Interface in Java Interview Questions and Answers for 7 years experience

1. What is an interface in Java?

   • Answer: An interface in Java is a reference type, similar to a class, that contains only constants and abstract methods. It defines a contract that classes must adhere to if they implement the interface. It cannot be instantiated directly.

2. What is the difference between an interface and an abstract class?

   • Answer: An abstract class can have both abstract and concrete methods, while an interface can only have abstract methods (before Java 8) and static/default methods (from Java 8 onwards). A class can extend only one abstract class, but it can implement multiple interfaces. An abstract class can have instance variables, while an interface can only have static constants.

3. Explain the concept of multiple inheritance in Java using interfaces.

   • Answer: Java doesn't support multiple inheritance of classes to prevent the diamond problem. However, it allows multiple inheritance of interfaces. A class can implement multiple interfaces, inheriting the behavior defined in each.

4. What are default methods in interfaces?

   • Answer: Introduced in Java 8, default methods provide a way to add new methods to existing interfaces without breaking existing implementations. They are implemented within the interface and provide a default behavior.

5. How do default methods resolve conflicts when multiple interfaces have the same default method?

   • Answer: If a class implements multiple interfaces with the same default method signature, the class must explicitly override the method to resolve the conflict. Otherwise, a compile-time error occurs.

6. What are static methods in interfaces?

   • Answer: Introduced in Java 8, static methods in interfaces are similar to static methods in classes. They belong to the interface itself, not to any specific implementation. They can be called directly using the interface name.

7. Explain the use of the `@FunctionalInterface` annotation.

   • Answer: The `@FunctionalInterface` annotation is used to indicate that an interface is a functional interface – meaning it has only one abstract method. This helps to avoid accidental addition of multiple abstract methods, ensuring the intended functional interface behavior. It's not mandatory but is good practice.

8. What is a marker interface? Give an example.

   • Answer: A marker interface is an interface with no methods (or just default methods which don't need implementation by implementing class). It serves as a tag to indicate that a class possesses a certain property. `java.io.Serializable` is a classic example; it marks a class as serializable.

9. Describe the significance of interfaces in achieving loose coupling.

   • Answer: Interfaces promote loose coupling by allowing classes to interact through a common contract without depending on the specific implementation. Changes to the implementation class won't affect classes that use the interface, as long as the contract remains the same.

10. How can interfaces be used for polymorphism?

    • Answer: Different classes implementing the same interface can be treated as objects of the interface type. This allows for flexibility and dynamic dispatch based on the actual object type at runtime.

11. Explain the concept of nested interfaces.

    • Answer: An interface can be defined inside another interface, similar to nested classes. This provides better organization and encapsulation.

12. How do you design interfaces for testability?

    • Answer: Well-designed interfaces with clear responsibilities make unit testing easier. Mocking frameworks can be used to create mock implementations of interfaces, allowing you to test individual components in isolation.

13. What are some best practices for designing interfaces?

    • Answer: Keep interfaces small and focused on a single responsibility (single responsibility principle). Avoid adding unnecessary methods. Use descriptive names. Favor interfaces over abstract classes when appropriate.

14. How can interfaces be used to achieve dependency injection?

    • Answer: Interfaces allow you to decouple dependencies. Instead of directly instantiating classes, you can pass interface references, allowing you to inject different implementations at runtime.

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