Java – Queue Example

Queue interface is in java.util package of java. Queue works like first in first out (FIFO) policy. Queue does not require any fix dimension like String array and int array. Queue contains many useful methods. To add element in Queue, we can use add() method of Queue interface.

To get value from Queue, Queue provides poll() and peek() method and Queue size() method. Size() method returns total number of elements in Queue. peek() method looks at the object at the head of this Queue without removing it from the Queue

Queue Example

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

public class QueueExample {

    public static void main(String[] args) {

        Queue<String> qe=new LinkedList<String>();

        qe.add("b");
        qe.add("a");
        qe.add("c");
        qe.add("e");
        qe.add("d");

        Iterator it=qe.iterator();

        System.out.println("Initial Size of Queue :"+qe.size());

        while(it.hasNext())
        {
            String iteratorValue=(String)it.next();
            System.out.println("Queue Next Value :"+iteratorValue);
        }

        // get value and does not remove element from queue
        System.out.println("Queue peek :"+qe.peek());

        // get first value and remove that object from queue
        System.out.println("Queue poll :"+qe.poll());

        System.out.println("Final Size of Queue :"+qe.size());
    }
}

Output

Initial Size of Queue :5
Queue Next Value :b
Queue Next Value :a
Queue Next Value :c
Queue Next Value :e
Queue Next Value :d
Queue peek :b
Queue poll :b
Final Size of Queue :4

Tags: Collections

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9 Responses to “Java – Queue Example”

gr says:

May 18, 2010 at 2:47 am

where is the implementation of the poll() and peek() methods?
gr says:

May 18, 2010 at 2:51 am

I got it now, the implementation is in the imported package: import java.util.Queue;
Don Bosco Antony says:

July 9, 2010 at 11:25 am

peek()
Retrieves, but does not remove, the head (first element) of this list

poll()
Retrieves and removes the head (first element) of this list.
Karina says:

January 27, 2011 at 8:24 am

thx a lot! this is so helpfull
Jhonn Allan says:

January 29, 2011 at 6:35 am

guysss can you help me . the user will input the item .can you help me to how to do that .. ? plssssssss ??reply with this ss
Malith says:

February 16, 2011 at 11:25 am

just use the this to add user input to the queue..
in this case its.

qe.add(the variable of users input);

for example..
qe.add(name);
where the name stores the value entered by the user..
Abu Dhachi says:

September 2, 2011 at 7:46 am

C:\QueueExample.java:9: ‘(’ or ‘[’ expected
Queue qe=new LinkedList();
^
1 error

Process completed.

This is my output….
What did I do wrong?
shei says:

October 11, 2011 at 9:47 pm

is it possible to display all the inputed items on the queue list? example, I inputted numbers 1 2 3 and 6 and placed it on queue – ithen i will print THE NUMBERS ON QUEUE are 1 2 3 6? is this possible? If yes.. HOW? please need some help.. ><
Noura says:

October 14, 2011 at 1:47 am

why it should be implemented by LinkedList