PriorityQueue in Java

PriorityQueue does not process elements in arrival order like a regular queue. Instead, it always processes the element with the highest priority first. By default, the smallest element has the highest priority (min-heap). You can reverse this or define any custom priority with a Comparator.

Basic Usage — Min-Heap (default)

Run this example to see its output.

Line by line:

• Adding elements in any order — 40, 10, 30, 20.
• pq.poll() — always returns the smallest: 10, then 20, then 30, then 40.
• pq.peek() — look at the smallest without removing.
• Printing pq directly does NOT show sorted order — it shows the internal heap array.

Max-Heap — Custom Comparator

Run this example to see its output.

Real Use Case: Task Scheduling by Priority

Run this example to see its output.

Tasks with priority 1 are processed first. Among tasks with the same priority, order is not guaranteed (heap is not a sort).

PriorityQueue With Strings (Natural Order = Alphabetical)

Run this example to see its output.

For Strings, "natural order" is alphabetical — poll() always gives the alphabetically smallest remaining string.

Common Mistake: Assuming Poll Returns Insertion Order

PriorityQueue is NOT a FIFO queue. It always returns the smallest (or highest priority) element, regardless of insertion order.

Practice Task

Task: Use PriorityQueue<Integer> to find the 3 smallest numbers from the array {42, 7, 18, 3, 99, 55, 12, 1} without sorting the full array.

Hint: Add all elements to a PriorityQueue (min-heap). Call poll() three times — each call gives the next smallest.

Solution:

Run this example to see its output.

What to observe: First poll() gives 1, second gives 3, third gives 7 — the three smallest in order. The rest of the array is untouched. For the 3 largest instead, use Collections.reverseOrder() as the comparator.