What's the difference between abstract classes and interfaces in programming?

Abstract classes provide a partial implementation which can be shared by multiple subclasses, while interfaces only declare methods without implementation.

In more detail, an abstract class in programming is a class that cannot be instantiated, meaning you cannot create an object from it directly. Instead, it is designed to be subclassed by other classes. Abstract classes can contain both abstract methods (methods declared without an implementation) and concrete methods (methods with an implementation). This allows you to define default behaviour for subclasses, which can be overridden if necessary.

On the other hand, an interface is a completely abstract class that contains only abstract methods. In other words, it declares methods but does not implement them. Any class that implements an interface must provide an implementation for all of its methods. Interfaces are used to define a contract or a behaviour that classes must follow. They are particularly useful when you want to ensure that unrelated classes implement the same methods, allowing them to be used interchangeably.

Another key difference between abstract classes and interfaces is that a class can extend only one abstract class, but it can implement multiple interfaces. This is because Java and many other object-oriented programming languages do not support multiple inheritance, which is the ability of a class to inherit from more than one superclass. However, they do support multiple interfaces, which allows a class to have behaviour from multiple sources.

In terms of when to use each, abstract classes are typically used when you have a hierarchy of classes that share common behaviour, but you want to prevent the instantiation of the superclass. Interfaces, meanwhile, are used when you want to guarantee that certain methods will be implemented by any class that uses the interface, regardless of where in the class hierarchy it is.

In summary, while both abstract classes and interfaces can be used to enforce certain behaviours in classes, they do so in different ways and are used in different scenarios. Understanding these differences is key to using them effectively in your code.