interface I { void print(); }

public I create() {
    return () -> { System.out.println("Hello!"); };
}

private I i = this::create;

public void test() {
    i.print();
}

public class Singleton {
    private Singleton() {}

    private static class SingletonHolder {
        private final static Singleton INSTANCE = new Singleton();
    }

    public static Singleton getInstance() {
        return SingletonHolder.INSTANCE;
    }
}

public class ThreadDemo {

    private int count = 1;

    public synchronized void doSomething() {
        for (int i = 0; i < 10; i++) {
            System.out.println(count++);
        }
    }

    public static void main(String[] args) {
        ThreadDemo demo = new ThreadDemo();
        Thread a1 = new A(demo);
        Thread a2 = new A(demo);
        a1.start();
        a2.start();
    }
}

class A extends Thread {
    ThreadDemo demo;

    public A(ThreadDemo td) {
        demo = td;
    }

    public void run() {
        demo.doSomething();
    }
}