The peek() method of ArrayBlockingQueue returns the head element of the queue. It retrieves but does not remove the element. It returns null if the queue is empty.

Syntax:

public E peek();

Program to Retrieve Element using the Peek Method

Let’s have a look at how to use the peek() method to retrieve the head element from the queue.

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;

public class ArrayBlockingQueuePeekExample {

        public static void main(String[] args) throws InterruptedException {

                int capacity = 5;
                BlockingQueue salQue = new ArrayBlockingQueue<>(capacity);
                salQue.add(20000);
                salQue.add(28000);
                salQue.add(12000);
                salQue.add(19000);
                salQue.add(29000);

                System.out.println("Queue elements: \n" + salQue);
                System.out.println("Head of the queue is: " + salQue.peek());
        }
}

Output:

Queue elements: 
[20000, 28000, 12000, 19000, 29000]
Head of the queue is: 20000

User-defined Object Example

The below example shows how to use the peek() method with the user-defined object.

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;

class Employee {
        private int employeeId;
        private String employeeName;

        public Employee() {
        }

        public Employee(int employeeId, String employeeName) {
                super();
                this.employeeId = employeeId;
                this.employeeName = employeeName;
        }

        @Override
        public String toString() {
                return "Employee [employeeId=" + employeeId + ", employeeName=" + employeeName + "]";
        }
}

public class PeekDemo {

        public static void main(String[] args) {

                int capacity = 5;
                BlockingQueue employeesQueue = new ArrayBlockingQueue<>(capacity);
                employeesQueue.add(new Employee(101, "John"));
                employeesQueue.add(new Employee(102, "Adam"));
                employeesQueue.add(new Employee(103, "Ricky"));
                employeesQueue.add(new Employee(104, "Shaun"));
                employeesQueue.add(new Employee(105, "Chris"));

                System.out.println("Queue elements: ");
                for (Employee employee : employeesQueue) {
                        System.out.println(employee);
                }
                System.out.println("Head of the queue is: " + employeesQueue.peek());
        }
}

Output:

Queue elements: 
Employee [employeeId=101, employeeName=John]
Employee [employeeId=102, employeeName=Adam]
Employee [employeeId=103, employeeName=Ricky]
Employee [employeeId=104, employeeName=Shaun]
Employee [employeeId=105, employeeName=Chris]
Head of the queue is: Employee [employeeId=101, employeeName=John]

Peek when ArrayBlockingQueue is Empty

If the queue is empty then peek returns null. Let’s have a look at the program.

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;

public class PeekEmptyQueueDemo {

        public static void main(String[] args) throws InterruptedException {

                int capacity = 5;
                BlockingQueue numbers = new ArrayBlockingQueue<>(capacity);
                numbers.add(25000);
                numbers.add(12000);
                numbers.add(45000);
                numbers.add(19000);
                numbers.add(70000);

                System.out.println("Queue elements: \n" + numbers);
                System.out.println("Head of the queue is: " + numbers.peek());

                numbers.clear();
                System.out.println("Queue after removing all elements: \n" + numbers);
                System.out.println("Head of the queue is: " + numbers.peek());
        }
}

Output:

Queue elements: 
[25000, 12000, 45000, 19000, 70000]
Head of the queue is: 25000
Queue after removing all elements: 
[]
Head of the queue is: null

That’s all about the peek() method of ArrayBlockingQueue. Please share if you find this article helpful.