Java HashMap How to get a key and value by index

Question

I am trying to use a HashMap to map a unique string to a string ArrayList like this   HashMap lt String  ArrayList lt String gt  gt    Basically  I want to be able to access the keys by number  not by using the key s name  And I want to be able to access said key s value  to iterate over it  I m imagining something like this   for all keys in my hashmap        for int i 0  i  lt  myhashmap currentKey getValue size    i                 do things with the hashmaps elements           Is there an easy way to do this

User · Answer

HashMaps are not ordered  unless you use a LinkedHashMap or SortedMap  In this case  you may want a LinkedHashMap   This will iterate in order of insertion  or in order of last access if you prefer   In this case  it would be  int index   0  for   Map Entry lt String ArrayList lt String gt  gt  e   myHashMap iterator   entrySet           String key   e getKey        ArrayList lt String gt  val   e getValue        index        There is no direct get index  in a map because it is an unordered list of key value pairs  LinkedHashMap is a special case that keeps the order

User · Answer

Here is the general solution if you really only want the first key s value  Object firstKey   myHashMap keySet   toArray   0   Object valueForFirstKey   myHashMap get firstKey

User · Answer

A solution is already selected  However  I post this solution for those who want to use an alternative approach      use LinkedHashMap if you want to read values from the hashmap in the same order as you put them into it private ArrayList lt String gt  getMapValueAt LinkedHashMap lt String  ArrayList lt String gt  gt  hashMap  int index        Map Entry lt String  ArrayList lt String gt  gt  entry    Map Entry lt String  ArrayList lt String gt  gt   hashMap entrySet   toArray   index       return entry getValue

User · Answer

You can do   for String key  hashMap keySet         for String value  hashMap get key                use the value here           This will iterate over every key  and then every value of the list associated with each key

User · Answer

HashMaps don t keep your key value pairs in a specific order   They are ordered based on the hash that each key s returns from its Object hashCode   method   You can however iterate over the set of key value pairs using an iterator with    for  String key   hashmap keySet           for  list   hashmap get key                   list toString

User · Answer

I came across the same problem  read a couple of answers from different related questions and came up with my own class   public class IndexableMap lt K  V gt  extends HashMap lt K  V gt         private LinkedList lt K gt  keyList   new LinkedList lt  gt           Override     public V put K key  V value            if   keyList contains key               keyList add key           return super put key  value               Override     public void putAll Map lt   extends K    extends V gt  m            for  Entry lt   extends K    extends V gt  entry   m entrySet                  put entry getKey    entry getValue                           Override     public void clear             keyList clear            super clear               public List lt K gt  getKeys             return keyList             public int getKeyIndex K key            return keyList indexOf key              public K getKeyAt int index            if  keyList size    gt  index              return keyList get index           return null             public V getValueAt int index            K key   getKeyAt index           if  key    null              return get key           return null            Example  types are differing from OPs question just for clarity    Map lt String  Double gt  myMap   new IndexableMap lt  gt      List lt String gt  keys   myMap getKeys    int keyIndex   myMap getKeyIndex  keyString    String key   myMap getKeyAt 2   Double value myMap getValueAt 2     Keep in mind that it does not override any of the complex methods  so you will need to do this on your own if you want to reliably access one of these   Edit  I made a change to the putAll   method  because the old one had a rare chance to cause HashMap and LinkedList being in different states

User · Answer

If you don t care about the actual key  a concise way to iterate over all the Map s values would be to use its values   method  Map lt String  List lt String gt  gt  myMap   for   List lt String gt  stringList   myMap values           for   String myString   stringList                process the string here           The values   method is part of the Map interface and returns a Collection view of the values in the map

User · Answer

You can use Kotlin extension function  fun LinkedHashMap lt String  String gt  getKeyByPosition position  Int            this keys toTypedArray   position    fun LinkedHashMap lt String  String gt  getValueByPosition position  Int            this values toTypedArray   position

User · Answer

You can iterate over keys by calling map keySet    or iterate over the entries by calling map entrySet    Iterating over entries will probably be faster   for  Map Entry lt String  List lt String gt  gt  entry   map entrySet          List lt String gt  list   entry getValue           Do things with the list     If you want to ensure that you iterate over the keys in the same order you inserted them then use a LinkedHashMap   By the way  I d recommend changing the declared type of the map to  lt String  List lt String gt  gt   Always best to declare types in terms of the interface rather than the implementation

User · Answer

for  Object key   data keySet          String lKey    String  key      List lt String gt  list   data get key

[java] Java HashMap: How to get a key and value by index?

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