Queue in Java

Queue is a collection interface for elements waiting to be processed. The head is the element that will be removed next. Many queues use first-in, first-out order, but the ordering rule depends on the implementation.

Why Do We Need This?

A help desk can process requests in arrival order. Other tasks may need a priority rule so the most urgent item is selected first. Both can use queues, but they need different implementations.

Main Ideas

Simple Working Example

Run this program on its own. All required imports and classes are included.

import java.util.ArrayDeque;
import java.util.Queue;
 
public class HelloWorld {
    public static void main(String[] args) {
        Queue<String> requests = new ArrayDeque<>();
        requests.offer("Print notes");
        requests.offer("Print exercise");
        System.out.println("Next: " + requests.peek());
        System.out.println("Processing: " + requests.poll());
        System.out.println("Remaining: " + requests.poll());
        System.out.println("Empty result: " + requests.poll());
    }
}

Output

Next: Print notes
Processing: Print notes
Remaining: Print exercise
Empty result: null

ArrayDeque behaves as a FIFO queue through these operations. peek leaves the first request in place; poll removes it. This implementation does not allow null elements and is not thread-safe.

Topics in This Section

• Queue Interface
• PriorityQueue
• Deque

Easy Way to Remember

Do not assume every queue is FIFO. Check the head-selection rule, empty behavior, capacity, and thread-safety requirements.