Queue peek() method in Java - GeeksforGeeks

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 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);    }}
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 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());    }}
Queue: []
Queue's head: null

Program 3: With the help of ArrayDeque.
// 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());    }}
Queue: [7855642, 35658786, 5278367, 74381793]
Queue's head: 7855642

Program 4: With the help of LinkedBlockingDeque.
// 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());    }}
Queue: [7855642, 35658786, 5278367, 74381793]
Queue's head: 7855642

Program 5: With the help of ConcurrentLinkedDeque.
// 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());    }}
Queue: [7855642, 35658786, 5278367, 74381793]
Queue's head: 7855642

Reference: https://docs.oracle.com/javase/8/docs/api/java/util/Queue.html#peek–
Java - util package
java-basics
Java-Collections
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