Functional Interfaces

About

• A functional interface is an interface that has exactly one abstract method. Functional interfaces are ideal for use with lambda expressions and method references in Java 8 and beyond.

• The @FunctionalInterface annotation is used to indicate a functional interface, but it is optional. When annotation is provided, the compiler ensures that a functional interface contains only one abstract method.

• Examples: Runnable, Callable, Comparator, and custom single-method interfaces.

• Lambda Expressions: Since Java 8, lambdas are used to instantiate functional interfaces, which greatly simplifies code and improves readability.

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

public class Main {
    public static void main(String[] args) {
        Greeting greeting = name -> System.out.println("Hello, " + name);
        greeting.sayHello("Alice");
    }
}

// Compilation error - Multiple non-overriding abstract methods found in interface test. SomeMain. Greeting
@FunctionalInterface
interface Greeting {
    void sayHello(String name);
    String sayBye();
}

For more details, refer to the below page -

Functional Interfaces