An interface in programming is a reference type that defines a contract for classes to implement. It specifies a set of methods and properties that must be included in any class that implements the interface. This allows for a consistent way to interact with different classes, as they all adhere to the same contract, even if their internal implementations differ.

In Java, interfaces can contain constants, method signatures, default methods, static methods, and nested types. However, they cannot be instantiated directly. Instead, classes implement the interface and provide concrete implementations for its abstract methods. A class can implement multiple interfaces, allowing for greater flexibility and reusability in code design ²³.

Interfaces are essential for achieving abstraction in object-oriented programming, as they allow developers to define behaviors without specifying how those behaviors are executed. This separation of definition and implementation promotes loose coupling between components, making the code easier to maintain and extend ⁴.