Java Collection Tutorial - Java Double Ended Queues

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A doubly ended queue or deque extends a queue to allow insertion and removal of elements from both ends.

An instance of Deque class represents a doubly ended queue. The Deque interface extends the Queue interface.

It declares additional methods to facilitate all the operations for a queue at the head as well as at the tail. It can be used as a FIFO queue or a LIFO queue.

The ArrayDeque and LinkedList classes are two implementation classes for the Deque interface.

The ArrayDeque class is backed by an array whereas the LinkedList class is backed by a linked list.

You should use the ArrayDeque as a Deque implementation if you are using a Deque as a stack.

The LinkedList implementation performs better if you use a Deque as a FIFO queue.

The following code shows how to use a Deque as a FIFO Queue.

import java.util.Deque;
import java.util.LinkedList;
/*from   w ww.j  a va 2  s . co m*/
public class Main {
  public static void main(String[] args) {
    Deque<String> deque = new LinkedList<>();
    deque.addLast("Oracle");
    deque.offerLast("Java");
    deque.offerLast("CSS");
    deque.offerLast("XML");

    System.out.println("Deque: " + deque);

    // remove elements from the Deque until it is empty
    while (deque.peekFirst() != null) {
      System.out.println("Head  Element: " + deque.peekFirst());
      deque.removeFirst();
      System.out.println("Removed one  element from  Deque");
      System.out.println("Deque: " + deque);
    }

    // the Deque is empty. Try to call its peekFirst(),
    // getFirst(), pollFirst() and removeFirst() methods
    System.out.println("deque.isEmpty(): " + deque.isEmpty());

    System.out.println("deque.peekFirst(): " + deque.peekFirst());
    System.out.println("deque.pollFirst(): " + deque.pollFirst());

    String str = deque.getFirst();
    System.out.println("deque.getFirst(): " + str);
    str = deque.removeFirst();
    System.out.println("deque.removeFirst(): " + str);

  }
}

The code above generates the following result.

Example

The following code shows how to use a Deque as a stack (or LIFO queue).

import java.util.ArrayDeque;
import java.util.Deque;
/*from www.j a va2  s  .c  om*/
public class Main {
  public static void main(String[] args) {
    // Create a Deque and use it as stack
    Deque<String> deque = new ArrayDeque<>();
    deque.push("Oracle");
    deque.push("HTML");
    deque.push("CSS");
    deque.push("XML");

    System.out.println("Stack: " + deque);

    // remove all elements from the Deque
    while (deque.peek() != null) {
      System.out.println("Element at  top:  " + deque.peek());
      System.out.println("Popped: " + deque.pop());
      System.out.println("Stack: " + deque);
    }

    System.out.println("Stack is  empty:  " + deque.isEmpty());
  }
}

The code above generates the following result.

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