Is it possible to get element from HashMap by its position

Question

How to retrieve an element from HashMap by its position  is it possible at all

User · Answer

Use LinkedHashMap      Hash table and linked list implementation of the Map interface  with predictable iteration order  This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries

User · Answer

If you want to maintain the order in which you added the elements to the map  use LinkedHashMap as opposed to just HashMap   Here is an approach that will allow you to get a value by its index in the map   public Object getElementByIndex LinkedHashMap map int index       return map get   map keySet   toArray     index

User · Answer

Another working approach is transforming map values into an array and then retrieve element at index  Test run of 100 000 element by index searches in LinkedHashMap of 100 000 objects using following approaches led to following results      My answer  public Particle getElementByIndex LinkedHashMap lt Point  Particle gt  map int index       return map values   toArray new Particle map values   size     index       68 965 ms    Syd Lambert s answer  public Particle getElementByIndex LinkedHashMap lt Point  Particle gt  map int index       return map get   map keySet   toArray     index          80 700 ms   All in all retrieving element by index from LinkedHashMap seems to be pretty heavy operation

User · Answer

By default  java LinkedHasMap does not support for getting value by position  So I suggest go with customized IndexedLinkedHashMap  public class IndexedLinkedHashMap lt K  V gt  extends LinkedHashMap lt K  V gt         private ArrayList lt K gt  keysList   new ArrayList lt  gt          public void add K key  V val            super put key  val           keysList add key              public void update K key  V val            super put key  val              public void removeItemByKey K key            super remove key           keysList remove key              public void removeItemByIndex int index            super remove keysList get index            keysList remove index              public V getItemByIndex int i            return  V  super get keysList get i               public int getIndexByKey K key            return keysList indexOf key             Then you can use this customized LinkedHasMap as  IndexedLinkedHashMap lt String UserModel gt  indexedLinkedHashMap new IndexedLinkedHashMap lt  gt       TO add Values   indexedLinkedHashMap add  key1  UserModel     To getValue by index  indexedLinkedHashMap getItemByIndex position

User · Answer

If you  for some reason  have to stick with the hashMap  you can convert the keySet to an array and index the keys in the array to get the values in the map like so   Object   keys   map keySet   toArray      You can then access the map like   map get keys i

User · Answer

You can try to implement something like that  look at    Map lt String  Integer gt  map   new LinkedHashMap lt String  Integer gt     map put  juan   2   map put  pedro   3   map put  pablo   5   map put  iphoncio  9   List lt String gt  indexes   new ArrayList lt String gt  map keySet         lt    Parse  System out println indexes indexOf  juan               gt  0 System out println indexes indexOf  iphoncio                gt  3   I hope this works for you

User · Answer

I m assuming by  position  you re referring to the order in which you ve inserted the elements into the HashMap   In that case you want to be using a LinkedHashMap   The LinkedHashMap doesn t offer an accessor method however  you will need to write one like  public Object getElementAt LinkedHashMap map  int index        for  Map Entry entry   map entrySet              if  index--    0                return entry value                        return null

User · Answer

HashMaps do not preserve ordering      This class makes no guarantees as to   the order of the map  in particular    it does not guarantee that the order   will remain constant over time    Take a look at LinkedHashMap  which guarantees a predictable iteration order

User · Answer

HashMaps don t allow access by position  it only knows about the hash code and and it can retrieve the value if it can calculate the hash code of the key  TreeMaps have a notion of ordering  Linkedhas maps preserve the order in which they entered the map

User · Answer

HashMap - and the underlying data structure - hash tables  do not have a notion of position  Unlike a LinkedList or Vector  the input key is transformed to a  bucket  where the value is stored   These buckets are not ordered in a way that makes sense outside the HashMap interface and as such  the items you put into the HashMap are not in order in the sense that you would expect with the other data structures

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HashMap has no concept of position so there is no way to get an object by position  Objects in Maps are set and get by keys

User · Answer

Use LinkedHashMap and use this function    private LinkedHashMap lt Integer  String gt  map   new LinkedHashMap lt Integer  String gt       Define like this and   private Entry getEntry int id           Iterator iterator   map entrySet   iterator            int n   0          while iterator hasNext                 Entry entry    Entry  iterator next                if n    id                   return entry                            n                       return null          The function can return the selected entry

User · Answer

you can use below code to get key    String     keys    String    item keySet   toArray new String 0     and get object or list that insert in HashMap with key of this item like this   item get keys position

User · Answer

Use a LinkedHashMap and when you need to retrieve by position  convert the values into an ArrayList   LinkedHashMap lt String String gt  linkedHashMap   new LinkedHashMap lt String String gt        Populate    linkedHashMap put  key0   value0    linkedHashMap put  key1   value1    linkedHashMap put  key2   value2       Get by position    int pos   1  String value    new ArrayList lt String gt  linkedHashMap values     get pos

[java] Is it possible to get element from HashMap by its position?

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