Getting NullPointer Exception when declaring LinkedBlockingQueue<String>

Question

This is the code I have. I have getting Null Pointer Exception in the last line of the constructor (workerQueue[i] = new LinkedBlockingQueue(100);):

public class QueueThreadPool {

  private BlockingQueue<String>[] workerQueue;
  private Thread[] workerThreads;
  private int numQueues;
  private int numThreads;

public QueueThreadPool(int numThreads, int numQueues) {
    this.numQueues = numQueues;
    this.numThreads = numThreads;

    for(int i=1; i<=numQueues; i++){
        workerQueue[i] = new LinkedBlockingQueue<String>(100);
    }
}
public static void main(String args[]){
    System.out.println("Start...");
    new QueueThreadPool(50, 11);
    System.out.println("End...");
}

Please help! Thanks!!

Subhrajyoti Majumder · Accepted Answer · 2013-07-23 04:29:55Z

2

Array workerQueue is not instantiated which you need to do.

private BlockingQueue<String>[] workerQueue;

workerQueue is a reference of BlockingQueue<String>[] type, not a Object.

But also you cannot create a generic array of BlockingQueue<String>. Instead of that create a List of BlockingQueue<String>. Ex -

private List<BlockingQueue<String>> workerQueue= new ArrayList<>();

you can also create the list Object at constructor.

private List<BlockingQueue<String>> workerQueue= new ArrayList<>();
public QueueThreadPool(int numThreads, int numQueues) {
    this.workerQueue = new ArrayList<>(numQueues);  // <-- initialize the field.
    this.numQueues = numQueues;
    this.numThreads = numThreads;
    ...

edited Jul 23, 2013 at 4:29

answered Jul 23, 2013 at 4:14

Subhrajyoti Majumder

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Pradeep Pati · Accepted Answer · 2013-07-23 04:14:54Z

0

You haven't initialized workerThreads. You have to do something like workerQueue= new BlockingQueue<String>[numQueues];

answered Jul 23, 2013 at 4:14

Pradeep Pati

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1 Comment

kajarigd Over a year ago

Thanks for the reply. As mentioned int he ab ove answer, this declaration creates a generic array, which again gives error.

Itay Maman · Accepted Answer · 2013-07-23 04:18:50Z

0

Two problems in the code: the field need to be initialized, the loop should go from 0 to the array-size's - 1. Here is how the fixed code should look like:

public class QueueThreadPool {

  private BlockingQueue<String>[] workerQueue;
  private Thread[] workerThreads;
  private int numQueues;
  private int numThreads;

public QueueThreadPool(int numThreads, int numQueues) {
    this.workerQueue = new BlockingQueue<String>[numQueues]  // <-- You need to initialize the field.
    this.numQueues = numQueues;
    this.numThreads = numThreads;

    for(int i=0; i < numQueues; i++){  // <-- Indexing into arrays goes from 0 to size-1     (inclusive).
        workerQueue[i] = new LinkedBlockingQueue<String>(100);
    }
}

answered Jul 23, 2013 at 4:18

Itay Maman

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1 Comment

kajarigd Over a year ago

Thanks for the reply. As mentioned in the above answer, this declaration creates a generic array, which again gives error.

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