What is Functional Interface in Java

💡 Concept: Functional Interface in Java

A functional interface is an interface with exactly one abstract method, designed to be implemented by a lambda expression or method reference.

📘 Quick Intro

Functional interfaces are a foundation of Java 8's functional programming features, enabling concise lambda syntax.

🧠 Analogy

Think of a functional interface as a single job description, perfect for handing off to a helper without ambiguity.

🔧 Technical Explanation

Has exactly one abstract method (SAM - Single Abstract Method).
Can have default and static methods.
Annotated with @FunctionalInterface (optional but recommended).
Used primarily with lambda expressions and method references.
Examples: Runnable, Callable, Comparator.

🎯 Use Cases

✅ Simplify anonymous class implementations.
✅ Enable functional programming constructs.
✅ Used extensively in Streams and other Java 8 APIs.

💻 Example: Functional Interface in Java


@FunctionalInterface
interface MyFunctionalInterface {
    void execute();
}

public class FunctionalInterfaceExample {
    public static void main(String[] args) {
        MyFunctionalInterface fi = () -> System.out.println("Executing functional interface");
        fi.execute();
    }
}

❓ Interview Q&A

Q1: What is a functional interface?
A: An interface with exactly one abstract method.

Q2: Is @FunctionalInterface annotation mandatory?
A: No, but recommended.

Q3: Can functional interfaces have default methods?
A: Yes.

Q4: Name some examples of functional interfaces.
A: Runnable, Callable, Comparator.

Q5: How do functional interfaces relate to lambda expressions?
A: Lambda expressions implement functional interfaces.

Q6: Can functional interfaces have multiple abstract methods?
A: No.

Q7: What is SAM?
A: Single Abstract Method.

Q8: Can functional interfaces extend other interfaces?
A: Yes, but must still have only one abstract method.

Q9: Are functional interfaces thread-safe?
A: Depends on implementation.

Q10: Can functional interfaces be used without lambdas?
A: Yes, with anonymous classes.

📝 MCQs

Q1. What is a functional interface?

Interface with multiple methods
Interface with one abstract method
Class
Enum

Q2. Is @FunctionalInterface annotation mandatory?

Yes
No
Sometimes
Always

Q3. Can functional interfaces have default methods?

No
Yes
Sometimes
Never

Q4. Name some functional interfaces

Runnable, Callable
Runnable, Callable, Comparator
Thread, Process
None

Q5. How do lambda expressions relate to functional interfaces?

Ignore them
Implement functional interfaces
Create classes
None

Q6. Can functional interfaces have multiple abstract methods?

Yes
No
Sometimes
Always

Q7. What is SAM?

Single Abstract Method
Static Abstract Method
Single Action Method
None

Q8. Can functional interfaces extend other interfaces?

No
Yes
Sometimes
Never

Q9. Are functional interfaces thread-safe?

Yes
No
Depends
Always

Q10. Can functional interfaces be used without lambdas?

No
Yes
Sometimes
Never

💡 Bonus Insight

Functional interfaces enable Java's powerful functional programming features, facilitating cleaner and more flexible code.

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