Puzzle solution.

Forrest.java

package ru;

// origin source: http://arhipov.blogspot.ru/2014/02/java-8-lambdas-unintentional-puzzle.html
// my gist with puzzle https://gist.github.com/iaveryanov/10990181
public class Forrest {
    public Runnable wrooom(){
        return new Runnable() {
            @Override
            public void run() {
                System.out.println("Hello, lambda!");
            }
        };
    }

    public static void main(String[] args) {

        Forrest forrest = new Forrest();

        Runnable runnable = forrest.wrooom();

        /**
         * Method reference forrest::wrooom will expand to
         */
        Runnable runnable2 = new Runnable() {
            @Override
            public void run() {
                // not print
                forrest.wrooom(); // return new Runnable, but will not be called run() with println!

                // will print
                // forrest.wrooom().run(); // return new Runnable and call run!
            }
        };

        runnable.run(); // print "Hello, lambda!"
        runnable2.run(); // print nothing, only create new Runnable!
    }
}