import java.util.*;
public class Singleton {

    private static volatile Singleton singleton;


    private Singleton() { }

    public synchronized Singleton getInstance() {
        // TODO: 3/29/20 Synchronize this
        if(singleton == null) {
            System.out.println("created singleton");
            singleton = new Singleton();
        }

        return singleton;
    }

    public static void main(String[] args) {
        // TODO: 3/29/20 Simulate the scenario when multiple threads call the method getInstance
        List<Thread> list = new ArrayList<>();
        Singleton s = new Singleton();
        for(int i = 0; i < 20; i++) {
            list.add(new Thread(new Runnable() {
                @Override
                public void run() {
                    s.getInstance();
                }
            }));
        }
        for(int i = 0; i < 20; i++) {
            list.get(i).start();
        }
        for(int i = 0; i < 20; i++) {
            try {
                list.get(i).join();
            }
            catch (InterruptedException e) {
                System.out.println(e);
            }
        }

    }

}