LinkedList Internal Working in Java with Examples

LinkedList is one of the most important collection implementations in Java. Unlike ArrayList, which stores elements in a dynamic array, LinkedList stores elements using nodes connected together. Understanding how LinkedList works internally is extremely important for Java interviews and for selecting the right collection in real-world applications.

In this article, we will explore the internal architecture of LinkedList, how insertion and deletion work, how nodes are connected, performance characteristics, and common interview questions.

What is LinkedList?

LinkedList is a class that implements both the List interface and Deque interface. It uses a doubly linked list internally to store and manage data.

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

technologies.add("Java");
technologies.add("Spring Boot");
technologies.add("Kafka");

System.out.println(technologies);

Output

[Java, Spring Boot, Kafka]

Important Characteristics of LinkedList

• Maintains insertion order.
• Allows duplicate elements.
• Allows null values.
• Uses a doubly linked list internally.
• Efficient insertions and deletions.
• Random access is slower compared to ArrayList.

Internal Structure of LinkedList

LinkedList stores data using nodes. Each node contains:

• Data
• Reference to previous node
• Reference to next node

Simplified node structure:

class Node<E> {
    E item;
    Node<E> next;
    Node<E> prev;
}

How Nodes Are Connected

NULL
  ^
  |
[Java] <-> [Spring] <-> [Kafka]
                         |
                         v
                       NULL

Each node knows its previous and next node, which makes insertion and deletion easier.

How add() Works Internally

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

list.add("Java");
list.add("Spring");
list.add("Kafka");

When a new element is added:

1. A new node is created.
2. The new node becomes the last node.
3. The previous last node is updated.
4. Links between nodes are adjusted.

Example

Before:

[Java] <-> [Spring]

After adding Kafka:

[Java] <-> [Spring] <-> [Kafka]

How get() Works Internally

Unlike ArrayList, LinkedList cannot directly access an element using an index.

list.get(2);

Java must traverse the list node by node until the required index is reached.

Index 0 -> Java
Index 1 -> Spring
Index 2 -> Kafka

This traversal makes random access slower.

How remove() Works Internally

list.remove("Spring");

Java performs:

1. Locate the target node.
2. Update previous node's next reference.
3. Update next node's previous reference.
4. Remove the target node.

Before Removal

Java <-> Spring <-> Kafka

After Removal

Java <-> Kafka

Adding Elements at the Beginning

LinkedList performs very efficiently when inserting elements at the start.

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

list.addFirst("Java");
list.addFirst("Spring");

Output

[Spring, Java]

Adding Elements at the End

list.addLast("Kafka");

LinkedList maintains references to both head and tail nodes, making this operation efficient.

LinkedList as Queue

Queue<String> queue =
        new LinkedList<>();

queue.offer("Java");
queue.offer("Spring");

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

Output

Java

LinkedList as Stack

LinkedList<String> stack =
        new LinkedList<>();

stack.push("Java");
stack.push("Spring");

System.out.println(stack.pop());

Output

Spring

Time Complexity

Operation	Complexity
Add First	O(1)
Add Last	O(1)
Remove First	O(1)
Remove Last	O(1)
Get By Index	O(n)
Search	O(n)

ArrayList vs LinkedList

Feature	ArrayList	LinkedList
Data Structure	Dynamic Array	Doubly Linked List
Random Access	Fast	Slow
Insertion	Slower	Faster
Deletion	Slower	Faster
Memory Usage	Lower	Higher

Real World Use Cases

• Queue implementations
• Browser navigation history
• Undo and redo operations
• Task scheduling systems
• Music playlist navigation

Common Interview Questions

Which data structure does LinkedList use internally?

LinkedList uses a doubly linked list internally.

Why is LinkedList slower than ArrayList for retrieval?

LinkedList requires traversal from node to node because elements are not stored in contiguous memory.

When should LinkedList be preferred over ArrayList?

When insertions and deletions occur frequently, especially at the beginning or middle of the collection.

Does LinkedList allow null values?

Yes. LinkedList allows multiple null values.

Is LinkedList synchronized?

No. LinkedList is not thread-safe.

Best Practices

• Use LinkedList when insertions and deletions are frequent.
• Use ArrayList when random access is more important.
• Avoid LinkedList for applications that perform frequent get(index) operations.
• Consider LinkedList when implementing queues and stacks.

Frequently Asked Questions (FAQ)

Can LinkedList contain duplicate elements?

Yes. LinkedList allows duplicate elements.

Can LinkedList contain null values?

Yes. Multiple null values can be stored.

Why does LinkedList consume more memory?

Each node stores additional previous and next references along with the actual data.

What is the biggest advantage of LinkedList?

Efficient insertion and deletion operations.

Conclusion

LinkedList is an important collection implementation that uses a doubly linked list internally. It excels in insertion and deletion operations but sacrifices random access performance. Understanding its internal structure, node management, and performance characteristics helps developers choose the correct collection and answer advanced interview questions confidently.

Keywords

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