Queue Data Structure Java | Fresher Interview

Both a queue and a stack store elements with O of one add and remove, and the fundamental difference is which end you remove from. A queue is first in, first out, FIFO. You add at the back, called enqueue, and you remove from the front, called dequeue, so the element that's been waiting longest comes out first. The everyday analogy is a checkout line: first come, first served. A stack is last in, first out, LIFO. You add and remove from the same end, the top, so the most recently added element comes out first, like a stack of plates. That ordering difference is what determines where you use each. A queue models fair, in-order processing, so it's the natural structure for breadth-first search, where you process nodes level by level, and for task scheduling, message buffers, and producer-consumer pipelines. A stack models most-recent-first behavior, so it's used for depth-first search and backtracking, for undo and redo, for the program's call stack, and for parsing expressions like matching parentheses. In Java, the practical advice is to use ArrayDeque for both. If I call push and pop I get stack semantics, and if I call offer and poll I get queue semantics. I avoid the legacy Stack class because it's synchronized and slower, and I avoid LinkedList as a queue because ArrayDeque is faster and cleaner. And if I need to add and remove at both ends, a Deque, a double-ended queue, gives me that flexibility.