Develop a multi-threaded application. Don't use synchronized word [duplicate]

Question

Free checkout. The fast food restaurant has several cash desks. Customers stand in line at a particular cash desk, but can move to another queue if the queue decreases or disappears.

In my code, I need to replace the "synchronized" method with some other

start:

package by.naumenka;

import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

public class Main {
public static void main(String[] args) throws InterruptedException {
List<Cashier> cashiers = new LinkedList<>();
Queue<Customer> customers = new LinkedList<Customer>();

for (int i = 0; i < 3; i++) {
    cashiers.add(new Cashier("Cashier "+ i, customers));
}

for (int i = 0; i < 10; i++) {
      synchronized (customers) {
        customers.add(new Customer("Customer " + i, 1 + (int) (9 * Math.random())));
        customers.notifyAll();
    }
}

synchronized (customers){
    while (!customers.isEmpty()){
        customers.wait();
    }
}

System.out.println("All customers have been served");

}
}

Cashier code:

package by.naumenka;

import java.util.Queue;

public class Cashier extends Thread {

volatile private Queue<Customer> customerQueue;

public Cashier(String name, Queue<Customer> customerQueue) {
super(name);
this.customerQueue = customerQueue;
this.setDaemon(true);
start();
}

@Override
public void run() {
while (true) {
    try {
        Customer currentCustomer = null;
        synchronized (customerQueue) {
            while (customerQueue.size() == 0) {
                customerQueue.wait();
            }
            currentCustomer = customerQueue.poll();
            customerQueue.notifyAll();
        }

        System.out.println(this + " have start to serve " + currentCustomer);
        Thread.sleep(500 * currentCustomer.getTaskQty());
        System.out.println(currentCustomer.getTaskQty()+ " tasks of " + 
currentCustomer + " was served by " + this);

    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}
}

@Override
public String toString() {
return getName();
}
}

Customer code:

package by.naumenka;

public class Customer  {

private String name;
private int taskQty;

public Customer(String name, int taskQty) {
this.name = name;
this.taskQty = taskQty;
}

public String getName() {
return name;
}

public int getTaskQty() {
return taskQty;
}


@Override
public String toString() {
return getName();
}
}

i tried to use a class that implements BlockingQueue, and the blocking operations put and take. These operations do not require synchronized as they are thread-safe. They also don't require use of wait() and notify.

 package by.naumenka;

 import java.util.LinkedList;
 import java.util.List;
 import java.util.concurrent.ArrayBlockingQueue;
 import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.TimeUnit;


 public class Main {
 public static void main(String[] args)  {
    BlockingQueue<String> customers = new ArrayBlockingQueue<String>(2);
    List<Cashier> cashiers = new LinkedList<>();

    new Thread(() -> {
        for (int i = 0; i < 3; i++) {
            try {
                TimeUnit.MILLISECONDS.sleep(1);
                cashiers.put(new Cashier("Cashier " + i, customers));
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }).start();
    new Thread(() -> {
        for (int i = 0; i < 10; i++) {
            try {
                TimeUnit.MILLISECONDS.sleep(1);
                customers.put(new Customer("Customer " + i, 1 + (int) (9 * 
  Math.random())));
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        }).start();

    new Thread(() -> {
    try (customers){
        while (!customers.isEmpty()){
            customers.wait();
        }
    }
    System.out.println("All customers have been served");
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    }
    }

But my code does not want to work and gives errors, please tell me what I'm doing wrong. I'm not good at programming, but I'm trying