Difference between ArrayList and LinkedList in Java

ArrayList and LinkedList are both part of the Java List interface, but they differ in their underlying data structures, performance characteristics, and use cases. Here’s a breakdown of the key differences between ArrayList and LinkedList:

Pictorial Representation of ArrayList and LinkedList:

1. Underlying Data Structure

• ArrayList: Internally uses a dynamic array (resizable array). When elements are added beyond the array’s capacity, a new larger array is created, and the old elements are copied into it.
• LinkedList: Internally uses a doubly linked list, where each element (node) holds a reference to the previous and next elements.

2. Access Time Complexity

• ArrayList: Provides constant-time (O(1)) access to elements by index since it’s backed by an array. You can directly access any element using its index, making random access very efficient.
• LinkedList: Provides linear-time (O(n)) access to elements by index. Since elements are stored in nodes that are linked together, you must traverse the list from the beginning or the end to reach a particular element. Use Case: If you need frequent access by index, ArrayList is better suited due to its faster random access.

3. Insertion/Deletion Time Complexity

• ArrayList:
  • Insertion at the end is usually O(1), but it can be O(n) when the array needs to be resized.
  • Insertion in the middle or at the beginning is O(n) because elements need to be shifted to make room for the new element.
  • Deletion requires shifting elements to fill the gap, resulting in O(n) time complexity.
• LinkedList:
  • Insertion/Deletion at the beginning or middle is O(1) (constant time) if you already have a reference to the node. It does not require shifting elements like an ArrayList.
  • However, finding the location where you want to insert/delete requires traversal, making it O(n) if you don’t already have a reference to the node.
  Use Case: For frequent insertions and deletions in the middle of the list, LinkedList performs better. However, if you mainly insert or delete at the end, ArrayList can be efficient as well.

4. Memory Overhead

• ArrayList: Has less memory overhead since it only stores the array of elements. However, when resizing, it needs additional memory to create a new array and copy the elements.
• LinkedList: Has higher memory overhead because each node stores both the element and two additional references (pointers) to the next and previous nodes. This extra space per node can add up if the list is large. Use Case: If memory consumption is a concern, ArrayList generally uses less memory compared to LinkedList.

5. Resizing

• ArrayList: Automatically resizes itself when its capacity is exceeded. Typically, when it grows, the new capacity is 1.5 times the previous size, which can involve reallocation and copying of elements.
• LinkedList: No resizing is needed since nodes are dynamically allocated. Each element is linked in the list as it is added. Use Case: If you expect your list to grow dynamically and frequently, LinkedList avoids the cost of array resizing. But for large data sets with infrequent resizing, ArrayList might be more efficient due to better cache locality.

6. Traversal Performance

• ArrayList: Supports efficient traversal using either a for-loop or an enhanced for-loop since elements are stored in contiguous memory locations.
• LinkedList: Traversal is slower because it involves following pointers between nodes, which can cause more cache misses and overhead due to dereferencing. Use Case: For large datasets where performance of traversal matters, ArrayList typically performs better.

7. Iterator Performance

• ArrayList: The Iterator for ArrayList is faster for traversal because it accesses elements directly via the array index.
• LinkedList: The Iterator for LinkedList needs to traverse through the nodes, making it slower in comparison to ArrayList.

8. Implementation of Additional Methods

• ArrayList: Not optimized for frequent insertions/deletions, especially in the middle of the list. Methods like add(index, element) or remove(index) are costly.
• LinkedList: Also implements Deque (Double Ended Queue) interface, making it more versatile for certain operations like addFirst(), addLast(), removeFirst(), removeLast(), etc. It’s well-suited for use as a stack, queue, or double-ended queue (deque).

9. Use as a Queue

• ArrayList: Can be used as a queue but isn’t designed for it, as removing from the front requires shifting all elements.
• LinkedList: Performs better as a queue due to its efficient insertion and deletion at both ends (addFirst(), addLast(), removeFirst(), etc.).

Summary of Key Differences

When to Use

• Use ArrayList when: You need fast random access, the list won’t change frequently (few insertions/deletions), and memory overhead is a concern.
• Use LinkedList when: You need frequent insertions and deletions, especially at the beginning or middle, and don’t require fast random access. Also useful when the list acts as a queue or stack.