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.
📄 PDF Download
Need a handy summary for your notes? Download this topic as a PDF!