If you know something about a category of things, you automatically know some things you can transfer to other categories.
If
you know something is an animal, you take for granted that it eats, sleeps, has
a way of being born, has a way of getting from one place to another... But
imagine that mammals, amphibians and reptiles are all animals. Also cows, dogs,
cats... are grouped in category mammals. Object-orientation refers to this as
inheritance.
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An inheritance hierarchy in the animal kingdom |
One
class (the child class or subclass) can inherit attributes and operations from
another (the parent class or superclass). The parent class is more general then
the child class.
In generalization, a child is substitutable for a parent. That is, anywhere the
parent appears, the child may appear. The reverse isn't true, however.
In
UML inheritance is represented with a line that connects the parent to a child
class, and on the parent's side you put an open triangle.
If we look from the association's side, the inheritance stands for is a kind of association.
What should analysts do to discover inheritance?
The analyst has to realize that the attributes and operations of one class are
general and apply to perhaps several other classes - which may add attributes
and operations of their own. Another possibility is that the analyst notes that
two or more classes have a number of common attributes and operations.
Classes
that provide no objects are said to be abstract classes. You indicate an
abstract class by writing its name in italics.
A
class can inherit attributes and operations from another class. The inheriting
class is the child of the parent class it inherits from. Abstract classes are
intended only as bases for inheritance and provide no objects of their own.