How does the implementation of Set collection differ from ArrayList in Java?

The main difference between Set and ArrayList collections in Java lies in their behavior and implementation:

• Set — a collection that does not allow duplicate elements. The order of elements is not guaranteed (for example, in HashSet), although there are implementations with ordering (TreeSet, LinkedHashSet).
• ArrayList — an ordered list that allows duplicates and preserves the order of insertion.

In terms of implementation:

• ArrayList is based on an array that dynamically expands when elements are added. It provides quick access by index.
• HashSet is usually implemented based on HashMap, where elements act as keys. This ensures fast lookup and uniqueness checking.

Example:

Set<String> set = new HashSet<>();
set.add("apple");
set.add("apple"); // will not be added a second time

List<String> list = new ArrayList<>();
list.add("apple");
list.add("apple"); // both elements will be added