What is PriorityQueue in Java?

A PriorityQueue is a queue where elements are ordered by their natural ordering (if they implement Comparable) or by a provided Comparator, not by insertion order. The head of the queue is always the smallest element (min-heap by default). Operations: offer() adds an element (O(log n)), poll() removes and returns the head (O(log n)), peek() returns the head without removing (O(1)). PriorityQueue does not permit null elements. Iteration over a PriorityQueue does not guarantee any particular order — only the head is guaranteed to be the minimum. Use it for scheduling algorithms, Dijkstra's shortest path, and any problem requiring "process the most important item first" semantics.