The ConcurrentLinkedDeque class in Java is a part of the Java Collection Framework and implements the Collection interface and the AbstractCollection class. It belongs to java.util.concurrent package. It is used to implement Deque with the help of LinkedList concurrently.
Features of ConcurrentLinkedDeque
- Iterators and spliterators are weakly consistent.
- Concurrent insertion, removal, and access operations execute safely across multiple threads.
- It does not permit null elements.
- size() method is not implemented in constant time. Because of the asynchronous nature of these deques, determining the current number of elements requires a traversal of the elements.
Class Hierarchy:
ConcurrentLinkedDeque in Java
Declaration:
public abstract class ConcurrentLinkedDeque<E>
extends AbstractCollection<E>
implements Deque<E>, Serializable
Here, E is the type of elements maintained by this collection.
It implements Serializable, Iterable<E>, Collection<E>, Deque<E>, Queue<E> interfaces.
Constructors of ConcurrentLinkedDeque:
1. ConcurrentLinkedDeque(): This constructor is used to construct an empty deque.
ConcurrentLinkedDeque<E> cld = new ConcurrentLinkedDeque<E>();
2. ConcurrentLinkedDeque(Collection<E> c): This constructor is used to construct a deque with the elements of the Collection passed as the parameter.
ConcurrentLinkedDeque<E> cld = new ConcurrentLinkedDeque<E>(Collection<E> c);
Below is the sample program to illustrate ConcurrentLinkedDeque in Java:
// Java Program to demonstrate ConcurrentLinkedDeque
import java.util.concurrent.*;
class ConcurrentLinkedDequeDemo {
public static void main(String[] args)
{
// Create a ConcurrentLinkedDeque
// using ConcurrentLinkedDeque()
// constructor
ConcurrentLinkedDeque<Integer>
cld = new ConcurrentLinkedDeque<Integer>();
// add element to the front
// using addFirst() method
cld.addFirst(12);
cld.addFirst(70);
cld.addFirst(2009);
cld.addFirst(575);
// Displaying the existing ConcurrentLinkedDeque
System.out.println("ConcurrentLinkedDeque: "
+ cld);
// Create a ConcurrentLinkedDeque
// using ConcurrentLinkedDeque(Collection c)
// constructor
ConcurrentLinkedDeque<Integer>
cld1 = new ConcurrentLinkedDeque<Integer>(cld);
// Displaying the existing ConcurrentLinkedDeque
System.out.println("ConcurrentLinkedDeque1: "
+ cld1);
}
}
result:
ConcurrentLinkedDeque: [575, 2009, 70, 12]
ConcurrentLinkedDeque1: [575, 2009, 70, 12]
Example:
// Java code to illustrate
// methods of ConcurrentLinkedDeque
import java.util.concurrent.*;
class ConcurrentLinkedDequeDemo {
public static void main(String[] args)
{
// Create a ConcurrentLinkedDeque
// using ConcurrentLinkedDeque() constructor
ConcurrentLinkedDeque<Integer>
cld = new ConcurrentLinkedDeque<Integer>();
// add element to the front
// using addFirst() method
cld.addFirst(12);
cld.addFirst(70);
cld.addFirst(2009);
cld.addFirst(575);
// Displaying the existing ConcurrentLinkedDeque
System.out.println("ConcurrentLinkedDeque: "
+ cld);
// Displaying the Last element
// using getLast() method
System.out.println("The Last element is: "
+ cld.getLast());
// Displaying the first element
// using peekFirst() method
System.out.println("First Element is: "
+ cld.peekFirst());
// Remove the Last element
// using removeLast() method
cld.removeLast();
// Displaying the existing ConcurrentLinkedDeque
System.out.println("ConcurrentLinkedDeque: "
+ cld);
}
}
result:
ConcurrentLinkedDeque: [575, 2009, 70, 12]
The Last element is: 12
First Element is: 575
ConcurrentLinkedDeque: [575, 2009, 70]
Basic Operations of ConcurrentLinkedDeque
1. Adding Elements
To add an element or Collection of elements, ConcurrentLinkedDeque provides methods like add(E e), addAll(Collection<? extends E> c), addFirst(E e), addLast(E e) methods. The example below explains these methods.
