Queue and Deque in Java

Introduction

In Java, the Queue and Deque interfaces are part of the java.util package. These interfaces provide ways to manage collections in a first-in-first-out (FIFO) or double-ended manner.

Queue: A collection designed for holding elements prior to processing. It follows FIFO order.

Deque: A double-ended queue that allows elements to be added or removed from both ends.

1. Queue

A Queue in Java is typically implemented using classes like LinkedList or PriorityQueue. Below is an example using LinkedList.

Example: Using Queue

    import java.util.LinkedList;
    import java.util.Queue;
    
    public class QueueExample {
        public static void main(String[] args) {
            // Create a Queue
            Queue queue = new LinkedList<>();
    
            // Add elements to the Queue
            queue.add("A");
            queue.add("B");
            queue.add("C");
    
            // Display the Queue
            System.out.println("Queue: " + queue);
    
            // Peek at the head of the Queue
            System.out.println("Head of Queue: " + queue.peek());
    
            // Remove an element from the Queue
            System.out.println("Removed: " + queue.poll());
    
            // Display the Queue after removal
            System.out.println("Queue after removal: " + queue);
        }
    }
        

2. Deque

A Deque (double-ended queue) can be implemented using classes like LinkedList or ArrayDeque. Below is an example using ArrayDeque.

Example: Using Deque

    import java.util.ArrayDeque;
    import java.util.Deque;
    
    public class DequeExample {
        public static void main(String[] args) {
            // Create a Deque
            Deque deque = new ArrayDeque<>();
    
            // Add elements to the front and end
            deque.addFirst("Front");
            deque.addLast("End");
            deque.addFirst("New Front");
    
            // Display the Deque
            System.out.println("Deque: " + deque);
    
            // Peek at elements
            System.out.println("First Element: " + deque.peekFirst());
            System.out.println("Last Element: " + deque.peekLast());
    
            // Remove elements from front and end
            System.out.println("Removed from Front: " + deque.pollFirst());
            System.out.println("Removed from End: " + deque.pollLast());
    
            // Display the Deque after removals
            System.out.println("Deque after removal: " + deque);
        }
    }
        

Differences Between Queue and Deque

Conclusion

Both Queue and Deque are useful in different scenarios:

• Queue: Use when you need FIFO operations.
• Deque: Use when you need operations at both ends.