What is the difference between Set and List?

Question

What is the fundamental difference between the Set<E> and List<E> interfaces?

Answer 1

List

1. Is an Ordered grouping of elements.
2. List is used to collection of elements with duplicates.
3. New methods are defined inside List interface.

Set

1. Is an Unordered grouping of elements.
2. Set is used to collection of elements without duplicates.
3. No new methods are defined inside Set interface, so we have to use Collection interface methods only with Set subclasses.

Answer 2

Few note worthy differences between List and Set in Java are given as following :

1) Fundamental difference between List and Set in Java is allowing duplicate elements. List in Java allows duplicates while Set doesn't allow any duplicate. If you insert duplicate in Set it will replace the older value. Any implementation of Set in Java will only contains unique elements.

2) Another significant difference between List and Set in Java is order. List is an Ordered Collection while Set is an unordered Collection. List maintains insertion order of elements, means any element which is inserted before will go on lower index than any element which is inserted after. Set in Java doesn't maintain any order. Though Set provide another alternative called SortedSet which can store Set elements in specific Sorting order defined by Comparable and Comparator methods of Objects stored in Set.

3) Popular implementation of List interface in Java includes ArrayList, Vector and LinkedList. While popular implementation of Set interface includes HashSet, TreeSet and LinkedHashSet.

Its pretty clear that if you need to maintain insertion order or object and you collection can contain duplicates than List is a way to go. On the other hand if your requirement is to maintain unique collection without any duplicates than Set is the way to go.