BarberShop.java

import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.*;


class BarberShop{
    public static void main(String[] args){

        AtomicInteger spaces = new AtomicInteger(15);
        
        final Semaphore barbers = new Semaphore(3, true);
        final Semaphore customers = new Semaphore(0, true);  
        ExecutorService openUp = Executors.newFixedThreadPool(3);

        Barber[]  employees = new Barber[3];
        
        System.out.println("Opening up shop");
        for(int i = 0; i < 3; i++) {
            employees[i]= new Barber(spaces, barbers, customers);
            openUp.execute(employees[i]);
        }

        while(true)
        {
            try { 
                Thread.sleep(ThreadLocalRandom.current().nextInt(100, 1000 + 100)); // Sleep until next person gets in
            }catch (InterruptedException e){}
            
            System.out.println("Customer walks in");
            
            if(spaces.get() >= 0){
                new Thread(new Customer(spaces, barbers, customers)).start();
            }
            else{
                System.out.println("Customer walks out, as no seats are available");
            }
        }
    }
}