// Java Program Demonstrate adding
// elements to the ConcurrentLinkedDeque
import java.util.concurrent.*;
class AddingElements {
public static void main(String[] args)
{
// create instance using ConcurrentLinkedDeque
ConcurrentLinkedDeque<Integer> cld1
= new ConcurrentLinkedDeque<Integer>();
// Add element to the tail using
// add or addLast methods
cld1.add(12);
cld1.add(110);
// Add element to the head
// using addFirst method
cld1.addFirst(55);
// Displaying the existing ConcurrentLinkedDeque
System.out.println("Initial Elements in "
+ "the LinkedDeque cld : "
+ cld1);
// create instance using ConcurrentLinkedDeque
ConcurrentLinkedDeque<Integer> cld2
= new ConcurrentLinkedDeque<Integer>();
// Add elements of cld1 to the
// cld2 using addAll method
cld2.addAll(cld1);
// Displaying the modified ConcurrentLinkedDeque
System.out.println("Initial Elements in "
+ "the LinkedDeque cld2: "
+ cld2);
}
}
result:
Initial Elements in the LinkedDeque cld : [55, 12, 110]
Initial Elements in the LinkedDeque cld2: [55, 12, 110]
2. Remove Elements
To remove an element, ConcurrentLinkedDeque provides methods like remove(), remove(Object o), removeFirst(), removeLast() etc. These methods are explained in the below example.
// Java Program to demonstrate removing
// elements of ConcurrentLinkedDeque
import java.util.concurrent.*;
class RemovingElements {
public static void main(String[] args)
{
// Create a ConcurrentLinkedDeque
// using ConcurrentLinkedDeque() constructor
ConcurrentLinkedDeque<Integer> cld
= new ConcurrentLinkedDeque<Integer>();
// Add elements using add() method
cld.add(40);
cld.add(50);
cld.add(60);
cld.add(70);
cld.add(80);
// Displaying the existing LinkedDeque
System.out.println(
"Existing ConcurrentLinkedDeque: " + cld);
// remove method removes the first
// element of ConcurrentLinkedDeque
// using remove() method
System.out.println("Element removed: "
+ cld.remove());
// Remove 60 using remove(Object)
System.out.println("60 removed: " + cld.remove(60));
// Displaying the existing ConcurrentLinkedDeque
System.out.println(
"Modified ConcurrentLinkedDeque: " + cld);
// Remove the first element
cld.removeFirst();
// Remove the Last element
cld.removeLast();
// Displaying the existing ConcurrentLinkedDeque
System.out.println(
"Modified ConcurrentLinkedDeque: " + cld);
}
}
Output
Existing ConcurrentLinkedDeque: [40, 50, 60, 70, 80]
Element removed: 40
60 removed: true
Modified ConcurrentLinkedDeque: [50, 70, 80]
Modified ConcurrentLinkedDeque: [70]
3. Iterating Elements
We can iterate the ConcurrentLinkedDeque using iterator() or descendingIterator() methods. The below code explains both these methods.
// Java code to illustrate iterating
// elements of ConcurrentLinkedDeque
import java.util.concurrent.*;
import java.util.*;
public class IteratingConcurrentLinkedDeque {
public static void main(String args[])
{
// Creating an empty ConcurrentLinkedDeque
ConcurrentLinkedDeque<String> deque
= new ConcurrentLinkedDeque<String>();
// Use add() method to add elements
// into the ConcurrentLinkedDeque
deque.add("Welcome");
deque.add("To");
deque.add("Prutor");
deque.add("4");
deque.add("Prutor");
// Displaying the ConcurrentLinkedDeque
System.out.println("ConcurrentLinkedDeque: "
+ deque);
// Creating an iterator
Iterator fitr = deque.iterator();
// Displaying the values
// after iterating through the ConcurrentLinkedDeque
System.out.println("The iterator values are: ");
while (fitr.hasNext()) {
System.out.println(fitr.next());
}
// Creating a desc_iterator
Iterator ditr = deque.descendingIterator();
// Displaying the values after iterating
// through the ConcurrentLinkedDeque
// in reverse order
System.out.println("The iterator values are: ");
while (ditr.hasNext()) {
System.out.println(ditr.next());
}
}
}
Output
ConcurrentLinkedDeque: [Welcome, To, Prutor, 4, Prutor]
The iterator values are:
Welcome
To
Prutor
4
Prutor
The iterator values are:
Prutor
4
Prutor
To
Welcome
4. Accessing Elements
To access the elements of ConcurrentLinkedDeque, it provides methods like getFirst(), getLast(), element() methods. The below example explains these methods.
