Java.Collections.Why does LinkedList implement both List and Deque?

✅ Short Answer:

Because LinkedList is both:

• A sequential list of elements (like an array),
• AND a double-ended queue (deque).

So, Java designers made LinkedList implement both List and Deque interfaces to give us the best of both worlds.

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📦 LinkedList in Java:

public class LinkedList<E> 
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, Serializable

✅ As a List:

• You can:
  • Access elements by index: get(i), set(i)
  • Add/remove elements at any position: add(i, e), remove(i)
  • Iterate in order

This supports typical sequential data access.

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✅ As a Deque:

• You can:
  • Add/remove at both ends: addFirst, addLast, removeFirst, removeLast
  • Use it as a queue (FIFO): offer, poll
  • Or as a stack (LIFO): push, pop

This supports queue-like or stack-like behavior.

🎯 Why it makes sense:

• LinkedList is built on a doubly linked list.
• This structure naturally supports:
  • Fast insertion/removal at both ends and in the middle.
  • Efficient sequential iteration.
• So it’s logical for it to implement both List and Deque.

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⚠️ Caveat:

• While LinkedList supports random access, it’s not efficient (O(n)) — unlike ArrayList (O(1)).

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🧠 Tip:

Use LinkedList when:

• You need frequent insertions/removals at the beginning or middle.
• Or you’re implementing queues, stacks, or deques.

Avoid it if you need fast random access — go with ArrayList instead.