Queue peek() method in Java
Last Updated :
26 Sep, 2018
The
peek() method of
Queue Interface returns the element at the front the container. It does not deletes the element in the container. This method returns the head of the queue. The method does not throws an exception when the Queue is empty, it returns
null instead.
Syntax:
E peek()
Returns: This method returns the
head of the Queue, it returns false when the Queue is empty
Below programs illustrate peek() method of Queue:
Program 1: With the help of
LinkedList.
Java
// Java Program Demonstrate peek()
// method of Queue
import java.util.*;
public class GFG {
public static void main(String[] args)
throws IllegalStateException
{
// create object of Queue
Queue<Integer> Q
= new LinkedList<Integer>();
// Add numbers to end of Queue
Q.add(7855642);
Q.add(35658786);
Q.add(5278367);
Q.add(74381793);
// print queue
System.out.println("Queue: " + Q);
// print head
System.out.println("Queue's head: " + Q.peek());
// print queue
System.out.println("Queue: " + Q);
}
}
Output:
Queue: [7855642, 35658786, 5278367, 74381793]
Queue's head: 7855642
Queue: [7855642, 35658786, 5278367, 74381793]
Program 2: To demonstrate peek() method of Queue when Queue is empty
Java
// Java Program Demonstrate peek()
// method of Queue when Queue is empty
import java.util.*;
public class GFG {
public static void main(String[] args)
throws IllegalStateException
{
// create object of Queue
Queue<Integer> Q
= new LinkedList<Integer>();
// print queue
System.out.println("Queue: " + Q);
// print head
System.out.println("Queue's head: " + Q.peek());
}
}
Output:
Queue: []
Queue's head: null
Program 3: With the help of
ArrayDeque.
Java
// Java Program Demonstrate peek()
// method of Queue
import java.util.*;
public class GFG {
public static void main(String[] args)
throws IllegalStateException
{
// create object of Queue
Queue<Integer> Q
= new ArrayDeque<Integer>();
// Add numbers to end of Queue
Q.add(7855642);
Q.add(35658786);
Q.add(5278367);
Q.add(74381793);
// print queue
System.out.println("Queue: " + Q);
// print head
System.out.println("Queue's head: " + Q.peek());
}
}
Output:
Queue: [7855642, 35658786, 5278367, 74381793]
Queue's head: 7855642
Program 4: With the help of
LinkedBlockingDeque.
Java
// Java Program Demonstrate peek()
// method of Queue
import java.util.*;
import java.util.concurrent.LinkedBlockingDeque;
public class GFG {
public static void main(String[] args)
throws IllegalStateException
{
// create object of Queue
Queue<Integer> Q
= new LinkedBlockingDeque<Integer>();
// Add numbers to end of Queue
Q.add(7855642);
Q.add(35658786);
Q.add(5278367);
Q.add(74381793);
// print queue
System.out.println("Queue: " + Q);
// print head
System.out.println("Queue's head: " + Q.peek());
}
}
Output:
Queue: [7855642, 35658786, 5278367, 74381793]
Queue's head: 7855642
Program 5: With the help of
ConcurrentLinkedDeque.
Java
// Java Program Demonstrate peek()
// method of Queue
import java.util.*;
import java.util.concurrent.ConcurrentLinkedDeque;
public class GFG {
public static void main(String[] args)
throws IllegalStateException
{
// create object of Queue
Queue<Integer> Q
= new ConcurrentLinkedDeque<Integer>();
// Add numbers to end of Queue
Q.add(7855642);
Q.add(35658786);
Q.add(5278367);
Q.add(74381793);
// print queue
System.out.println("Queue: " + Q);
// print head
System.out.println("Queue's head: " + Q.peek());
}
}
Output:
Queue: [7855642, 35658786, 5278367, 74381793]
Queue's head: 7855642
Reference: https://docs.oracle.com/javase/8/docs/api/java/util/Queue.html#peek--