ArrayDeque is a resizable array implementation of the Deque interface, which stands for double-ended queue. It allows elements to be added or removed from both ends efficiently. It can be used as a stack (LIFO) or a queue (FIFO).
- ArrayDeque grows dynamically.
- It generally provides faster operations than LinkedList because it does not have the overhead of node management.
- Operations like addFirst(), addLast(), removeFirst(), removeLast() are all done in constant time O(1).
- The ArrayDeque class implements these two interfaces Queue interface and Deque interface. support concurrent access by multiple threads.
Example: This example demonstrates how to use an ArrayDeque to add elements to both ends of the deque and then remove them from both ends using addFirst(), addLast(), removeFirst() and removeLast() methods.
Java
import java.util.ArrayDeque;
import java.util.Deque;
public class Geeks {
public static void main(String[] args) {
Deque<Integer> d = new ArrayDeque<>();
d.addFirst(1);
d.addLast(2);
int f = d.removeFirst();
int l = d.removeLast();
System.out.println("First: " + f + ", Last: " + l);
}
}
ArrayDeque Hierarchy
The below image demonstrates the inheritance hierarchy of ArrayDeque, how it implements the Deque interface, which extends the Queue and how both are part of the Collection interface.
ArrayDeque HierarchyDeclaration of ArrayDeque
In Java, the declaration of ArrayDeque can be done as:
ArrayDeque<Type> deque = new ArrayDeque<>();
Note: Here, "Type" is the type of the element the deque will hold (e.g. Integer, String).
Constructors
1. ArrayDeque(): This constructor is used to create an empty ArrayDeque and by default holds an initial capacity to hold 16 elements.
ArrayDeque<Integer> deque = new ArrayDeque<>();
Example 1: This example demonstrates how to create an empty ArrayDeque, add elements to it and print the deque's contents.
Java
import java.util.ArrayDeque;
public class Geeks {
public static void main(String[] args) {
// Empty d with default capacity
ArrayDeque<Integer> d = new ArrayDeque<>();
d.add(10);
d.add(20);
System.out.println("ArrayDeque: " + d);
}
}
OutputArrayDeque: [10, 20]
2. ArrayDeque(Collection<? extends E> c): This constructor is used to create an ArrayDeque containing all the elements the same as that of the specified collection.
Syntax:
ArrayDeque<Integer> deque = new ArrayDeque<>(list);
Example 2: This example demonstrates how to initialize an ArrayDeque with elements from a collection and print its contents.
Java
import java.util.ArrayDeque;
import java.util.Arrays;
public class Geeks {
public static void main(String[] args)
{
ArrayDeque<Integer> d = new ArrayDeque<>(Arrays.asList(1, 2, 3, 4));
System.out.println("ArrayDeque: " + d);
}
}
OutputArrayDeque: [1, 2, 3, 4]
3. ArrayDeque(int numofElements): This constructor is used to create an empty ArrayDeque and holds the capacity to contain a specified number of elements.
Synatx:
ArrayDeque<String> deque = new ArrayDeque<>(25); // capacity = 25
Example 3: This example demonstrates how to create an ArrayDeque with a specified initial capacity (10) and add elements to it.
Java
import java.util.ArrayDeque;
public class Geeks {
public static void main(String[] args)
{
ArrayDeque<Integer> d = new ArrayDeque<>(10);
d.add(5);
d.add(15);
System.out.println("ArrayDeque: " + d);
}
}
OutputArrayDeque: [5, 15]
1. Adding Element: We can use methods like add(), addFirst(), addLast(), offer(), offerFirst(), offerLast() to insert element to the ArrayDeque.
Example: This example demonstrates how to use various methods add(), addFirst(), addLast(), offer(), offerFirst(), offerLast() to insert elements in the Deque.
Java
import java.io.*;
import java.util.*;
public class Geeks {
public static void main(String[] args)
{
// Initializing a deque since deque is an interface. it is assigned the ArrayDeque class
Deque<String> d = new ArrayDeque<String>();
// add() method to insert
d.add("The");
d.addFirst("To");
d.addLast("Geeks");
// offer() method to insert
d.offer("For");
d.offerFirst("Welcome");
d.offerLast("Geeks");
System.out.println("ArrayDeque : " + d);
}
}
OutputArrayDeque : [Welcome, To, The, Geeks, For, Geeks]
2. Accessing Elements: We can use methods like getFirst(), getLast(), peek(), peekFirst(), peekLast() to access elements of the ArrayDeque.
Example: This example demonstrates how to access the elements of the ArrayDeque using getFirst() and getLast() method.
Java
import java.io.*;
import java.util.*;
public class Geeks {
public static void main(String args[])
{
// Creating an empty ArrayDeque
ArrayDeque<String> d = new ArrayDeque<String>();
// Using add() method to add elements into the Deque Custom input elements
d.add("Welcome");
d.add("To");
d.add("Geeks");
d.add("for");
d.add("Geeks");
// Displaying the ArrayDeque
System.out.println("ArrayDeque: " + d);
// Displaying the First element
System.out.println("The first element is: " + d.getFirst());
// Displaying the Last element
System.out.println("The last element is: " + d.getLast());
}
}
OutputArrayDeque: [Welcome, To, Geeks, for, Geeks]
The first element is: Welcome
The last element is: Geeks
3. Removing Elements: We can use various methods like remove(), removeFirst(), removeLast(), poll(), pollFirst(), pollLast(), pop() to remove elements from the ArrayDeque.
