Functional Interfaces

01What is a Functional Interface?

A functional interface is an interface that contains exactly one abstract method. It's the type of interface that lambda expressions are designed to implement directly.

02Why is it used?

Functional interfaces provide the foundation that makes lambda expressions possible — the lambda's short code becomes the implementation of that one single abstract method defined in the interface.

03Real-Life Example

Think of a job role with exactly one single core responsibility, like "Deliver the package." Because there's only one clear task, anyone can quickly describe how they'll do it, without needing a long, detailed job description — that's the simplicity a functional interface provides.

java
@FunctionalInterface
interface InterfaceName {
    void methodName(); // exactly one abstract method
}

java
@FunctionalInterface
interface Calculator {
    int operate(int a, int b);
}

public class FunctionalInterfaceDemo {
    public static void main(String[] args) {
        Calculator addition = (a, b) -> a + b;
        System.out.println("Sum: " + addition.operate(5, 10));
    }
}

Sum: 15