GEOG 485:
GIS Programming and Software Development

1.5.4 Inheritance


Another important feature of object-oriented languages is inheritance. Classes are arranged in a hierarchical relationship, such that each class inherits its properties and methods from the class above it in the hierarchy (its parent class or superclass). A class also passes along its properties and methods to the class below it (its child class or subclass). A real-world analogy involves the classification of animal species. As a species, we have many characteristics that are unique to humans. However, we also inherit many characteristics from classes higher in the class hierarchy. We have some characteristics as a result of being vertebrates. We have other characteristics as a result of being mammals. To illustrate the point, think of the ability of humans to run. Our bodies respond to our command to run not because we belong to the "human" class, but because we inherit that trait from some class higher in the class hierarchy.

Back in the programming context, the lesson to be learned is that it pays to know where a class fits into the class hierarchy. Without that piece of information, you will be unaware of all of the operations available to you. This information about inheritance can often be found in informational posters called object model diagrams.

Here's an example of an object model diagram for the ArcGIS Python Geoprocessor at 10.x. Take a look at the green(ish) box titled FeatureClass Properties and notice at the middle column, second from the top, it says Dataset Properties. This is because FeatureClass inherits all properties from Dataset. Therefore, any properties on a Dataset object, such as Extent or SpatialReference, can also be obtained if you create a FeatureClass object. Apart from all the properties it inherits from Dataset, the FeatureClass has its own specialized properties such as FeatureType and ShapeType (in the top box in the left column).