What is the difference between the data structure LinkedList and the collection HashSet in Java?

LinkedList and HashSet are different data structures with different purposes and characteristics:

• LinkedList is an ordered list implemented as a doubly linked list. It allows storing elements in insertion order, supports duplication of elements, and provides fast insertion and deletion operations in the middle of the list (O(1) with a reference to the node). Index-based access is slow (O(n)).

• HashSet is a collection that stores unique elements without a specific order. It is based on a hash table, which provides fast search, addition, and removal of elements (average O(1)). Duplicates are not allowed.

Example:

LinkedList<String> list = new LinkedList<>();
list.add("apple");
list.add("apple"); // duplicate is allowed

HashSet<String> set = new HashSet<>();
set.add("apple");
set.add("apple"); // second add will not add a new element

Summary: LinkedList is an ordered list with duplicates, HashSet is an unordered set of unique elements.