When to use LinkedList over ArrayList in Java?

In Java, choosing between a LinkedList and an ArrayList depends on the specific use case and the operations you intend to perform frequently. Here’s a detailed comparison and guidance on when to use LinkedList over ArrayList:

Use LinkedList When:

1. Frequent Insertions/Deletions at the Beginning or Middle:
   • A LinkedList is implemented as a doubly-linked list, so adding or removing elements from the beginning or middle is more efficient (O(1) or O(n), depending on the location).
   • Example:

     LinkedList<Integer> list = new LinkedList<>();
     list.addFirst(1);  // Efficient insertion at the beginning
     list.add(1, 2);    // Efficient insertion at index
     

2. Memory Utilization is Not a Concern:
   • A LinkedList requires extra memory to store pointers for each node (two pointers per node in a doubly-linked list). If memory is a constraint, an ArrayList may be preferable.
3. Need for Efficient Iteration in Non-Contiguous Operations:
   • If your application involves frequent use of iterators or ListIterator for traversal, a LinkedList might work well since traversal using an iterator avoids array resizing issues.
4. When Random Access is Not Required:
   • LinkedList does not support efficient random access (O(n) complexity for accessing elements by index). Use it if sequential access is sufficient.

Avoid LinkedList When:

1. Frequent Random Access of Elements is Needed:
   • Accessing elements in a LinkedList by index requires traversing from the start or end (O(n)).
   • Example:

     LinkedList<Integer> list = new LinkedList<>();
     list.add(10);
     System.out.println(list.get(0)); // Slower compared to ArrayList
     

2. Memory Overhead is a Concern:
   • Each node in a LinkedList requires additional memory for storing pointers (next and previous references).

Why Use ArrayList Instead?

Choose ArrayList if:

• You need frequent random access (O(1) complexity).
• The majority of operations involve appending elements to the end.
• You require lower memory overhead, as ArrayList does not store pointers.

Summary of Operations

When to Use LinkedList: Examples

1. Queue/Deque Implementation:
   • LinkedList is often used to implement queues and deques because of its efficient insertion and deletion at both ends.

     LinkedList<Integer> queue = new LinkedList<>();
     queue.addLast(10); // Enqueue
     queue.removeFirst(); // Dequeue
     

2. When Frequent Insertions and Deletions Are Required:
   • Example: A real-time system where tasks are added and removed frequently.

General Rule of Thumb:

• Use LinkedList for frequent insertions and deletions.
• Use ArrayList for frequent random access and when memory is a concern.