ArrayBlockingQueue peek() 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.

The peek() method is used to return the head of the queue. It retrieves but does not remove, the head of this queue. If the queue is empty then this method returns null.
Syntax:

public E peek()

Parameters: The method does not take any parameters.
Return Value: The method returns the element present at the head of this queue.
Below programs illustrate peek() method of ArrayBlockingQueue. 
Program 1:

Java

// Program to demonstrate peek() 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 element to ArrayBlockingQueue
    queue.add(23);
    queue.add(32);
    queue.add(45);
    queue.add(12);

    // Print queue after adding numbers
    System.out.println("After adding numbers queue is ");
    System.out.println(queue);

    // Print head of queue using peek() method
    System.out.println("Head of queue "+queue.peek());

}
}

Output:

After adding numbers queue is 
[23, 32, 45, 12]
Head of queue 23

Program 2:

Java

// Program to demonstrate peek( ) method of ArrayBlockingQueue

import java.util.concurrent.ArrayBlockingQueue;

public class GFG {

    // Create a User Object with name and age as the 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.offerExample();

    }

    // Method to give example of offer function
    public void offerExample() {

        // Define 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 user2=new User("Amar","23");
            User user3=new User("Sanjeet","25");
            User user4=new User("Suvo","26");
            User user5=new User("Ravi","22");

            // Add Objects to ArrayBlockingQueue
            queue.offer(user1);
            queue.offer(user2);
            queue.offer(user3);
            queue.offer(user4);
            queue.offer(user5);

            // Find peek of queue
           User head=queue.peek();

            // Print head
            System.out.println("Details of First User Inserted");
            System.out.println("User Name : "+head.name);
            System.out.println("User Age : "+head.age);
    }
}

Output:

Details of First User Inserted
User Name : Aman
User Age : 24

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