Queue & Deque

Syntax

Queue<Type> queue = new LinkedList<>();\nDeque<Type> deque = new ArrayDeque<>();

Examples

import java.util.LinkedList;
import java.util.Queue;

Queue<String> line = new LinkedList<>();
line.offer("Customer A");
line.offer("Customer B");
line.offer("Customer C");

System.out.println(line.poll());  // Customer A - first in, first out
System.out.println(line.peek());   // Customer B - look without removing
System.out.println(line);          // [Customer B, Customer C]

import java.util.ArrayDeque;
import java.util.Deque;

Deque<String> stack = new ArrayDeque<>();
stack.push("first");
stack.push("second");
stack.push("third");

System.out.println(stack.pop());  // third - last in, first out
System.out.println(stack.pop());  // second

import java.util.ArrayDeque;
import java.util.Deque;

Deque<Integer> deque = new ArrayDeque<>();
deque.addFirst(2);
deque.addLast(3);
deque.addFirst(1);
deque.addLast(4);

System.out.println(deque);  // [1, 2, 3, 4]

System.out.println(deque.removeFirst());  // 1
System.out.println(deque.removeLast());    // 4
System.out.println(deque);                  // [2, 3]

import java.util.ArrayDeque;
import java.util.Queue;

Queue<Integer> queue = new ArrayDeque<>();

queue.offer(1);  // returns false on failure, instead of throwing
boolean added = queue.offer(2);
System.out.println(added);  // true

Integer result = queue.poll();  // returns null if empty, instead of throwing
System.out.println(result);      // 1

// On an empty queue:
Queue<Integer> empty = new ArrayDeque<>();
System.out.println(empty.poll());  // null, no exception
// empty.remove();  // would throw NoSuchElementException instead

Best practices

• Prefer ArrayDeque over the legacy Stack and Vector classes, and over LinkedList, for both stack and queue use cases - it is faster and more modern
• Use offer()/poll()/peek() (the safe, non-throwing methods) in most cases, reserving add()/remove()/element() for situations where a failure genuinely indicates a bug
• Use a Deque when you need flexibility to add/remove from both ends, and a plain Queue interface when you only need strict FIFO behavior
• Check for null after poll()/peek() on an empty collection, since these methods return null rather than throwing in that case

At a glance

Purpose

General-purpose application development

File extension

.java

Runs in

Java Virtual Machine

Usually used with

JDK and Java libraries

Specifications & further reading

• Java Language Specification

Related Java documentation