Java - Queue peek() Method

Description

The Java Queue peek() method retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.

Declaration

Following is the declaration for java.util.Queue.peek() method

public E peek()

Parameters

NA

Return Value

This method returns the head of the list represented by this queue, or null if this queue is empty.

Exception

NA

Example 1

The following example shows the usage of Java Queue peek() method with Integers. We're creating an LinkedList of Integers, adding some elements, print it and then use peek() method to get the first element. Queue remains untouched.

package com.tutorialspoint;

import java.util.LinkedList;
import java.util.Queue;

public class QueueDemo {
   public static void main(String[] args) {
      
      // create an empty queue
      Queue<Integer> queue = new LinkedList<>();

      // use add() method to add elements in the queue
      queue.add(25);
      queue.add(30);
      queue.add(20);
      queue.add(18);        

      // let us print all the elements available in queue
      System.out.println("Queue = " + queue);

      // it will retrieve first element
      System.out.println("Retrieved Element is = " + queue.peek());
	  
      // let us print all the elements available in queue again
      System.out.println("Queue = " + queue);
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Queue = [25, 30, 20, 18]
Retrieved Element is = 25
Queue = [25, 30, 20, 18]

Example 2

The following example shows the usage of Java Queue peek() method with Strings. We're creating an LinkedList of Strings, adding some elements, print it and then use peek() method to get the first element. Queue remains untouched.

package com.tutorialspoint;

import java.util.LinkedList;
import java.util.Queue;

public class QueueDemo {
   public static void main(String[] args) {
      
      // create an empty queue
      Queue<String> queue = new LinkedList<>();

      // use add() method to add elements in the queue
      queue.add("A");
      queue.add("B");
      queue.add("C");
      queue.add("D");        

      // let us print all the elements available in queue
      System.out.println("Queue = " + queue);

      // it will retrieve first element
      System.out.println("Retrieved Element is = " + queue.peek());
	  
      // let us print all the elements available in queue again
      System.out.println("Queue = " + queue);
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Queue = [A, B, C, D]
Retrieved Element is = A
Queue = [A, B, C, D]

Example 3

The following example shows the usage of Java Queue peek() method with Student objects. We're creating an LinkedList of Student objects, adding some students, print it and then use peek() method to get the first student. Queue remains untouched.

package com.tutorialspoint;

import java.util.LinkedList;
import java.util.Queue;

public class QueueDemo {
   public static void main(String[] args) {
      
      // create an empty queue
      Queue<Student> queue = new LinkedList<>();

      // use add() method to add elements in the queue
      queue.add(new Student(1, "Julie"));
      queue.add(new Student(2, "Robert"));
      queue.add(new Student(3, "Adam"));       

      // let us print all the elements available in queue
      System.out.println("Queue = " + queue);

      // it will retrieve first element
      System.out.println("Retrieved Element is = " + queue.peek());
	  
      // let us print all the elements available in queue again
      System.out.println("Queue = " + queue);
   }
}
class Student {
   int rollNo;
   String name;

   Student(int rollNo, String name){
      this.rollNo = rollNo;
      this.name = name;
   }

   @Override
   public String toString() {
      return "[ " + this.rollNo + ", " + this.name + " ]";
   }
   
   @Override
   public boolean equals(Object obj) {
      Student s = (Student)obj;
      return this.rollNo == s.rollNo && this.name.equalsIgnoreCase(s.name);
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Queue = [[ 1, Julie ], [ 2, Robert ], [ 3, Adam ]]
Retrieved Element is = [ 1, Julie ]
Queue = [[ 1, Julie ], [ 2, Robert ], [ 3, Adam ]]