How PriorityQueue works ?

PriorityQueue

• When items are added to a PriorityQueue they are not order by FIFO
• All items in a PriorityQueue must be comparable , because PriorityQueue adds and removes based on Comparable.
• A priority queue does not permit null elements.
• It does not permit insertion of non-comparable objects ; may result in ClassCastException
• Iterator does't go through elements based on priority, but you can use poll() method.

Example 1

// queue of capacity 1

PriorityQueue queue = new PriorityQueue();

// insert elements to queue

queue.offer(3.2);

queue.offer(9.8) ;

queue.offer(5.4);

System.out.print("Polling from queue: ");

// display elements in queue

while(queue.size() > 0) {

   System.out.printf("%.1f ", queue.peek());   // view top element

   queue.poll();

}

Output

Polling from queue: 3.2   5.4   9.8

Example 2

PriorityQueue queue = new PriorityQueue();

queue.offer("CCC-1");

queue.offer("BBB");

queue.offer("AAA");

queue.offer("CCC-2");

out.println("1. " + queue.poll());    // removes

out.println("2. " + queue.poll());    // removes

out.println("3. " + queue.peek());

out.println("4. " + queue.peek());

out.println("5. " + queue.remove());  // removes

out.println("6. " + queue.remove());  // removes

out.println("7. " + queue.peek());

out.println("8. " + queue.element()); // Throws NoSuchElementException !

Output

1. AAA

2. BBB

3. CCC-1

4. CCC-1

5. CCC-1

6. CCC-2

7. null

Exception in thread "main" java.util.NoSuchElementException