An interface in Java defines a contract—a set of rules a class must follow. It specifies what a class should do, not how to achieve it. This approach is fundamental for achieving 100% abstraction and enabling multiple inheritance in Java's single-inheritance model, creating cleaner, more modular code architecture.

Traditional interfaces contained only abstract methods and public static final variables. Since Java 8, they evolved to include default methods with bodies and static methods. This evolution solved the multiple inheritance problem, allowing developers to provide shared behavior without forcing rigid class hierarchies. Java 9 further introduced private methods for internal code reuse.

Modern Java interfaces now support complex design patterns. Developers can call default methods from implementing classes using syntax like `A.super.show()`. This flexibility maintains backward compatibility while expanding functionality. Understanding these rules is essential for building scalable, maintainable applications in object-oriented programming.