Functional interface in Java

Quick example

 * Declare our functional interface
public interface CustomLogger {
    public void log(String message);

class Demo {
    public static String message = "Hello World!";

    public static logMessage(CustomLogger customLogger) {

Demo.logMessage(message -> {

If this wasn't a functional interface then in that case
we would have required to create a class that implements
CustomLogger, then override log method and 
then pass a new instance of it to logMessage

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