What is the difference between Set and List

Question

What is the fundamental difference between the Set lt E gt  and List lt E gt  interfaces

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Set   Cannot have duplicate values Ordering depends on implementation  By default it is not ordered Cannot have access by index  List   Can have duplicate values Ordered by default Can have access by index

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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

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List  List allows duplicate elements and null values  Easy to search using the corresponding index of the elements and also it will display elements in insertion order  Example  linkedlist   import java util     public class ListExample     public static void main String   args           TODO Auto-generated method stub      List lt Integer gt  l new LinkedList lt Integer gt         l add 001       l add 555       l add 333       l add 888       l add 555       l add null       l add null        Iterator lt Integer gt  il l iterator         System out println l get 0         while il hasNext             System out println il next                for Integer str   l           System out println  Value   str                 Output   1 1 555 333 888 555 null null Value 1 Value 555 Value 333 Value 888 Value 555 Value null Value null  Set  Set isn t allow any duplicate elements and it allow single null value It will not maintain any order to display elements Only TreeSet will display in ascending order   Example  TreeSet   import java util TreeSet   public class SetExample     public static void main String   args           TODO Auto-generated method stub      TreeSet lt String gt  set   new TreeSet lt String gt         try           set add  hello            set add  world            set add  welcome            set add  all             for  String num   set                System out println  num                      set add null         catch  NullPointerException e            System out println e           System out println  Set doesn t allow null value and duplicate value                   Output   all hello welcome world java lang NullPointerException Set doesn t allow null value and duplicate value

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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

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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

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List   Lists generally allow duplicate objects  Lists must be ordered  and are therefore accessible by index   Implementation classes include  ArrayList  LinkedList  Vector  Set   Sets  do not allow duplicate objects  Most implementations are unordered  but it is implementation specific   Implementation classes include   HashSet  unordered     LinkedHashSet  ordered    TreeSet  ordered by natural order or by provided comparator

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1 List allows duplicate values and  set does nt allow duplicates  2 List maintains the order in which you inserted elements in to the list Set does nt maintain order  3 List is an ordered sequence of elements whereas Set is a distinct list of elements which is unordered

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List is an ordered sequence of elements whereas Set is a distinct list of elements which is unordered  thank you  Quinn Taylor    List lt E gt       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 lt E gt       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

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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

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Set  A Set cannot have Duplicate elements in its collections  it is also an unordered collection  To access the data from Set  it is required to use Iterator only and index based retrieve is not possible for it  It is mainly used whenever required uniqueness collection   List  A List can have duplicate elements  with the natural ordered as it is inserted  Thus  it can be retrieved data based on index or iterator  It is widely used to store collection which needs to access based on index

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Ordering    a list has an order  a set does not

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List and Set both are interfaces  They both extends Collection interface  The important differences between set and list are   Duplicate Objects  The main difference between List and Set is that List allows duplicates while Set doesn t allow duplicates   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    Null elements  List allows any number of null elements  Set can have only a single null elements at most

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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

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Like the answer as SET don t have duplicate value and List can  Of course  order is another one thing to different them apart

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As we are talking about the Java interfaces  why not look at the Javadoc      A List is an ordered collection  sequence   which typically allows duplicates A Set a is collection that contains no duplicate elements  iteration order may be guaranteed by the implementation   There is NO mention about lack of order concerning Sets  it depends on the implementation

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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

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------------------------------------------------------------------------                                  List                       Set                   ------------------- ---------------------- -----------------------------          Duplicates                YES                       NO                    ------------------- ---------------------- -----------------------------            Order                ORDERED            DEPENDS ON IMPLEMENTATION       ------------------- ---------------------- -----------------------------      Positional Access            YES                        NO                    ------------------------------------------------------------------------

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Here ist a clear example with groovy  i create a set and a list  then i try to store 20 randomly generated value within each list  the generated value can be in range 0 to 5  s      as Set l       max   5 print  random Numbers    20 times  e    int Math random   max s  lt  lt  e l  lt  lt  e print   e        println   n  println  Set    s   println  list    l     The result    random Numbers  4  1  4  0  1  2  4  0  0  3  4  3  2  0  4  0  1  3  1  3  Set    4  1  0  2  3    list    4  1  4  0  1  2  4  0  0  3  4  3  2  0  4  0  1  3  1  3    You can see that the difference is that    Set does not allow duplicate values   List allow duplicate values

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List   Is an Ordered grouping of elements   List is used to collection of elements with duplicates  New methods are defined inside List interface    Set   Is an Unordered grouping of elements  Set is used to collection of elements without duplicates  No new methods are defined inside Set interface  so we have to use Collection interface methods only with Set subclasses

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Hi So many answers are already given  Let me point out some points which are not mentioned so far    Most of the List implementations  ArrayList Vector  implement RandomAccess interface which is a marker interface for faster access  None of the Set implementations do that  List uses one special Iterator called ListIterator which supports iteration in both directions  Set uses Iterator which supports only 1 way iteration HashSet takes 5 5 times more memory than ArrayList for storing same number of elements

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

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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

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List    Allowed duplicates  Ordered in grouping elements  In other words having definite order No need to sorted in ascending order    Set    Not allowed duplicates  Unordered in grouping elements  In other words having no definite order It might or might not arranged in ascending order

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List Vs Set  1  Set does not allow duplicates  List allows duplicate  Based on the implementation of Set  It also maintains the insertion Order    eg   LinkedHashSet  It maintains the insertion order Please refer click here  2  contains method  By nature of the Set it will give better performance to access  Best case its o 1   But List has performance issue to invoke contains

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List    List is ordered grouping elements List provides positional access in the elements of the collections  We can store duplicate elements in the list  List implements ArrayList  LinkedList  Vector and Stack  List can store multiple null elements   Set    Unorderd grouping elements  Set doesn   t provides positional access in the elements of the collections  We can   t store duplicate elements in the set  Set implement HashSet and LinkedHashSet interfaces  Set can store only one null elements

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Set lt E gt  and List lt E gt  are both used to store elements of type E   The difference is that Set is stored in unordered way and does not allow duplicate values  List is used to store elements in ordered way and it does allow duplicate values   Set elements cannot be accessed by an index position  and List elements can be accessed with an index position

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TOPIC Name  List VS Set  I have just gone through Java s most important topic called Collections Framework  I thought to share my little knowledge about Collections with you  List  Set  Map are the most important topic of it  So let s start with List and Set   Difference between List and Set    List is a collection class which extends AbstractList class where as Set is a collection class which extends AbstractSet class but both implements Collection interface  List interface allows duplicate values  elements  whereas Set interface does not allow duplicate values  In case of duplicate elements in Set  it replaces older values  List interface allows NULL values where as Set interface does not allow Null values  In case of using Null values in Set it gives NullPointerException   List interface maintains insertion order  That means the way we add the elements in the List in the same way we obtain it using iterator or for-each style  Whereas Set implementations do not necessarily maintain insertion order   Although SortedSet does using TreeSet  and LinkedHashSet maintains insertion order   List interface has its own methods defined whereas Set interface does not have its own method so Set uses Collection interface methods only  List interface has one legacy class called Vector whereas Set interface does not have any legacy class Last but not the least    The listIterator   method can only be used to cycle through the elements within List Classes whereas we can use iterator   method to access Set class elements   Anything else can we add  Please let me know   Thanks

[java] What is the difference between Set and List?

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