What is an abstract class, and how does it relate to objects?

An abstract class is a blueprint for other classes, providing a common interface but cannot be instantiated itself.

In object-oriented programming, an abstract class is a class that cannot be instantiated, meaning you cannot create an object of an abstract class. It is designed to act as a base or blueprint for other classes, allowing you to define methods and variables that must be created within any child classes built from the abstract class.

Abstract classes are crucial in large functional units of an application where you want to provide a common interface for different implementations and subclasses. They are used when you want to provide a generalised form of a concept, which can then be made more specific by subclasses. For instance, you might have an abstract class called 'Animal' with methods like 'eat' and 'sleep'. You could then have subclasses like 'Dog' and 'Cat' which provide their own implementations of these methods.

The relationship between abstract classes and objects is that an abstract class provides a template for creating objects. However, because an abstract class is incomplete or 'abstract', you cannot create an object directly from it. Instead, you create objects from its subclasses. These objects will have the properties and methods defined in the abstract class, as well as any additional properties and methods defined in the subclass.

In Java, for example, you define an abstract class using the 'abstract' keyword. Any method in an abstract class that does not have a body is also abstract, and must be implemented in any subclass. If a subclass does not provide implementations for all of the abstract methods in its parent class, then that subclass must also be declared abstract.

In conclusion, abstract classes are a fundamental part of object-oriented programming, providing a way to define common interfaces and behaviours for related classes. They provide a high level of abstraction, allowing you to focus on what a set of related classes have in common, and to specify how they should be structured and behave.