// Java Program to Demonstrate accessing
// elements of ConcurrentLinkedDeque
import java.util.concurrent.*;
import java.util.*;
class Accessing {
public static void main(String[] args)
{
// Creating an empty ConcurrentLinkedDeque
ConcurrentLinkedDeque<String> cld
= new ConcurrentLinkedDeque<String>();
// Add elements into the ConcurrentLinkedDeque
cld.add("Welcome");
cld.add("To");
cld.add("Prutor");
cld.add("4");
cld.add("Prutor");
// Displaying the ConcurrentLinkedDeque
System.out.println("Elements in the ConcurrentLinkedDeque: " + cld);
// Displaying the first element
System.out.println("The first element is: "
+ cld.getFirst());
// Displaying the Last element
System.out.println("The Last element is: "
+ cld.getLast());
// Displaying the head of ConcurrentLinkedDeque
System.out.println("The Head of ConcurrentLinkedDeque is: "
+ cld.element());
}
}
result:
Elements in the ConcurrentLinkedDeque: [Welcome, To, Prutor, 4, Prutor]
The first element is: Welcome
The Last element is: Prutor
The Head of ConcurrentLinkedDeque is: Welcome
Methods of ConcurrentLinkedDeque
Here, E is the type of element.
METHOD
DESCRIPTION
- add(E e) Inserts the specified element at the tail of this deque.
- addAll(Collection<? extends E> c) Appends all of the elements in the specified collection to the end of this deque, in the order that they are returned by the specified collection’s iterator.
- addFirst(E e) Inserts the specified element at the front of this deque.
- addLast(E e) Inserts the specified element at the end of this deque.
- clear() Removes all of the elements from this deque.
- contains(Object o) Returns true if this deque contains the specified element.
- descendingIterator() Returns an iterator over the elements in this deque in reverse sequential order.
- element() Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this 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() Retrieves, but does not remove, the first element of this deque.
- getLast() Retrieves, but does not remove, the last element of this deque.
- isEmpty() Returns true if this collection contains no elements.
- iterator() Returns an iterator over the elements in this deque in the proper sequence.
- offer(E e) Inserts the specified element at the tail of this deque.
- offerFirst(E e) Inserts the specified element at the front of this deque.
- offerLast(E e) Inserts the specified element at the end of this deque.
- pop() Pops an element from the stack represented by this deque.
- push(E e) Pushes an element onto the stack represented by this deque (in other words, at the head of this deque) if it is possible to do so immediately without violating capacity restrictions, throwing an
- IllegalStateException if no space is currently available.
- remove() Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque).
- remove(Object o) Removes the first occurrence 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() Retrieves and removes the first element of this deque.
- removeFirstOccurrence(Object o) Removes the first occurrence of the specified element from this deque.
- removeIf(Predicate<? super E> filter) Removes all of the elements of this collection that satisfy the given predicate.
- removeLast() Retrieves and removes the last element of this deque.
- removeLastOccurrence(Object o) Removes the last occurrence of the specified element from this deque.
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() Returns a 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 last element).
- toArray(T[] a) Returns an array containing all of the elements in this deque, in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array.
Methods declared in class java.util.AbstractCollection
METHOD
DESCRIPTION
- containsAll(Collection<?> c) Returns true if this collection contains all of the elements in the specified collection.
- toString() Returns a string representation of this collection.
Methods declared in interface java.util.Collection
METHOD
DESCRIPTION
- containsAll(Collection<?> c) Returns true if this collection contains all of the elements in the specified collection.
- equals(Object o) Compares the specified object with this collection for equality.
- hashCode() Returns the hash code value for this collection.
- parallelStream() Returns a possibly parallel Stream with this collection as its source.
- stream() Returns a sequential Stream with this collection as its source.
- toArray(IntFunction<T[]> generator) Returns an array containing all of the elements in this collection, using the provided generator function to allocate the returned array.
Methods declared in interface java.util.Deque
METHOD
DESCRIPTION
- peek() Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque), or returns null if this deque is empty.
- 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.
- poll() Retrieves and removes the head of the queue represented by this deque (in other words, the first 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.