Java Queue interface

In Java Queue can be created by simply using the Queue interface in java.uitl. package
The Queue interface is available in java.util package and extends the Collection interface. The queue collection is used to hold the elements about to be processed and provides various operations like the insertion, removal etc. It is an ordered list of objects with its use limited to insert elements at the end of the list and deleting elements from the start of list i.e. it follows the FIFO or the First-In-First-Out principle.

Methods in Queue:

1. add()- This method is used to add elements at the tail of queue. More specifically, at the last of linked-list if it is used, or according to the priority in case of priority queue implementation.
2. peek()- This method is used to view the head of queue without removing it. It returns Null if the queue is empty.
3. element()- This method is similar to peek(). It throws NoSuchElementException when the queue is empty.
4. remove()- This method removes and returns the head of the queue. It throws NoSuchElementException when the queue is empty.
5. poll()- This method removes and returns the head of the queue. It returns null if the queue is empty.
6. size()- This method return the no. of elements in the queue.

Below is a simple Java program to demonstrate these methods:


// Java program to demonstrate working of Queue 
// interface in Java 
import java.util.LinkedList; 
import java.util.Queue; 

public class QueueExample 
{ 
   public static void main(String[] args) 
   { 
Queue<Integer> q = new LinkedList<>(); 

// Adds elements {0, 1, 2, 3, 4} to queue 
for (int i=0; i<5; i++) 
q.add(i); 

// Display contents of the queue. 
System.out.println("Elements of queue-"+q); 

// To remove the head of queue. 
int removedele = q.remove(); 
System.out.println("removed element-" + removedele); 

System.out.println(q); 

// To view the head of queue 
int head = q.peek(); 
System.out.println("head of queue-" + head); 

// Rest all methods of collection interface, 
// Like size and contains can be used with this 
// implementation. 
int size = q.size(); 
System.out.println("Size of queue-" + size); 
   } 

} 

Output:
Elements of queue-[0, 1, 2, 3, 4]
removed element-0
[1, 2, 3, 4]
head of queue-1

Size of queue-4