What is a LinkedList in Java and How is it Used?

Introduction to LinkedList

A `LinkedList` is a type of data structure in Java that consists of a chain of nodes where each node contains two parts: the data and a reference (or link) to the next node in the sequence. This makes `LinkedList` a dynamic and flexible alternative to arrays. Unlike arrays, which have a fixed size, a `LinkedList` can grow or shrink in size as elements are added or removed.

The `LinkedList` class in Java is part of the java.util package, and it implements both the List and Deque interfaces, offering methods for accessing, modifying, and deleting elements. This makes it an ideal choice for scenarios where you need frequent insertions and deletions from any position in the list.

Java’s `LinkedList` is different from other collections like arrays, which have a fixed size and offer efficient access to elements by index. A `LinkedList` uses pointers to link elements together, which allows for efficient insertion and deletion of elements from both ends of the list.

Why Use LinkedList?

The primary reason to use a `LinkedList` over other data structures like arrays or ArrayList is its ability to perform operations like insertion and deletion efficiently. For example, when you add or remove an element at the beginning or middle of an ArrayList, the rest of the elements need to be shifted to accommodate the change, making these operations time-consuming. In contrast, a `LinkedList` can quickly adjust the pointers of neighboring nodes without requiring a shift of all other elements.

Here are some key scenarios where a `LinkedList` can be more beneficial than other data structures:

• When you need fast insertions and deletions at the beginning or middle of the list.
• When the size of the list frequently changes, and you don’t know the size upfront.
• When you need to implement a queue or a stack where elements are added and removed from the front or back.

Basic Operations with LinkedList

The `LinkedList` class in Java provides various methods for performing common operations. These include adding, removing, and accessing elements, among others. Let’s take a look at some of these operations with code examples:

1. Creating a LinkedList

import java.util.LinkedList;

public class LinkedListExample {
    public static void main(String[] args) {
        // Create a LinkedList of Strings
        LinkedList list = new LinkedList<>();
        
        // Add elements to the LinkedList
        list.add("Java");
        list.add("Python");
        list.add("C++");
        
        // Display the LinkedList
        System.out.println("LinkedList: " + list);
    }
}
        

2. Adding Elements

list.addFirst("JavaScript");  // Adds element at the start
list.addLast("Ruby");         // Adds element at the end
list.add(2, "Go");            // Adds element at index 2
        

3. Removing Elements

list.removeFirst();  // Removes the first element
list.removeLast();   // Removes the last element
list.remove(2);      // Removes element at index 2
        

4. Accessing Elements

String firstElement = list.getFirst();  // Get first element
String lastElement = list.getLast();   // Get last element
String elementAtIndex = list.get(1);   // Get element at index 1
        

Types of LinkedLists in Java

There are different types of LinkedLists that you can use in Java:

• Single LinkedList: Each node has a reference to the next node in the sequence, and the last node points to null.
• Doubly LinkedList: Each node contains a reference to both the next and previous nodes, allowing for efficient traversal in both directions.
• Circular LinkedList: The last node points back to the first node, creating a circular chain.

Java’s built-in `LinkedList` class is a doubly linked list, meaning that each node contains references to both the next and previous nodes.

Performance Considerations

One of the biggest advantages of a `LinkedList` is its efficiency in certain operations, but it’s important to understand the trade-offs:

• Time Complexity: Inserting or removing elements at the beginning or middle of the list is generally faster than in an array because no elements need to be shifted.
• Space Complexity: A `LinkedList` uses more memory than an array because each node needs extra space for storing the reference to the next (and possibly previous) node.
• Access Time: Accessing elements in a `LinkedList` is slower than an array, because it requires traversing the list from the beginning (or end) to the desired node.