Java 8 FAQ: Top Questions

A functional interface is an interface that contains only one abstract method. These interfaces are used primarily to enable lambda expressions, which provide a way to implement the method without using a class.

🔍 Key Concept:

Functional interfaces act as targets for lambda expressions and method references.

🗺️ Step-by-Step Instructions:

1. Use the @FunctionalInterface annotation for clarity (optional but recommended).
2. Ensure the interface has only one abstract method.
3. Use lambda expressions to implement the method inline.

📥 Example Input:

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

🏆 Lambda Usage:

Greeting greet = name -> System.out.println("Hello " + name);
greet.sayHello("Java");

✅ Java 8 Solution:

public class FunctionalInterfaceExample {
  public static void main(String[] args) {
    Greeting greet = name -> System.out.println("Hello " + name);
    greet.sayHello("Java");
  }
}

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

📘 Detailed Explanation:

• Only one abstract method: Allows the compiler to infer the lambda body.
• Built-in examples: Runnable, Callable, Comparator, Predicate, Function.
• @FunctionalInterface: Annotation to enforce the contract and help documentation.

🛠️ Use Cases:

• Event listeners and command patterns.
• Lambda expression support in APIs.
• Defining concise single-method logic units.