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Inheritance and polymorphism (discussed below) are two sides of the same coin and represent very foundational concepts in object-oriented programming. The union design pattern is an expression of these relationships and enables us to talk about them in a more tangible manner.
Consider the following "is-a" or inheritance relationships between "concrete" entities Coca Cola, sulfuric acid, milk and the "abstract" liquid (named "
ALiquid " by convention since it is an abstract entity):
Union of liquids
ALiquid is the union of
CocaCola ,
SulfuricAcid and
Milk
Another way that we can express the notions depicted by the diagram is to say that the abstract
ALiquid superclass represents the union of Coca Cola, sulfuric acid and milk. That is,
a superclass represents the union of all of its subclasses.or in other words
a superclass represents all that is abstractly equivalent about its subclasses.For instance, the notion of an abstract liquid embodies all the behaviors and attributes such as having no definite shape, randomized atomic positions, freezing and boiling points that are common to Coca Cola, sulphuric acid and milk. Note the fine distinction between having a value and having the same value.
The above diagram illustrating the relationship betwen a superclass and its subclasses is called the Union Design Pattern . The union pattern shows us inheritance in how the Coca Cola, sulfuric acid and milk will all inherit the abstract behaviors of liquids, such as the lack of a definite shape and freezing/boiling points. Conversely, it also shows that if a situation utilizes a liquid, either Coca Cola, milk or sulphuric acid can be used as they are all abstractly equivalent as liquids. Note that this does not imply that all three will act identically! For instance, the human throat can swallow any liquid because it is made to work with fluids that can flow. However, the reaction of the throat to sulphuric acid is markedly different than it reaction to milk! This ability to substitute any subclass for its superclass and get different behavior is called polymorphism .
A subtle but extremely important point is that
Commonality does not imply abstract equivalence.Just because a feature is common to every item in a set, does not necessarily mean that it represents some sort of abstract feature of those elements. For instance, cats, dogs, humans, and rats are all mammals where a mammal is defined as an animal that produces milk to feed its young. One could thus make a class model where a superclass
Mammal has subclasses
Cat ,
Dog ,
Human and
Rat . One common feature is behavior is that cats, dogs, humans and rats all give live birth of their young. So it is tempting to say that the
Mammal class should also embody the "live birth" behavior. However, as one wanders the world discovering new mammals, in the backwaters of Australia one finds the
duck-billed platypus which produces milk and is therefore clearly a mammal. However, the duck-billed platypus also lays eggs. Thus the "live birth" behavior does not belong in the
Mammal superclass as it was only a coincidence that it was common to our initial set of subclasses. More importantly, being able to give live birth was never part of the abstract definition of a mammal and thus should never have been included in the
Mammal superclass in the first place.
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