In this chapter you will learn:

1. What is Deque in Java?
2. Declaration of Deque
3. Programming Example

Keyword Deque stands for “Double Ended Queue” which allows insertion and removal of both sides. You can add and remove element both sides as head and tail. However Deque has not size limit but using interface you can restrict its size limit. There are some special methods which allow element’s insertion and removal in both sides. Deque can also be used as Stack. It can behave like LIFO and FIFO both.

Methods

Compare Methods (Queue Vs. Deque)

Deque d=new LinkedList();

Programming Example

import java.util.*;
class DeQue_Example
{
  public static void main(String[] args)
  {
    Deque d=new LinkedList();
    
    d.addLast("Tail1");
    d.addLast("Tail2");
    d.addFirst("Head1");
    d.addFirst("Head2");
    
    //Print Deque using Iterator
    Iterator itr=d.iterator();
    while(itr.hasNext())
    {
      System.out.print(itr.next() + " ");
    }
    System.out.println("\nPeek Last Value : "+d.peekLast());
    System.out.println("Remove Last Value : "+d.pollLast());
    //Adding new value to last
    d.addLast("Hello");
    System.out.println("printing : "+d);
  }
}

Output

Summary

In this program you can clearly see that Deque allows insertion and removal of elements both sides. It has some special methods that allows programmer to use deque as stack. In the next chapter you will learn about ArrayDeque Class.