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Interface in Java

In this video, we dive into *Interface in Java**. Building on our previous discussion of **Abstract Classes**, we explore the **key differences* between interfaces and abstract classes. You'll learn that while an abstract class provides an incomplete blueprint with a mix of abstract and concrete methods, variables, and constructors (but cannot be instantiated directly), an interface is more like a *contract**. It traditionally contains only **abstract methods* and *final variables (constants)**, and **cannot have instance attributes* or **constructors**. A crucial distinction highlighted is that a class can **only extend one abstract class**, but it can **implement multiple interfaces**. This capability is Java's way of achieving **multiple inheritance of behavior**. We also discuss how, starting with *Java 8**, interfaces gained the ability to include **default and static methods**, allowing for concrete implementations directly within the interface. We demonstrate how interface methods are **implicitly abstract and public by default**, so you don't need the `abstract` keyword, but you *do need the *`default` keyword* for concrete methods. Through a practical example creating a "Hospital" project with an `Operation` interface, we show how to: Create an interface. Define abstract methods like `execute` and `getDescription`. Add a default method like `status`. Create a concrete class (`Doctors`) that *implements* the interface using the **`implements` keyword**. Provide the required implementations for the abstract methods in the implementing class. Call both the implemented abstract methods and the default interface methods from an object of the class. Demonstrate *implementing multiple interfaces* by having the `Doctors` class implement both `Operation` and `StaffAction`, reinforcing that a class can implement **as many interfaces as needed**. We also briefly touch upon some *built-in interfaces in Java* like `Comparable`, `Runnable`, `Serializable`, and those from the collections framework (`List`, `Set`, `Map`). By the end, you'll have a solid understanding of what Java interfaces are, how they differ from abstract classes, and how they are used to define contracts and enable multiple inheritance of behavior in your Java programs. We hope this video is useful for expanding your knowledge in Java programming.