AbstractQueue in Java with Examples

AbstractQueue is a part of the Java Collection Framework that provides a skeletal implementation of the Queue interface for building queue-based data structures.

• Offers default methods like add(), remove(), element() using offer(), poll(), peek()
• Does not support blocking operations (unlike BlockingQueue)
• Ordering depends on implementation (FIFO or priority-based)

AbstractQueue : [1, 2, 3]

Explanation: Elements are inserted into the queue using the add() method and finally, the queue is printed to display the stored elements.

Hierarchy of AbstractQueue

This class extends AbstractCollection and implements the Queue interface. Since Queue extends Collection and Collection extends Iterable, it indirectly supports all their methods.

Implementation Classes of AbstractQueue

• ArrayBlockingQueue: A bounded blocking queue backed by an array with fixed capacity.
• ConcurrentLinkedQueue: A thread-safe, non-blocking queue based on linked nodes.
• DelayQueue: A queue where elements can only be taken after their specified delay has expired.
• LinkedBlockingDeque: A blocking double-ended queue that allows insertion and removal from both ends.
• LinkedBlockingQueue: A thread-safe blocking queue based on linked nodes with optional capacity limit.
• LinkedTransferQueue: A concurrent queue that allows producers to transfer elements directly to consumers.
• PriorityBlockingQueue: A thread-safe blocking queue where elements are ordered based on priority.
• PriorityQueue: A non-thread-safe queue where elements are ordered according to their priority (min-heap by default).

Declaration of AbstractQueue

public abstract class AbstractQueue<E> extends AbstractCollection<E>
implements Queue<E> 

Constructors in Java AbstractQueue

• protected AbstractQueue() – Default constructor of the AbstractQueue class.
• Since AbstractQueue is an abstract class, it cannot be instantiated directly.
• Objects must be created using its subclasses such as LinkedBlockingQueue, PriorityQueue, or ArrayBlockingQueue.

AbstractQueue<E> objName = new ArrayBlockingQueue<E>();

AbstractQueue contains: [10, 20, 30, 40, 50]

Basic Operations

1. Adding Elements

• add(E e)– Inserts an element into the queue. Returns true if successful; throws IllegalStateException if the queue has no space.
• addAll(Collection<? extends E> c)– Adds all elements from the specified collection to the queue.

AbstractQueue contains : [10, 20, 30, 40, 50]
AbstractQueue2 initially contains : []
AbstractQueue1 after addition contains : [10, 20, 30, 40, 50]

2. Remove the Elements 

• The remove() method returns and removes the head of this queue.
• The clear() method removes all the elements from this queue. The queue will be empty after this call returns.

AbstractQueue1 contains : [10, 20, 30, 40, 50]
head : 10
AbstractQueue1 after removal of head : [20, 30, 40, 50]
AbstractQueue1 : []

3. Accessing the Elements

The element() method of AbstractQueue retrieves but does not remove, the head of this queue.

AbstractQueue1 contains : [10, 20, 30, 40, 50]
head : 10

Methods of AbstractQueue

Methods declared in class java.util.AbstractCollection

Methods declared in interface java.util.Collection

Methods declared in interface java.lang.Iterable

Methods declared in interface java.util.Queue