Java Deque Interface

The interface called Deque is present in java.util package. It is the subtype of the interface queue. The Deque supports the addition as well as the removal of elements from both ends of the data structure. Therefore, a deque can be used as a stack or a queue. We know that the stack supports the Last In First Out (LIFO) operation, and the operation First In First Out is supported by a queue. As a deque supports both, either of the mentioned operations can be performed on it. Deque is an acronym for "double ended queue".

Deque Interface declaration

Methods of Java Deque Interface

Method	Description
boolean add(object)	It is used to insert the specified element into this deque and return true upon success.
boolean offer(object)	It is used to insert the specified element into this deque.
Object remove()	It is used to retrieve and removes the head of this deque.
Object poll()	It is used to retrieve and removes the head of this deque, or returns null if this deque is empty.
Object element()	It is used to retrieve, but does not remove, the head of this deque.
Object peek()	It is used to retrieve, but does not remove, the head of this deque, or returns null if this deque is empty.
Object peekFirst()	The method returns the head element of the deque. The method does not remove any element from the deque. Null is returned by this method, when the deque is empty.
Object peekLast()	The method returns the last element of the deque. The method does not remove any element from the deque. Null is returned by this method, when the deque is empty.
Boolean offerFirst(e)	Inserts the element e at the front of the queue. If the insertion is successful, true is returned; otherwise, false.
Object offerLast(e)	Inserts the element e at the tail of the queue. If the insertion is successful, true is returned; otherwise, false.

ArrayDeque class

We know that it is not possible to create an object of an interface in Java. Therefore, for instantiation, we need a class that implements the Deque interface, and that class is ArrayDeque. It grows and shrinks as per usage. It also inherits the AbstractCollection class.

The important points about ArrayDeque class are:

Unlike Queue, we can add or remove elements from both sides.
Null elements are not allowed in the ArrayDeque.
ArrayDeque is not thread safe, in the absence of external synchronization.
ArrayDeque has no capacity restrictions.
ArrayDeque is faster than LinkedList and Stack.

ArrayDeque Hierarchy

The hierarchy of ArrayDeque class is given in the figure displayed at the right side of the page.

ArrayDeque class declaration

Let's see the declaration for java.util.ArrayDeque class.

public class ArrayDeque<E> extends AbstractCollection<E> implements Deque<E>, Cloneable, Serializable

Java ArrayDeque Example

FileName: ArrayDequeExample.java

import java.util.*;
public class ArrayDequeExample {
   public static void main(String[] args) {
   //Creating Deque and adding elements
   Deque<String> deque = new ArrayDeque<String>();
   deque.add("Ravi");  
   deque.add("Vijay");   
   deque.add("Ajay");  
   //Traversing elements
   for (String str : deque) {
   System.out.println(str);
   }
   }
}

Output:

Ravi
Vijay
Ajay

Java ArrayDeque Example: offerFirst() and pollLast()

FileName: DequeExample.java

import java.util.*;
public class DequeExample {
public static void main(String[] args) {
	Deque<String> deque=new ArrayDeque<String>();
	deque.offer("arvind");
	deque.offer("vimal");
	deque.add("mukul");
	deque.offerFirst("jai");
	System.out.println("After offerFirst Traversal...");
	for(String s:deque){
		System.out.println(s);
	}
	//deque.poll();
	//deque.pollFirst();//it is same as poll()
	deque.pollLast();
	System.out.println("After pollLast() Traversal...");
	for(String s:deque){
		System.out.println(s);
	}
}
}

Output:

After offerFirst Traversal...
jai
arvind
vimal
mukul
After pollLast() Traversal...
jai
arvind
vimal

Java ArrayDeque Example: Book

FileName: ArrayDequeExample.java

import java.util.*;  
class Book {  
int id;  
String name,author,publisher;  
int quantity;  
public Book(int id, String name, String author, String publisher, int quantity) {  
    this.id = id;  
    this.name = name;  
    this.author = author;  
    this.publisher = publisher;  
    this.quantity = quantity;  
}  
}  
public class ArrayDequeExample {  
public static void main(String[] args) {  
	Deque<Book> set=new ArrayDeque<Book>();  
    //Creating Books  
    Book b1=new Book(101,"Let us C","Yashwant Kanetkar","BPB",8);  
    Book b2=new Book(102,"Data Communications & Networking","Forouzan","Mc Graw Hill",4);  
    Book b3=new Book(103,"Operating System","Galvin","Wiley",6);  
    //Adding Books to Deque 
    set.add(b1);  
    set.add(b2);  
    set.add(b3);  
    //Traversing ArrayDeque
    for(Book b:set){  
    System.out.println(b.id+" "+b.name+" "+b.author+" "+b.publisher+" "+b.quantity);  
    }  
}  
}  

Output:

101 Let us C Yashwant Kanetkar BPB 8
102 Data Communications & Networking Forouzan Mc Graw Hill 4
103 Operating System Galvin Wiley 6

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