Interfaces and Abstract Classes

# CHAPTER 18

Interfaces and Abstract Classes #

1. Introduction #

2. Defining and Implementing Interfaces #

interface Flyable {
    void fly();        // Abstract by default
    int MAX_ALTITUDE = 40000; // public static final by default
}

interface Swimmable {
    void swim();
}

class Duck implements Flyable, Swimmable {
    @Override
    public void fly() { System.out.println("Duck is flying!"); }
    @Override
    public void swim() { System.out.println("Duck is swimming!"); }
}

3. Default Methods (Java 8+) #

interface Logger {
    void log(String message);
    default void logError(String error) {
        System.err.println("ERROR: " + error);
    }
}

4. Static Methods in Interfaces #

interface MathUtils {
    static int square(int n) { return n * n; }
}
MathUtils.square(5); // 25

5. Functional Interfaces (Java 8+) #

@FunctionalInterface
interface Greeting {
    void sayHello(String name);
}

Greeting g = (name) -> System.out.println("Hello, " + name);
g.sayHello("Alice");

6. Interface vs Abstract Class #

7. MCQ Quiz with Answers #

8. Interview Questions #

• Q: When would you use an interface vs an abstract class?

• Q: What is a marker interface (e.g., Serializable)?

• Q: Can an interface extend another interface?

• Q: What are default methods and why were they added in Java 8?

9. Summary #

10. Next Chapter Recommendation #