What is the main difference between an abstract class and an interface?

The main distinction between an abstract class and an interface lies in the ability of the abstract class to contain methods with implementations, while an interface traditionally focuses on declaring methods without any implementation.

An abstract class can define both abstract methods (those without an implementation) and concrete methods (those with an implementation). This allows for shared functionality among subclasses, enabling them to inherit specific behaviors that are common to all derived classes. For instance, an abstract class can provide a base functionality that subclasses can either utilize directly or override as needed.

In contrast, an interface was traditionally designed to define a contract without imposing any specific behavior. All methods in an interface are abstract by default, meaning they do not have any body or implementation. Although recent updates in programming languages have allowed interfaces to include default methods (with an implementation), this remains less common compared to the concept of concrete methods in abstract classes.

This distinction is fundamental in object-oriented programming, as it dictates how classes are designed and how they interact with one another, emphasizing the role of an abstract class as a more flexible construct compared to an interface.