Java.Collections.What can PriorityQueue do?

The PriorityQueue in Java is a powerful tool for managing elements based on priority rather than insertion order — super useful in many real-world scenarios like scheduling, simulations, and pathfinding algorithms (like Dijkstra!).

---

🎯 What PriorityQueue Can Do

✅ Maintains elements in priority order, not insertion order.

• By default, it’s a min-heap — the smallest element is always at the head.
• You can provide a custom comparator to change the order (e.g., max-heap).

✅ Efficient operations:

Operation	Time Complexity	Description
add(e) / offer(e)	O(log n)	Insert new element, auto-reorders heap
poll()	O(log n)	Removes and returns highest priority
peek()	O(1)	Views head (highest priority)

---

🧪 Example: Default (Min-Heap)

import java.util.PriorityQueue;

PriorityQueue<Integer> pq = new PriorityQueue<>();
pq.add(5);
pq.add(1);
pq.add(3);

while (!pq.isEmpty()) {
    System.out.println(pq.poll()); // Output: 1, 3, 5
}

✅ Elements come out sorted, smallest first.

🔧 Example: Max-Heap (Custom Comparator)

PriorityQueue<Integer> maxPQ = new PriorityQueue<>((a, b) -> b - a);
maxPQ.add(5);
maxPQ.add(1);
maxPQ.add(3);

while (!maxPQ.isEmpty()) {
    System.out.println(maxPQ.poll()); // Output: 5, 3, 1
}

🧠 Use Cases for PriorityQueue

Use Case	Why It Fits
Task scheduling (OS, jobs)	Execute high-priority tasks first
Dijkstra’s algorithm	Visit node with smallest distance
Event-driven simulation	Process events in time order
Real-time systems	Handle critical events sooner
K largest/smallest elements	Keeps top-k efficiently

---

⚠️ Limitations

• ❌ No random access (get(i) not supported).
• ❌ Not thread-safe — use PriorityBlockingQueue if you need concurrency.
• ❌ Iterating doesn’t give sorted order — you need to poll().

---

🤖 TL;DR:

PriorityQueue is a heap-based structure for managing elements by priority, not position. It’s great for efficient priority-based processing in O(log n) time.