Deque is the abbreviation of “Double Ended Queue”. A Collection that allows us to add (or) remove elements at both ends. Deque supports total size of collection for both fixed and unspecified size limits.
Example:
import java.util.ArrayDeque;
import java.util.Iterator;
public class DequeExample
{
public static void main(String as[])
{
ArrayDeque adObj = new ArrayDeque();
//Insertion by using various methods
adObj.add("Oracle");
adObj.addFirst("DB2");
adObj.offerFirst("MySQL"); //returns boolean - true R false
adObj.offerLast("Postgres"); //returns boolean - true R false
//Retrievals
System.out.println("Retrieving First Element :" + adObj.peekFirst());
System.out.println("Retrieving Last Element :"+ adObj.peekLast());
//Removals
System.out.println("Removing First Element :"+ adObj.pollFirst());
System.out.println("Removing Last Element :"+ adObj.pollLast());
//Reverse traversal
System.out.println("Remaining Elements :");
Iterator it = adObj.descendingIterator();
while(it.hasNext())
{
System.out.println(it.next());
}
}
}
Output:
Retrieving First Element :MySQL
Retrieving Last Element :Postgres
Removing First Element :MySQL
Removing Last Element :Postgres
Remaining Elements :
Oracle
DB2
For more please visit- http://download.oracle.com/javase/6/docs/api/java/util/ArrayDeque.html
Deque implementation can be used as Stack(Last in first out ) or Queue(First in First Out). For each insertion, retrieval and removal of elements from deque there exists methods in 2 flavours. One will throw exception if it fails in an operation and another one returns status or special value for each operation.
| Operation | Special value method | Exception throwing method |
| Insertion at head | offerFirst(e) | addFirst(e) |
| Removal at head | pollFirst() | removeFirst() |
Retrieval at Head
| peekFirst() | getFirst() |
| Insertion at Tail | offerLast(e) | addLast(e) |
| Removal at Tail | pollLast() | removeLast() |
Retrieval at Tail
| peekLast() | getLast() |
Implementation of Deque doesn’t require preventing the insertion of null, but when we are using special value method null is return to indicate that collection is empty. So it is recommendable not to allow insertion of null.
ArrayDeque is a class that implements Deque. It has no capacity restrictions. It will perform faster than stack when used as stack and faster than linked list when used as queue. ArrayDeque is not thread Safe. The following example explains how to write program using ArrayDeque.
import java.util.ArrayDeque;
import java.util.Iterator;
public class DequeExample
{
public static void main(String as[])
{
ArrayDeque adObj = new ArrayDeque();
//Insertion by using various methods
adObj.add("Oracle");
adObj.addFirst("DB2");
adObj.offerFirst("MySQL"); //returns boolean - true R false
adObj.offerLast("Postgres"); //returns boolean - true R false
//Retrievals
System.out.println("Retrieving First Element :" + adObj.peekFirst());
System.out.println("Retrieving Last Element :"+ adObj.peekLast());
//Removals
System.out.println("Removing First Element :"+ adObj.pollFirst());
System.out.println("Removing Last Element :"+ adObj.pollLast());
//Reverse traversal
System.out.println("Remaining Elements :");
Iterator it = adObj.descendingIterator();
while(it.hasNext())
{
System.out.println(it.next());
}
}
}
Output:
Retrieving First Element :MySQL
Retrieving Last Element :Postgres
Removing First Element :MySQL
Removing Last Element :Postgres
Remaining Elements :
Oracle
DB2
For more please visit- http://download.oracle.com/javase/6/docs/api/java/util/ArrayDeque.html
