Question

public class ObjQueue<E> extends Vector<E>
{...}

ObjQueue<Customer>[] line=new ObjQueue[numServices];
...
if(line[waitingLine].isEmpty()) --emptyLines;

NullPointerException is for if(line[waitingLine].isEmpty()) --emptyLines; What’s wrong?

Answer 1

This:

new objQueue[numServices];

creates an array with all of its elements set to null — not an array of empty queues. So this:

line[waitingLine].isEmpty()

will invoke isEmpty() on null, unless you’ve explicitly initialized the elements of line in some way.

You probably want to follow this:

objQueue<Customer> [] line=new objQueue[numServices];

with this:

for(int i = 0; i < numServices; ++i)
    line[i] = new objQueue<Customer>();

to initialize each element to a new, distinct queue.