What is the difference between Set and List?

List is an ordered sequence of elements whereas Set is a distinct list of elements which is unordered (thank you, Quinn Taylor).

List<E>:

An ordered collection (also known as a sequence). The user of this interface has precise control over where in the list each element is inserted. The user can access elements by their integer index (position in the list), and search for elements in the list.

Set<E>:

A collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element. As implied by its name, this interface models the mathematical set abstraction.

Ordered lists of element (unique or not)
Conform to Java's interface named List
Can be accessed by index

Implemented using

• LinkedList
• ArrayList

Lists of unique elements:
Conform to Java's interface named Set
Can not be accessed by index

Implemented using

• HashSet (unordered)
• LinkedHashSet (ordered)
• TreeSet (sorted by natural order or by provided comparator)

Both interfaces Set and List conform to Java's interface named Collection

A Set cannot contain duplicate elements while a List can. A List (in Java) also implies order.

• A List is an ordered grouping of items
• A Set is an unordered grouping of items with no duplicates allowed (usually)

Conceptually we usually refer to an unordered grouping that allows duplicates as a Bag and doesn't allow duplicates is a Set.

List and Set both are interfaces. They both extends Collection interface. The important differences between set and list are:

1. Duplicate Objects

The main difference between List and Set is that List allows duplicates while Set doesn't allow duplicates.

2. Order

List is an ordered collection it maintains the insertion order, which means upon displaying the list content it will display the elements in the same order in which they got inserted into the list.

Set is an unordered collection, it doesn’t maintain any order. There are few implementations of Set which maintains the order such as LinkedHashSet (It maintains the elements in insertion order).

3. Null elements

List allows any number of null elements. Set can have only a single null elements at most.

A set is an unordered group of distinct objects — no duplicate objects are allowed. It is generally implemented using the hash code of the objects being inserted. (Specific implementations may add ordering, but the Set interface itself does not.)

A list is an ordered group of objects which may contain duplicates. It could be implemented with an ArrayList, LinkedList, etc.

This might not be the answer you're looking for, but the JavaDoc of the collections classes is actually pretty descriptive. Copy/pasted:

An ordered collection (also known as a sequence). The user of this interface has precise control over where in the list each element is inserted. The user can access elements by their integer index (position in the list), and search for elements in the list.

Unlike sets, lists typically allow duplicate elements. More formally, lists typically allow pairs of elements e1 and e2 such that e1.equals(e2), and they typically allow multiple null elements if they allow null elements at all. It is not inconceivable that someone might wish to implement a list that prohibits duplicates, by throwing runtime exceptions when the user attempts to insert them, but we expect this usage to be rare.

All of the List classes maintain the order of insertion. They use different implementations based on performance and other characteristics (e.g. ArrayList for speed of access of a specific index, LinkedList for simply maintaining order). Since there is no key, duplicates are allowed.

The Set classes do not maintain insertion order. They may optionally impose a specific order (as with SortedSet), but typically have an implementation-defined order based on some hash function (as with HashSet). Since Sets are accessed by key, duplicates are not allowed.

The biggest different is the basic concept.

From the Set and List interface. Set is mathematics concept. Set method extends collection.however not add new method. size() means cardinality(more is BitSet.cardinality, Linear counter，Log Log，HyperLogLog). addAll() means union. retainAll() means intersection. removeAll() means difference.

However List lack of these concepts. List add a lot of method to support sequence concept which Collection interface not supply. core concept is INDEX. like add(index,element),get(index),search(indexOf()),remove(index) element. List also provide "Collection View" subList. Set do not have view. do not have positional access. List also provide a lot of algorithms in Collections class. sort(List),binarySearch(List),reverse(List),shuffle(List),fill(List). the method params is List interface. duplicate elements are just the result of concepts. not the essential difference.

So the essential difference is concept. Set is mathematics set concept.List is sequence concept.

Ordering... a list has an order, a set does not.