Example: This example demonstrates removing elements from the ArrayDeque using pop(), poll() and pollFirst() method.
Java
import java.util.*;
public class Geeks {
public static void main(String[] args) {
// Initializing a deque
Deque<String> d = new ArrayDeque<String>();
// Adding elements
d.add("Java");
d.addFirst("C++");
d.addLast("Python");
// Printing initial elements
System.out.println("Initial Deque: " + d);
// Removing elements as a stack from top/front
System.out.println("Removed element using pop(): " + d.pop());
// Removing an element from the front
System.out.println("Removed element using poll(): " + d.poll());
// Removing an element from the front using pollFirst
System.out.println("Removed element using pollFirst(): " + d.pollFirst());
// The deque is empty now
System.out.println("Final Deque: " + d);
}
}
OutputInitial Deque: [C++, Java, Python]
Removed element using pop(): C++
Removed element using poll(): Java
Removed element using pollFirst(): Python
Final Deque: []
4.Iterating Elements: We can use various methods like remove(), iterator(), descendingIterator() to iterate over the elements of ArrayDeque.
Example: This example demonstrates iterating through a Deque in both forward and reverse order using the Iterator and descendingIterator() method.
Java
import java.util.*;
public class Geeks {
public static void main(String[] args)
{
// Declaring and initializing an deque
Deque<String> d = new ArrayDeque<String>();
// Adding elements at the back using add() method
d.add("For");
// Adding element at the front using addFirst() method
d.addFirst("Geeks");
// add element at the last using addLast() method
d.addLast("Geeks");
d.add("is so good");
// Iterate using Iterator interface from the front of the queue
System.out.println("Iterating in ForwardOrder:");
for (Iterator i = d.iterator(); i.hasNext();) {
System.out.print(i.next() + " ");
}
System.out.println();
// Iterate in reverse sequence in a queue
System.out.println("Iterating in ReverseOrder:");
for (Iterator i = d.descendingIterator();
i.hasNext();) {
System.out.print(i.next() + " ");
}
}
}
OutputIterating in ForwardOrder:
Geeks For Geeks is so good
Iterating in ReverseOrder:
is so good Geeks For Geeks
Methods in ArrayDeque class
Method | Description |
|---|
| add(Element e) | The method inserts a particular element at the end of the deque. |
| addAll(Collection<? extends E> c) | Adds all of the elements in the specified collection at the end of this deque, as if by calling addLast(E) on each one, in the order that they are returned by the collection's iterator. |
| addFirst(Element e) | The method inserts particular element at the start of the deque. |
| addLast(Element e) | The method inserts a particular element at the end of the deque. It is similar to the add() method |
| clear() | The method removes all deque elements. |
| clone() | The method copies the deque. |
| contains(Obj) | The method checks whether a deque contains the element or not |
| element() | The method returns element at the head of the deque |
| forEach(Consumer<? super E> action) | Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception. |
| getFirst() | The method returns first element of the deque |
| getLast() | The method returns last element of the deque |
| isEmpty() | The method checks whether the deque is empty or not. |
| iterator() | Returns an iterator over the elements in this deque. |
| offer(Element e) | The method inserts element at the end of deque. |
| offerFirst(Element e) | The method inserts element at the front of deque. |
| offerLast(Element e) | The method inserts element at the end of the deque. |
| peek() | The method returns head element without removing it. |
| poll() | The method returns head element and also removes it |
| pop() | The method pops out an element for stack represented by deque |
| push(Element e) | The method pushes an element onto stack represented by deque |
| remove() | The method returns head element and also removes it |
| remove(Object o) | Removes a single instance of the specified element from this deque. |
| removeAll(Collection<?> c) | Removes all of this collection's elements that are also contained in the specified collection (optional operation). |
| removeFirst() | The method returns the first element and also removes it |
| removeFirstOccurrence(Object o) | Removes the first occurrence of the specified element in this deque (when traversing the deque from head to tail). |
| removeIf(Predicate<? super Element> filter) | Removes all of the elements of this collection that satisfy the given predicate. |
| removeLast() | The method returns the last element and also removes it |
| removeLastOccurrence(Object o) | Removes the last occurrence of the specified element in this deque (when traversing the deque from head to tail). |
| retainAll(Collection<?> c) | Retains only the elements in this collection that are contained in the specified collection (optional operation). |
| size() | Returns the number of elements in this deque. |
| spliterator() | Creates a late-binding and fail-fast Spliterator over the elements in this deque. |
| toArray() | Returns an array containing all of the elements in this deque in proper sequence (from first to the last element). |
| toArray(T[] a) | Returns an array containing all of the elements in this deque in proper sequence (from first to the last element); the runtime type of the returned array is that of the specified array. |
Methods Declared in Interface java.util.Deque
Method | Action Performed |
|---|
| descendingIterator() | Returns an iterator over the elements in this deque in reverse sequential order. |
| peekFirst() | Retrieves, but does not remove, the first element of this deque or returns null if this deque is empty. |
| peekLast() | Retrieves, but does not remove, the last element of this deque or returns null if this deque is empty. |
| pollFirst() | Retrieves and removes the first element of this deque or returns null if this deque is empty. |
| pollLast() | Retrieves and removes the last element of this deque or returns null if this deque is empty. |
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