Queue

tags: Collection, Data Structures

What is the Queue interface in Java?

• Queue is a subinterface of Collection that represents a FIFO (First-In-First-Out) data structure.
• It is used for holding elements before processing them.

What are the key features of Queue?

• Follows FIFO order (unless it’s a PriorityQueue)
• Supports adding at the tail and removing from the head
• Often used in scenarios like task scheduling, order processing, and message queues

Which classes implement the Queue interface?

• LinkedList
• PriorityQueue
• ArrayDeque
• ConcurrentLinkedQueue
• LinkedBlockingQueue
• DelayQueue (from java.util.concurrent)

What are the common methods in the Queue interface?

Method	Description
add(E e)	Adds element to the queue, throws exception if fails
offer(E e)	Adds element, returns false if fails
remove()	Removes and returns the head, throws exception if empty
poll()	Removes and returns head, returns null if empty
element()	Returns head without removing, throws exception if empty
peek()	Returns head without removing, returns null if empty

Which implementation of Queue should you use for general-purpose use?

• LinkedList if you need a simple Queue (it implements both List and Queue)
• ArrayDeque for efficient non-blocking FIFO/stack operations
• PriorityQueue for sorted elements
• ConcurrentLinkedQueue for thread-safe operations

What is PriorityQueue?

A PriorityQueue is a queue where elements are ordered based on priority.
It does not follow FIFO; the element with the highest priority (lowest value) comes out first.

Queue<Integer> pq = new PriorityQueue<>();
pq.add(10);
pq.add(5);
pq.add(20);
poll() //returns 5

Does PriorityQueue allow null values?

No. PriorityQueue does not allow null elements.
Adding null will throw NullPointerException.

Is Queue thread-safe?

• Basic implementations like LinkedList, ArrayDeque and PriorityQueue are not thread-safe.
• Use ConcurrentLinkedQueue, LinkedBlockingQueue, or other concurrent implementations for thread safety.

What is Deque in Java?

Deque stands for Double-Ended Queue.
It allows elements to be added or removed from both ends.

Implementations:

• ArrayDeque (non-thread-safe, efficient)
• LinkedBlockingDeque (thread-safe)

What is the difference between Queue and Deque?

Feature	Queue	Deque
Insertion	End only	Both ends
Removal	Front only	Both ends
Use Cases	FIFO tasks	Stacks, deques, double-ended logic

What is ArrayDeque?

• ArrayDeque is a resizable array implementation of Deque.
• It is faster than Stack and LinkedList for stack/queue operations.
• It does not allow null elements.

How to implement a stack using Queue or Deque?

Deque<Integer> stack = new ArrayDeque<>();
stack.push(10); // addFirst()
stack.pop();    // removeFirst()

What is BlockingQueue?

BlockingQueue is an interface that supports blocking operations for producers and consumers.

• If the queue is empty, take() will wait.
• If the queue is full, put() will wait.

Which classes implement BlockingQueue?

• LinkedBlockingQueue
• ArrayBlockingQueue
• PriorityBlockingQueue
• DelayQueue

What is the difference between ConcurrentLinkedQueue and LinkedBlockingQueue?

Feature	ConcurrentLinkedQueue	LinkedBlockingQueue
Blocking support	No	Yes
Bounded	No	Yes (optionally)
Use case	Non-blocking, high concurrency	Producer-consumer

How to safely iterate a Queue?

You can use an Iterator, but do not modify the queue structure during iteration.
For concurrent use, prefer ConcurrentLinkedQueue.

Can you store null in a Queue?

• Not recommended in most implementations
• PriorityQueue, ArrayDeque, and concurrent queues do not allow null

How to convert a Queue to a List?

Queue<String> queue = new LinkedList<>();
List<String> list = new ArrayList<>(queue);

When should you use a Queue in real applications?

• Task scheduling
• Order processing
• Message passing between threads
• Rate-limiting requests