Thread: Please Help (Queues)

Hi!
so i'm trying to make a program with a customer class that keeps how many minutes each customer needs to be served. The driver class CustomerQueue is a queue of all customers that haven't been served yet. Each iteration of the program is one minute, and there is a 25% chance of a new customer entering the queue each minute. Each customer takes 1-5 minutes to be completely served ad removed from queue.

My Code:

import java.util.Queue;
import java.util.LinkedList;
import java.util.Random;
 
public class CustomerQueue
{
	public static void main(String[] args) 
	{
		int chance;
		Queue<Customer> customerQueue = new  LinkedList<Customer>();
 
		//Simulating 60 minutes
		for (int i = 0; i<60; i++)
		{
			//25% chance of a new customer, to do this:
			//get a random number from 1-4, if you get 4, you add a customer
			Random rand = new Random();
			chance = rand.nextInt(4)+1;
 
			if(chance == 4)
			(
				Customer newCustomer = new Customer();
				customerQueue.add(newCustomer);
				system.out.println("A new customer just entered the queue.\nTotal number of customers in the Queue is: "+customerQueue.size());
			)
 
			//decrease a minute from the firs customer and check if he's fully servieced
			if(customerQueue(0).timeLeft() == 0)
			(
				customerQueue.remove();
				System.out.println("Customer serviced and removed from queue.\nQueue lenght is now: "+customerQueue.size());
			)
 
			System.out.println("-------------------one minute elapsed---------------------------");
 
		}
	}
}

import java.util.Random;
 
public class Customer
{
	private int serviceTime;
 
	public void customer()
	{
		Random rand = new Random();
		serviceTime = rand.nextInt(5)+1;
	}
 
	private int timeLeft()
	{
		serviceTime =-1;
		return serviceTime;
	}
 
 
}

I'm having a problem adding and removing Customer objects from the queue with these errors:
CustomerQueue.java:22: error: ')' expected
Customer newCustomer = new Customer();
^
CustomerQueue.java:25: error: illegal start of expression
)
^
CustomerQueue.java:30: error: ')' expected
customerQueue.remove();
^
CustomerQueue.java:29: error: not a statement
(
^
CustomerQueue.java:32: error: illegal start of expression
)
^
5 errors

oops!
thanks for that. It compiles now but i get an exception when running it. could you take a look at the new code?

import java.util.Queue;
import java.util.LinkedList;
import java.util.Random;
 
public class CustomerQueue
{
	public static void main(String[] args) 
	{
		int chance;
		Queue<Customer> customerQueue = new  LinkedList<Customer>();
 
		//Simulating 60 minutes
		for (int i = 0; i<60; i++)
		{
			//25% chance of a new customer, to do this:
			//get a random number from 1-4, if you get 4, you add a customer
			Random rand = new Random();
			chance = rand.nextInt(4)+1;
 
			if(chance == 4)
			{
				Customer newCustomer = new Customer();
				customerQueue.add(newCustomer);
				System.out.println("A new customer just entered the queue.\nTotal number of customers in the Queue is: "+customerQueue.size());
			}
 
			//decrease a minute from the firs customer and check if he's fully servieced
			customerQueue.peek().minutePassed();
 
			//check if customer fully served
			if(customerQueue.peek().timeLeft() == 0)
			{
				customerQueue.remove();
				System.out.println("Customer serviced and removed from queue.\nQueue lenght is now: "+customerQueue.size());
			}
 
			System.out.println("time left for first customer: "+customerQueue.peek().timeLeft());
			System.out.println("-------------------one minute elapsed---------------------------");
 
		}
	}
}

import java.util.Random;
 
public class Customer
{
	private int serviceTime;
 
	public Customer()
	{
		Random rand = new Random();
		serviceTime = rand.nextInt(5)+1;
	}
 
	public void minutePassed()
	{
		serviceTime =-1;
	}
 
	public int timeLeft()
	{
		return serviceTime;
	}
 
 
}

am I using the peek method of the class correctly? I'm not sure how to update the first customer object in the queue or see the timeLeft in it. I also tried to create another Customer variable and assigning the first Customer object to it and then getting the timeLeft and using minutePassed but I get the same error.
Please help?

Originally Posted by husain2213

...It compiles now but i get an exception when running it. ....

You should *always* also post the full text of the exception and indicate by an obvious comment on the code which line(s) throw the exception. Please do this as this will greatly help us to figure out exactly what is wrong. Also, part of what you have to learn is how to interpret these exception stack traces. It's not hard to do once you've had a little practice.

Exception in thread "main" java.lang.NullPointerException
at CustomerQueue.main(CustomerQueue.java:28)

that's the exact exception. it points to this line:
customerQueue.peek().minutePassed();

Originally Posted by husain2213

Exception in thread "main" java.lang.NullPointerException
at CustomerQueue.main(CustomerQueue.java:28)

that's the exact exception. it points to this line:
customerQueue.peek().minutePassed();

So what is null? consider changing this:

customerQueue.peek().minutePassed();

to this:

// add before checking the minutes passed:
System.out.println("customerQueue is null: " + (customerQueue == null));
System.out.println("customerQueue.peek() is null: " + (customerQueue.peek() == null));
 
// then try to get the minutes passed:
customerQueue.peek().minutePassed();

and letting us know what you see.

customerQueue is null: false
customerQueue.peek() is null: true
Exception in thread "main" java.lang.NullPointerException
at CustomerQueue.main(CustomerQueue.java:30)

is this happening because there might be no Customers in the queue? if that's the case, how can I keep the program running even if there are no customers?

Originally Posted by husain2213

customerQueue is null: false
customerQueue.peek() is null: true
Exception in thread "main" java.lang.NullPointerException
at CustomerQueue.main(CustomerQueue.java:30)

is this happening because there might be no Customers in the queue? if that's the case, how can I keep the program running even if there are no customers?

Yep, that's what the Queue API states:

Returns the head of this queue, or null if this queue is empty

You could call peek and check that it's not null before proceeding:

Customer cust = customerQueue.peek();
if (cust != null) {
   // do check the minutes passed here
   cust.minutesPassed();
   if (cust.timeLeft == 0) {
     //...
   }
}

Thanks a lot,, its working now just needs some tweaking
really appreciate the help

You're quite welcome! Glad you've got it working.