Queue and Deque: ArrayDeque as stack and queue

Queue: first-in, first-out

Queue<E> exposes two sets of methods for each operation — one that throws on failure and one that returns a sentinel:

| Operation | Throws | Returns null/false | |---|---|---| | Insert | add(e) | offer(e) | | Remove head | remove() | poll() | | Inspect head | element() | peek() |

Prefer offer/poll/peek — they communicate failure via return value rather than an exception, which is more useful in conditional logic.

Deque: double-ended queue

Deque<E> extends Queue and adds operations at both ends. ArrayDeque is the recommended concrete class:

// Used as a QUEUE (FIFO):
Deque<String> queue = new ArrayDeque<>();
queue.offer("first");
queue.offer("second");
String head = queue.poll(); // "first"

// Used as a STACK (LIFO):
Deque<Integer> stack = new ArrayDeque<>();
stack.push(1);   // addFirst
stack.push(2);   // addFirst
int top = stack.pop();   // 2  — removeFirst
int peek = stack.peek(); // 1  — peekFirst

Prefer ArrayDeque over Stack and LinkedList:

- java.util.Stack extends Vector (a legacy synchronized class from Java 1.0). It is effectively deprecated for new code. - Using LinkedList as a queue works but ArrayDeque has better cache locality and is generally faster. - ArrayDeque is the modern, unsynchronized, unbounded alternative for both stack and queue use cases.