subclass automatically possesses certain behaviors and/or attributes simply because it is classified as being a subclass of an entity that possesses those behaviors and/or attributes. That is, a cherry can be said to automatically contain a seed because it is a subclass of Fruit and all fruit contain seeds.

Inheritance is useful from a code reuse perspective. Any (non-private) code in the superclass does not have to be replicated in any of the subclasses because they will automatically inherit those behaviors and attributes. However, one must be very careful when transferring common code from the subclasses to the superclass (a process called " hoisting "), as the proper abstraction represented by the superclass may be broken (see note above).