ArrayBlockingQueue: access to specific element

Question

I have an ArrayBlockingQueue declared like this:

private BlockingQueue<E> queue = new ArrayBlockingQueue<E>();

now I have to access to a specific element of this queue. I have a method

public E takeElement(int j)
{
       //some code
}

and I have to take the j-element of the queue. How can I do that?? thx

If you need indexed access, why are you using a BlockingQueue? Its intended usage is processing items in order, with the possibility of waiting (blocking) if the queue is empty. — yole
– yole, Commented Feb 4, 2015 at 9:18
I have a queue and some method... somethimes I have to take head-element, somethimes I have to take j element. — Frank Cunningham
– Frank Cunningham, Commented Feb 4, 2015 at 9:27
What is the problem that you are really trying to solve? You don't need to use BlockingQueue in a way that it was never meant to be used: You need a different data structure/algorithm that solves some problem. What is the problem? — Solomon Slow
– Solomon Slow, Commented Feb 4, 2015 at 14:55

DennisW · Accepted Answer · 2015-02-04 09:16:21Z

2

You could write:

E element = (E) queue.toArray()[j];

This will probably give a warning, but that is safe to ignore.

answered Feb 4, 2015 at 9:16

DennisW

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4 Comments

Frank Cunningham Over a year ago

thx. Now I have the opposite question... what I have to do if I have to put an element in j-position??

DennisW Over a year ago

You can't. If you need access like this, you shouldn't be using a Queue. Consider ArrayList instead.

Frank Cunningham Over a year ago

I do this but I'm not sure that is a thread-dafe operation: queue.toArray()[j] = element;

DennisW Over a year ago

Assinging an element to the array has no effect on the Queue.

Evgeniy Dorofeev · Accepted Answer · 2015-02-04 11:34:29Z

0

try this

public E takeElement(int j) {
    for(int i = 0; i < j; i++) {
        E e = queue.poll();
        if (i == j - 1) {
            return e;
        }
    }
    return null;
}

or iterator based version:

public E takeElement(int j) {
    Iterator<E> it = queue.iterator();
    for(int i = 0; it.hasNext() && i < j; i++) {
        E e = it.next();
        if (i == j - 1) {
            return e;
        }
    }
    return null; 
}

edited Feb 4, 2015 at 11:34

answered Feb 4, 2015 at 9:27

Evgeniy Dorofeev

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4 Comments

DennisW Over a year ago

This would remove all elements in front of the sought element. I don't think that's what OP is looking for. Granted, it does adhere better to the purpose of Queue.

Frank Cunningham Over a year ago

Poll remove all head element, I don't want this.

DennisW Over a year ago

You really should be using a List. Just type list.get(0); when you need the head element.

Simon Forsberg Over a year ago

It's a bit funny when a 57.2k user posts an answer that gets auto-flagged as Low Quality. Try explaining your answer a bit more in the future and it won't have to show up in LQ-queue.

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