Java Collections Framework: LinkedList

Reading time: 15 minutes

Properties

Properties of LinkedList in Java Collection Framework is as follows:

• Java LinkedList interface is a member of the Java Collections Framework.
• It is an implementation of the List and Deque interfaces.
• Internally, it is an implemented using Doubly Linked List Data Structure.
• It supports duplicate elements.
• It stores or maintains it’s elements in Insertion order.
• We can add any number of null elements.
• It is not synchronised that means it is not Thread safe.
• We can create a synchronised LinkedList using Collections.synchronizedList() method.
• In Java applications, we can use it as a List, stack or queue.
• It does not implement RandomAccess interface. So we can access elements in sequential order only. It does not support accessing elements randomly.
• When we try to access an element from a LinkedList, searching that element starts from the beginning or end of the LinkedList based on where that elements is available.
• We can use ListIterator to iterate LinkedList elements.
• From Java SE 8 on-wards, we can convert a LinkedList into a Stream and vice-versa.
• Java SE 9 is going to add couple of factory methods to create an Immutable LinkedList.

Java LinkedList Methods

Some of the useful methods of Linked List is as follows:

• int size(): to get the number of elements in the list.

• boolean isEmpty(): to check if list is empty or not.

• boolean contains(Object o): Returns true if this list contains the specified element.

• Iterator iterator(): Returns an iterator over the elements in this list in proper sequence.

• Object[] toArray(): Returns an array containing all of the elements in this list in proper sequence.

• boolean add(E e): Appends the specified element to the end of this list.

• boolean remove(Object o): Removes the first occurrence of the specified element from this list.

• boolean retainAll(Collection c): Retains only the elements in this list that are contained in the specified collection.

• void clear(): Removes all the elements from the list.

• E get(int index): Returns the element at the specified position in the list.

• E set(int index, E element): Replaces the element at the specified position in the list with the specified element.

• ListIterator listIterator(): Returns a list iterator over the elements in the list.

• List subList(int fromIndex, int toIndex): Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive. The returned list is backed by this list, so non-structural changes in the returned list are reflected in this list, and vice-versa.

• void addFirst(E e): Inserts the specified element at the beginning of this list.

• void addLast(E e)**==: Inserts the specified element at the end of this list.

• E getFirst(): Retrieves, but does not remove, the first element of this list. This method differs from peekFirst only in that it throws an exception if this list is empty.

• E getLast(): Retrieves, but does not remove, the last element of this list. This method differs from peekLast only in that it throws an exception if this list is empty.

• E remvoeFirst(): Removes and returns the first element from this list.

• E removeLast(): Removes and returns the last element from this list.

• boolean offerFirst(E e): Inserts the specified element at the front of this list.

• boolean offerLast(E e): Inserts the specified element at the end of this list.

• E pollFirst(): Retrieves and removes the first element of this list, or returns null if this list is empty.

• E pollLast(): Retrieves and removes the last element of this list, or returns null if this list is empty.

• E peekFirst(): Retrieves, but does not remove, the first element of this list, or returns null if this list is empty.

• E peekLast(): Retrieves, but does not remove, the last element of this list, or returns null if this list is empty.

Implementations

Simple usage example of LinkedList class in Java:

import java.util.LinkedList;
import java.util.List;
public class LinkedListDemo 
{
  public static void main(String[] args) 
  {
    List data = new LinkedList();
    data.add("1");
    data.add(2);
    data.add(3);
    data.add("4");
    System.out.println("LinkedList content: " + data);
    System.out.println("LinkedList size: " + data.size());
  }
}

With generics:

import java.util.LinkedList;
import java.util.List;
public class LinkedListGenericsDemo
{
  public static void main(String[] args) 
  {
    List data = new LinkedList<>();
    data.add("OpenGenus");
    data.add("is");
    data.add("for");
    data.add("you");
    System.out.println("LinkedList content: " + data);
    System.out.println("LinkedList size: " + data.size());
  }
}