HashSet vs LinkedHashSet

Question

What is the difference between them  I know that      A LinkedHashSet is an ordered version of HashSet that   maintains a doubly-linked List across all elements  Use this class instead of HashSet   when you care about the iteration order  When you iterate through a HashSet the   order is unpredictable  while a LinkedHashSet lets you iterate through the elements   in the order in which they were inserted     But in sourcecode of LinkedHashSet there are only calling constructors of HashSet  So where is double-linked List and insertion order

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HashSet   The underlined data structure is Hashtable  Duplicate objects are not allowed insertion order is not preserved and it is based on hash code of objects  Null insertion is possible only once   It implements Serializable  Clonable but not RandomAccess interface  HashSet is best choose if frequent operation is search operation      In HashSet duplicates are not allowed if users are trying to insert duplicates when we won t get any compile or runtime exceptions  add method returns simply false    Constructors      HashSet h new HashSet    creates an empty HashSet object with default initial capacity 16 and default fill ratio Load factor  is 0 75        HashSet h new HashSet int initialCapacity   creates an empty HashSet object with specified initialCapacity and default fill ration is 0 75       HashSet h new HashSet int initialCapacity  float fillRatio        HashSet h new HashSet Collection c   creates an equivalent HashSet object for the given collection  This constructor meant for inter conversion between collection object    LinkedHashSet   It is a child class of HashSet  it is exactly same as HashSet including Constructors and Methods  except the following differences   Differences HashSet    The underlined data structure is Hashtable  Insertion order is not preserved  introduced 1 2 version    LinkedHashSet    The underlined data structure is a combination of LinkedList and Hashtable  Insertion order is preserved  Indroduced in 1 4 version

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All Methods and constructors are same but only one difference is LinkedHashset will maintain insertion order but it will not allow duplicates   Hashset will not maintain any insertion order  It is combination of List and Set simple

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HashSet don t maintain the order of insertion item LinkedHashSet maintain the order of insertion item  Example  Set lt String gt  set          using new HashSet lt  gt    OR new LinkedHashSet lt  gt    set add  2    set add  1    set add  ab    for String value   set      System out println value         HashSet output  1 ab 2   LinkedHashSet output  2 1 ab

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HashSet is unordered and unsorted Set  LinkedHashSet is the ordered version of HashSet     The only difference between HashSet and LinkedHashSet is that  LinkedHashSet maintains the insertion order     When we iterate through a HashSet  the order is unpredictable while it is predictable in case of LinkedHashSet     The reason for how LinkedHashSet maintains insertion order is that  The underlying used data structure is Doubly-Linked-List

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HashSet  Unordered actually  if u passing the parameter means  Set lt Integer gt  set new HashSet lt Integer gt     for int i 0 i lt set length i        SOP set  enter code here      Out Put  May be 2 1 3 not predictable  next time another order   LinkedHashSet   which produce FIFO Order

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I suggest you to use LinkedHashSet most of the time  because it has better performance overall     Predictable iteration order LinkedHashSet  Oracle  LinkedHashSet is more expensive for insertions than HashSet  In general slightly better performance than HashMap  because the most of the time we use Set structures for iterating       Performance tests    ------------- TreeSet -------------  size       add  contains   iterate    10       746       173        89   100       501       264        68  1000       714       410        69 10000      1975       552        69 ------------- HashSet -------------  size       add  contains   iterate    10       308        91        94   100       178        75        73  1000       216       110        72 10000       711       215       100 ---------- LinkedHashSet ----------  size       add  contains   iterate    10       350        65        83   100       270        74        55  1000       303       111        54 10000      1615       256        58   You can see source test page here  The Final Performance Testing Example

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If you take a look at the constructors called from the LinkedHashSet class you will see that internally it s a LinkedHashMap that is used for backing purpose

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LinkedHashSet s constructors invoke the following base class constructor   HashSet int initialCapacity  float loadFactor  boolean dummy      map   new LinkedHashMap lt E  Object gt  initialCapacity  loadFactor       As you can see  the internal map is a LinkedHashMap  If you look inside LinkedHashMap  you ll discover the following field   private transient Entry lt K  V gt  header    This is the linked list in question

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The answer lies in which constructors the LinkedHashSet uses to construct the base class   public LinkedHashSet int initialCapacity  float loadFactor        super initialCapacity  loadFactor  true            lt -- boolean dummy argument         public LinkedHashSet int initialCapacity        super initialCapacity   75f  true                  lt -- boolean dummy argument         public LinkedHashSet         super 16   75f  true                               lt -- boolean dummy argument         public LinkedHashSet Collection lt   extends E gt  c        super Math max 2 c size    11    75f  true         lt -- boolean dummy argument     addAll c       And  one example of  a HashSet constructor that takes a boolean argument is described  and looks like this          Constructs a new  empty linked hash set    This package private    constructor is only used by LinkedHashSet   The backing    HashMap instance is a LinkedHashMap with the specified initial    capacity and the specified load factor         param      initialCapacity   the initial capacity of the hash map     param      loadFactor        the load factor of the hash map     param      dummy             ignored  distinguishes this                constructor from other int  float constructor       throws     IllegalArgumentException if the initial capacity is less                than zero  or if the load factor is nonpositive     HashSet int initialCapacity  float loadFactor  boolean dummy        map   new LinkedHashMap lt E Object gt  initialCapacity  loadFactor

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You should look at the source of the HashSet constructor it calls    it s a special constructor that makes the backing Map a LinkedHashMap instead of just a HashMap

[java] HashSet vs LinkedHashSet

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