Queue in java

What is a Queue Data Structure?

A Queue is a linear data structure that stores elements in a sequence and follows the FIFO principle (First In, First Out). This means the first element that enters the queue is the first one to be removed. It works just like a line of people waiting for a service.

Types of Queues

1. Simple/Linear Queue                 Basic FIFO queue with front and rear
2. Circular Queue                                 Last position connects to first to avoid wasted space
3. Priority Queue                                Elements removed based on priority, not FIFO
4. Deque (Double-Ended Queue) Insert/remove from both ends
5. Input-Restricted Queue                 Enqueue allowed only at one end
6. Output-Restricted Queue         Dequeue allowed only at one end
7. Double Queue                                 Special queue allowing two rear or two front operations

Advantages of Queue

• Maintains correct processing order (FIFO)
• Very efficient for task scheduling
• Helps manage real-time data flow
• Easy to implement using arrays or linked lists
• Useful in asynchronous systems (threads, I/O, networks)

Disadvantages of Queue

• Linear queues can waste memory (solved by circular queue)
• Random access is not allowed—you must dequeue sequentially
• Fixed-size array queues have limited capacity
• Can cause overflow or underflow errors if not managed properly

Common Queue Methods / Operations

1. Enqueue(x)            Add element x to the rear of the queue
2. Dequeue()            Remove and return the front element
3. Peek() / Front()    View the front element without removing it
4. isEmpty()            Returns true if the queue has no elements
5. isFull()                    For array queues, checks if queue is full
6. Size()                    Returns total number of elements in the queue
7. Clear()                    Removes all elements

Real-World Examples of Queue

• People waiting in a line
• Printer job scheduling
• Task scheduling in operating systems
• CPU process scheduling
• Network packet handling
• Customer support systems
• Ticket booking systems
• Messaging systems (e.g., Kafka, RabbitMQ)