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An interface is a collection of abstract methods (methods without a body) that can be implemented by any class. Interfaces provide a an outline or structure for classes to follow, helping to ensure that the classes implementing the interface will have the expected functionalities. In most programming languages, interfaces are defined using the keyword "interface".

An abstract class is a class that cannot be instantiated (i.e., you cannot create objects of an abstract class). It can contain both abstract methods (methods without a body) and concrete methods (methods with a body). Abstract classes are meant to serve as a blueprint for other classes to extend and implement the required abstract methods. In most programming languages, abstract classes are defined using the keyword "abstract".

Both interfaces and abstract classes play a role in providing structure to the classes that implement or extend them. 1. They both contain abstract methods, which must be implemented by any class using them. 2. Interfaces and abstract classes cannot be instantiated; they can only be used by other classes. 3. Both help to achieve abstraction in object-oriented programming, which allows a separation of implementation from interface. 4. They establish a contract or guideline for classes to ensure an expected set of functionalities.

Although interfaces and abstract classes share some similarities, they have important differences: 1. Interfaces can only contain abstract methods, while abstract classes can contain both abstract and concrete (implemented) methods. 2. A class can implement multiple interfaces, but it can only extend a single abstract class. 3. Interface methods have public visibility by default, whereas abstract class methods can have different access modifiers (public, protected, or private). 4. Interfaces typically focus on defining a contract or "what" a class should do, while abstract classes can also provide guidance on "how" a class should do something, by including implementation details in its concrete methods. In conclusion, an interface is similar to an abstract class because they both provide structure and abstraction in object-oriented programming. However, they differ in terms of their capabilities and the mechanisms they use to achieve abstraction.