Java 8: Functional Interface

Definition: 

A function interface in java is an interface with a single abstract method. They are used to type lambda expressions.

Example: 

In the following example, 'Cat' is a functional interface.

 interface Cat {
    void meow();
 }

 public static void main(String... args) {
    Cat cat = () -> System.out.println("Meowwwwww");    
    cat.meow();
 }

It is good practice to annotate functional interfaces with the annotation @FunctionalInterface .

Java 8: Runnables with Lambdas

Old (anonymous inner classes)

 Runnable meow = new Runnable() {
    @Override    
    public void run() {
        System.out.println("meow");    
    }
 };
 meow.run();

Output: 
    meow

    Process finished with exit code 0

New.1 (lambda)

 Runnable rawr = () -> System.out.println("rawr");
 rawr.run();

Output: 
    rawr

    Process finished with exit code 0

New.2 (passing function call)

 public static void main(String... args) {
    Runnable lalala = Main::LaLaLa;    
    lalala.run();
 }

 public static void LaLaLa() {
    System.out.println("lalala");
 }

Output: 
    lalala

    Process finished with exit code 0