Abstract Classes

We built class hierarchies where every class could be instantiated. However, in the real world, some concepts are too general to exist as standalone objects. Consider a “Payment.” You can make a credit card payment or a bank transfer, but you cannot simply make a “generic payment” without knowing the specific method.

In Java, we use abstract classes to represent these high-level concepts. They allow us to define a strict template for subclasses while preventing code from creating incomplete objects.

The need for abstraction

When we design a class hierarchy, the superclass often acts as a category rather than a specific entity. If we define a class named Shape, we intend for it to be a parent for Circle or Rectangle.

If Shape is a normal (concrete) class, nothing stops us from writing new Shape(). This creates a meaningless object that doesn't represent any actual shape. We need a way to tell the compiler: This class is just a blueprint for other classes. Do not allow anyone to create a direct instance of it.

Defining an abstract class

We declare a class as abstract using the abstract keyword. This signals the compiler ...