// 메서드가 하나인 함수형 인터페이스 Runable
@FunctionalInterface
public interface Runnable {
    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object's
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see     java.lang.Thread#run()
     */
    public abstract void run();
}

// Thread의 생성자
public Thread(Runnable target) {
    init(null, target, "Thread-" + nextThreadNum(), 0);
}

// Thread의 Lambda expression 사용
new Thread(()->{
	System.out.println("Hello World.");
}).start();