Java LinkedList

The LinkedList class in Java is part of the Java Collections Framework and implements both the List and Deque interfaces. It provides a doubly-linked list data structure.

1. What is a LinkedList?#

• LinkedList is a doubly-linked list.
• It allows elements to be added or removed from both ends.
• It can store duplicate and null elements.

List<String> list = new LinkedList<>();

2. Internal Working#

• Each node contains a data element, a reference to the previous node, and a reference to the next node.
• Enables constant-time insertions and removals using iterators.

3. Constructors#

LinkedList()                          // Default constructor
LinkedList(Collection<? extends E>)   // Constructs a list containing elements of the given collection

4. Commonly Used Methods with Examples#

1. add(E e) – Adds element at the end#

LinkedList<String> list = new LinkedList<>();
list.add("Apple");
System.out.println(list);

Output:

[Apple]

2. add(int index, E element) – Inserts at a specific index#

list.add(0, "Banana");
System.out.println(list);

Output:

[Banana, Apple]

3. addFirst(E e) – Adds to the beginning#

list.addFirst("Mango");
System.out.println(list);

Output:

[Mango, Banana, Apple]

4. addLast(E e) – Adds to the end#

list.addLast("Grapes");
System.out.println(list);

Output:

[Mango, Banana, Apple, Grapes]

5. get(int index) – Retrieves element at a given index#

String item = list.get(2);
System.out.println(item);

Output:

Apple

6. getFirst() – Gets the first element#

System.out.println(list.getFirst());

Output:

Mango

7. getLast() – Gets the last element#

System.out.println(list.getLast());

Output:

Grapes

8. remove() – Removes and returns the first element#

list.remove();
System.out.println(list);

Output:

[Banana, Apple, Grapes]

9. remove(int index) – Removes element at a given index#

list.remove(1);
System.out.println(list);

Output:

[Banana, Grapes]

10. removeFirst() – Removes the first element#

list.removeFirst();
System.out.println(list);

Output:

[Grapes]

11. removeLast() – Removes the last element#

list.removeLast();
System.out.println(list);

Output:

[]

12. contains(Object o) – Checks if an element exists#

Output:

true

13. size() – Returns the number of elements in the list#

System.out.println(list.size());

Output:

1

14. isEmpty() – Checks if the list is empty#

System.out.println(list.isEmpty());

Output:

false

15. clear() – Removes all elements#

list.clear();
System.out.println(list);

Output:

[]

16. offer(E e) – Adds element at the end (queue style)#

list.offer("Kiwi");
System.out.println(list);

Output:

[Kiwi]

17. peek() – Retrieves the head element without removing it#

System.out.println(list.peek());

Output:

Kiwi

18. poll() – Retrieves and removes the head element#

System.out.println(list.poll());
System.out.println(list);

Output:

Kiwi
[]

19. descendingIterator() – Iterates in reverse order#

list.add("One");
list.add("Two");
list.add("Three");
Iterator<String> desc = list.descendingIterator();
while (desc.hasNext()) {
    System.out.print(desc.next() + " ");
}

Output:

Three Two One

5. Performance#

6. Thread Safety#

• LinkedList is not synchronized.
• For a thread-safe version:

List<String> syncList = Collections.synchronizedList(new LinkedList<>());

7. When to Use LinkedList?#

• When frequent insertion or removal is required.
• When random access is not a priority.
• For implementing queues, stacks, and deques.

8. Best Practices#

• Prefer using the List interface:

List<String> names = new LinkedList<>();

• Avoid frequent index-based access when performance is critical.

Conclusion#

The LinkedList is a versatile structure for dynamic data handling, especially where frequent modifications are needed. It shines in scenarios requiring sequential access and frequent insertions or deletions.