The Queue interface present in the java.util package and extends the Collection interface is used to hold the elements about to be processed in FIFO(First In First Out) order. It is an ordered list of objects with its use limited to insert elements at the end of the list and deleting elements from the start of the list, (i.e.), it follows the FIFO or the First-In-First-Out principle.
Being an interface the queue needs a concrete class for the declaration and the most common classes are the PriorityQueue and LinkedList in Java.It is to be noted that both the implementations are not thread safe. PriorityBlockingQueue is one alternative implementation if thread safe implementation is needed.
Declaration: The Queue interface is declared as:
public interface Queue extends Collection
Creating Queue Objects
Since Queue is an interface, objects cannot be created of the type queue. We always need a class which extends this list in order to create an object. And also, after the introduction of Generics in Java 1.5, it is possible to restrict the type of object that can be stored in the Queue. This type-safe queue can be defined as:
// Obj is the type of the object to be stored in Queue
Queue<Obj> queue = new PriorityQueue<Obj> ();
Example of a Queue:
Elements of queue [0, 1, 2, 3, 4] removed element-0 [1, 2, 3, 4] head of queue-1 Size of queue-4
Operations on Queue Interface
Let’s see how to perform a few frequently used operations on the queue using the Priority Queue class.
1. Adding Elements: In order to add an element in a queue, we can use the add() method. The insertion order is not retained in the PriorityQueue. The elements are stored based on the priority order which is ascending by default.
[For, Geeks, Geeks]
2. Removing Elements: In order to remove an element from a queue, we can use the remove() method. If there are multiple such objects, then the first occurrence of the object is removed. Apart from that, poll() method is also used to remove the head and return it.
Initial Queue [For, Geeks, Geeks] After Remove [For, Geeks] Poll Method For Final Queue [Geeks]
3. Iterating the Queue: There are multiple ways to iterate through the Queue. The most famous way is converting the queue to the array and traversing using the for loop. However, the queue also has an inbuilt iterator which can be used to iterate through the queue.
For Geeks Geeks
Characteristics of a Queue: The following are the characteristics of the queue:
- The Queue is used to insert elements at the end of the queue and removes from the beginning of the queue. It follows FIFO concept.
- The Java Queue supports all methods of Collection interface including insertion, deletion etc.
- LinkedList, ArrayBlockingQueue and PriorityQueue are the most frequently used implementations.
- If any null operation is performed on BlockingQueues, NullPointerException is thrown.
- The Queues which are available in java.util package are Unbounded Queues.
- The Queues which are available in java.util.concurrent package are the Bounded Queues.
- All Queues except the Deques supports insertion and removal at the tail and head of the queue respectively. The Deques support element insertion and removal at both ends.
Classes which implement the Queue Interface:
1. PriorityQueue: PriorityQueue class which is implemented in the collection framework provides us a way to process the objects based on the priority. It is known that a queue follows First-In-First-Out algorithm, but sometimes the elements of the queue are needed to be processed according to the priority, that’s when the PriorityQueue comes into play. Let’s see how to create a queue object using this class.
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2. LinkedList: LinkedList is a class which is implemented in the collection framework which inherently implements the linked list data structure. It is a linear data structure where the elements are not stored in contiguous locations and every element is a separate object with a data part and address part. The elements are linked using pointers and addresses. Each element is known as a node. Due to the dynamicity and ease of insertions and deletions, they are preferred over the arrays or queues. Let’s see how to create a queue object using this class.
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3. PriorityBlockingQueue: It is to be noted that both the implementations, the PriorityQueue and LinkedList are not thread-safe. PriorityBlockingQueue is one alternative implementation if thread-safe implementation is needed. PriorityBlockingQueue is an unbounded blocking queue that uses the same ordering rules as class PriorityQueue and supplies blocking retrieval operations.
Since it is unbounded, adding elements may sometimes fail due to resource exhaustion resulting in OutOfMemoryError. Let’s see how to create a queue object using this class.
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Methods of Queue Interface
The queue interface inherits all the methods present in the collections interface while implementing the following methods:
|add(element)||This method is used to add elements at the tail of queue. More specifically, at the last of linked-list if it is used, or according to the priority in case of priority queue implementation.|
|element()||This method is similar to peek(). It throws NoSuchElementException when the queue is empty.|
|offer(element)||This method is used to insert an element in the queue. This method is preferable to add() method since this method does not throws an exception when the capacity of the container is full since it returns false.|
|peek()||This method is used to view the head of queue without removing it. It returns Null if the queue is empty.|
|poll()||This method removes and returns the head of the queue. It returns null if the queue is empty.|
|remove()||This method removes and returns the head of the queue. It throws NoSuchElementException when the queue is empty.|
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