Barber

import java.util.LinkedList;
import java.util.Queue;
import java.util.Random;
import java.util.concurrent.Semaphore;



public class BarberShop {

    static int freeChairs = 5;
    static Queue<Integer> waiting = new LinkedList<>();
    static Semaphore chairsLock = new Semaphore(1);
    static Semaphore customerReady = new Semaphore(0);
    static Semaphore barberReady = new Semaphore(0);

    static class Barber extends Thread{
        @Override
        public void run() {
            try {
                execute();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        public void execute() throws InterruptedException {
            while (true){
                while (!waiting.isEmpty()){
                    chairsLock.acquire();
                    freeChairs++;
                    int id = waiting.poll();
                    barberReady.release();
                    chairsLock.release();
                    System.out.println("Barber give haircut to customer "+id);
                    customerReady.tryAcquire();
                }
                System.out.println("Barber goes to sleep");
                customerReady.acquire();
            }
        }
    }

    static class Customer extends Thread{
        int id;

        public Customer(int id) {
            this.id = id;
        }

        @Override
        public void run() {
            try {
                execute();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        public void execute() throws InterruptedException {
            chairsLock.acquire();
            if(freeChairs>0){
                System.out.println("Customer "+id+" sits in.");
                waiting.add(this.id);
                freeChairs--;
                customerReady.release();
                chairsLock.release();
                barberReady.acquire();
            }
            chairsLock.release();
        }
    }

    public static void main(String[] args) throws InterruptedException {
        // TODO: 3/29/20 Synchronize the scenario
        Thread t = new Barber();
        t.start();
        Random random = new Random();
        for (int i = 0; i < 100; i++) {
            t = new Customer(i+1);
            t.start();
        }
    }
}