Abstract Classes and Interfaces

So far, we have looked at ordinary classes, sometimes referred to as “concrete” classes. You can have classes in a hierarchy, where an open class can be extended by subclasses. However, while a class can declare what is open to be overridden, a concrete class cannot stipulate something that must be implemented in a subclass. While a subclass can elect to override an open function, it does not have to do so.

Abstract classes and interfaces offer two ways to provide “contracts” between classes and their subclasses. These work similar to their Java counterparts, allowing you to declare functions with no default implementation, where the subclass is required by the compiler to provide an implementation.

The Objective: Contracts

Sometimes, we want to be able to get information about an object, but where the type of the object does not have a way of knowing that information.

For example, let’s go back to the class hierarchy that we saw in the chapter on classes, adding in one more species:

open class Animal

class Frog : Animal()

class Axolotl : Animal()

class KomodoDragon : Animal()

We might want to know if an Animal has gills. Whether any given type of Animal has gills depends partly on the species and partly on circumstances:

• A Komodo dragon has no gills
• An axolotl has external gills
• A frog may have external gills briefly after hatching, but they get absorbed back into the body shortly thereafter

As a result, we cannot implement a hasGills() function or hasGills property on Animal very easily… unless we use abstract classes or interfaces. Then, we can ask an Animal if it has gills, but the actual implementation would be delegated to Frog, Axolotl, or KomodoDragon.