Designing a General Class Before Specific Ones

Many things in the world sort naturally into categories that sit inside other categories. A vehicle can be a truck, and a truck can be a fire engine. A face drawn on a screen can be built from a circle, and that circle is itself a shape. When a program needs to represent this kind of nesting, it needs a way to record which category sits inside which. Class hierarchies are the tool for doing that.

The chain fire engine, truck, vehicle is the simplest case of a class hierarchy. Each step in the chain is a link from a more specific category to a more general one. Read it from the bottom up: a fire engine is a kind of truck, and a truck is a kind of vehicle. Read it from the top down and the same chain says that vehicle has a truck category inside it, and that truck has a fire engine category inside it.

The diagram keeps the most general class at the top and places more specific classes lower down. Following the links upward from the fire engine reaches the truck and then the vehicle, and that upward path is exactly what you will trace later when you find the ancestors of a class.

A class hierarchy is a set of classes ordered in a lattice created by derivation, and it is used to represent concepts that have hierarchical relationships. Derivation is the act of defining a more specific class from a more general one. When a class is derived from another, it sits one level below that class in the hierarchy.

The word lattice describes why a hierarchy looks like a branching structure rather than a single straight line. A simple list has one line of items, but a lattice allows classes to sit at several levels at once and to branch out from one another. Two properties describe the shape. A hierarchy is deep when many levels separate the most general class from the most specific one. It is wide when a single class has many direct children. Real hierarchies can be both deep and wide, sometimes holding hundreds of classes.

The source's main semi-realistic example uses shapes on a screen. A smiley face is a kind of circle, and a circle is a kind of shape. Written as a diagram, the general class sits at the top and the specific classes sit beneath it. The diagram below shows this simplest form. A real shapes program would usually hold more classes at the same level as Circle, such as other kinds of shapes, but this chain is enough to show how the hierarchy grows downward from a general class toward specific ones.

In a class hierarchy diagram, each line is drawn as an arrow, and each arrow stands for an inheritance relationship. The source states that class Circle is derived from class Shape. The arrow connects the derived class to the class it is derived from. This means an arrow always links a more specific class to a more general one, and the arrow points toward the general class.

The ancestors of a class are the classes you reach by following its arrows upward toward the general class. Tracing them is a repeatable procedure: start at the class you are given, follow its arrow to the class it is derived from, record that class, and repeat until no arrow remains.

Notice that Shape is an ancestor of Smiley face even though no arrow connects them directly. The relationship passes along the chain, just as the fire engine was known to be a vehicle through the truck. Every class above a given class in the chain counts as an ancestor of it.

Before writing any code for a hierarchy, the general class must be specified first. It defines the properties common to every member of the category, such as the properties common to every shape. More specific classes such as Circle are then derived from it. Thinking in this order, from general to specific, is a useful habit when designing a hierarchy, because each derived class builds on a base that already exists.