ArrayBlockingQueue poll() Method in Java

ArrayBlockingQueue is bounded, blocking queue that stores the elements internally backed by an array.

• ArrayBlockingQueue class is a member of the Java Collections Framework.
• Bounded means it will have a fixed size, you can not store number the elements more than the capacity of the queue.
• The queue also follows FIFO (first-in-first-out) rule for storing and removing elements from the queue.
• If you try to put an element into a full queue or to take an element from an empty queue then the queue will block you.

There are two types of poll() method depending upon no of parameter passed.

1. The poll() method retrieves and removes element from head of this queue.If queue is empty then method will return null.
   Syntax:

   public E poll()

   Return Value: The method returns the element from the head of this queue, or null if this queue is empty.

   Below programs illustrate poll() method of ArrayBlockingQueue.

   Program 1:

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   /* *Program Demonstrate poll() method of ArrayBlockingQueue. */    import java.util.concurrent.ArrayBlockingQueue;    public class GFG {        public static void main(String[] args)     {         // define capacity of ArrayBlockingQueue         int capacity = 5;            // create object of ArrayBlockingQueue         ArrayBlockingQueue<Integer> queue = new                 ArrayBlockingQueue<Integer>(capacity);            // Add elements to ArrayBlockingQueue         queue.offer(423);         queue.offer(233);         queue.offer(356);            // print elements         System.out.println("Queue Contains" + queue);            // try to poll  elements         System.out.println("Removing From head: " +                                         queue.poll());         System.out.println("Queue Contains" + queue);         System.out.println("Removing From head: " +                                         queue.poll());         System.out.println("Queue Contains" + queue);         System.out.println("Removing From head: " +                                         queue.poll());         System.out.println("Queue Contains" + queue);         System.out.println("Removing From head: " +                                         queue.poll());         System.out.println("Queue Contains" + queue);     } }

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   Output:

   Queue Contains[423, 233, 356]
   Removing From head: 423
   Queue Contains[233, 356]
   Removing From head: 233
   Queue Contains[356]
   Removing From head: 356
   Queue Contains[]
   Removing From head: null
   Queue Contains[]
   

   Program 2:

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   /* * Program Demonstrate poll() method of ArrayBlockingQueue. */    import java.util.concurrent.ArrayBlockingQueue;    public class GFG {        // Create a User Object with name and age as an attribute     public class User {            public String name;         public String age;         User(String name, String age)         {             this.name = name;             this.age = age;         }     }        // Main Method     public static void main(String[] args)     {         GFG gfg = new GFG();         gfg.pollMethodExample();     }        // Method to give example of contains function     public void pollMethodExample()     {            // Define the capacity of ArrayBlockingQueue         int capacity = 5;            // Create object of ArrayBlockingQueue         ArrayBlockingQueue<User> queue =                 new ArrayBlockingQueue<User>(capacity);            // Create user objects         User user1 = new User("Aman", "24");         User user3 = new User("Sanjeet", "25");            // Add Objects to ArrayBlockingQueue         queue.offer(user1);         queue.offer(user3);            // Poll users from queue         User user = queue.poll();         System.out.println("removing user having name = "                                             + user.name);         user = queue.poll();         System.out.println("removing user having name = "                                             + user.name);            // Now queue is empty         // Try to remove it will return null         user = queue.poll();         System.out.println("removing user having name = "                                                  + user);     } }

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   Output:

   removing user having name = Aman
   removing user having name = Sanjeet
   removing user having name = null
   

2. The poll(long timeout, TimeUnit unit) method retrieves and removes element from head of this queue. If the queue is empty then it will, wait till a specified time for an element to become available.
   Syntax:

   public E poll(long timeout, TimeUnit unit) throws InterruptedException

   Parameters: The method takes two parameters:

   • timeout (long) – how long to wait before giving up, in units of unit.
   • unit (TimeUnit)- a TimeUnit determining how to interpret the timeout parameter.

   Return Value: The method returns the head of this queue, or null if the specified waiting time elapses before an element is available.
   Exception: The method throws InterruptedException if interrupted while waiting.

   Below program illustrates poll(long timeout, TimeUnit unit)method of ArrayBlockingQueue.

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   /* * Program Demonstrate offer(E e, long timeout, TimeUnit unit) * method of ArrayBlockingQueue. */    import java.util.concurrent.ArrayBlockingQueue; import java.util.concurrent.TimeUnit;    public class GFG {        public static void main(String[] args)                         throws InterruptedException     {         // Define capacity of ArrayBlockingQueue         int capacity = 5;            // Create object of ArrayBlockingQueue         ArrayBlockingQueue<Integer> queue =              new ArrayBlockingQueue<Integer>(capacity);            // Add elements to ArrayBlockingQueue         queue.offer(423);         queue.offer(233);         queue.offer(356);            // Print elements         System.out.println("Queue Contains" + queue);            // Try to poll  elements         System.out.println("Removing From head: "                    + queue.poll(10, TimeUnit.SECONDS));         System.out.println("Queue Contains" + queue);         System.out.println("Removing From head: "                    + queue.poll(10, TimeUnit.SECONDS));         System.out.println("Queue Contains" + queue);         System.out.println("Removing From head: " +                      queue.poll(10, TimeUnit.SECONDS));         System.out.println("Queue Contains" + queue);         System.out.println("Removing From head: " +                     queue.poll(10, TimeUnit.SECONDS));     } }

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   Output:

   Queue Contains[423, 233, 356]
   Removing From head: 423
   Queue Contains[233, 356]
   Removing From head: 233
   Queue Contains[356]
   Removing From head: 356
   Queue Contains[]
   Removing From head: null
   

Reference:
https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ArrayBlockingQueue.html#poll()

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