How can I create method to generate new LinkedList?

Question

I am trying to create a method that creates new LinkedLists. I want to pass a String parameter to use as the new LinkedList identifier but I'm getting an error "java: variable s is already defined in method createQueue(java.lang.String)"

Is there anyway to use a String to create the new LinkedList like this? I need to do it this way for an assignment so I can't change the method declaration.

public void createQueue(String s){

   LinkedList<obj> s = new LinkedList<obj>();

}

I may be looking at this the wrong way also. I'm just trying to create the linkedList atm. But my requirements are as follows:

boolean addQueue(String)

This method will have a String parameter. It will return a boolean. It will add a new queue specified by the parameter. E.g. addQueue(“ready”) would create a new queue called “ready” to the list of queues. If there is already a queue by the specified name, then this method will return false. E.g. If you already have a queue named “ready” and you call addQueue(“ready”), it will return false. Otherwise, it will create the queue and return true.

Answer 1

You have to maintain a collection of queues. Because each queue has a unique name, the most appropriate collection is Map:

public class QueueManager {

    private Map<String, List<Pcb>> queues = new HashMap<String, List<Pcb>>();

    public boolean addQueue(String queueName) {
        if (queues.containsKey(queueName)) {
            // There is already a queue with that name
            return false;
        } else {
            queues.put(queueName, new ArrayList<Pcb>());
            return true;
        }
    }
}

Here I made the assumption that the queue is implemented with an ArrayList, but of course LinkedListwould work similarly. Then the method addPcb()is quite obvious:

public void addPcb(Pcb pcb, String queueName) {
    List<Pcb> queue = queues.get(queueName);
    if (queue != null) {
        queue.add(pcb);
    } else {
        throw new IllegalArgumentException("Queue does not exist: " + queueName);
    }
}

An alternative implementation of addPcb(), using addQueue()could be:

public void addPcb(Pcb pcb, String queueName) {
    addQueue(queueName);
    List<Pcb> queue = queues.get(queueName);
    queue.add(pcb);
}