How to include duplicate keys in HashMap

Question

I need linked and  multiple keys in key set  I tried this   LinkedHashMap lt Integer  String gt  map   new LinkedHashMap lt  Integer String gt      map put  -1505711364  4    map put 294357273   15    map put -1593134417   28    map put -1231165758   45    map put 121046798   58    map put 294357273   71    map put -1593134417   82    map put -1231165758   95    map put 121046798   108      I need duplicate keys which is order preserved  What is the way to do this

User · Answer

You can t have duplicate keys in a Map  You can rather create a Map lt Key  List lt Value gt  gt   or if you can  use Guava s Multimap  Multimap lt Integer  String gt  multimap   ArrayListMultimap create    multimap put 1   quot rohit quot    multimap put 1   quot jain quot     System out println multimap get 1        Prints -  rohit  jain   And then you can get the java util Map using the Multimap asMap   method

User · Answer

Use Map lt Integer  List lt String gt  gt    Map lt Integer  List lt String gt  gt  map   new LinkedHashMap lt  Integer  List lt String gt  gt      map put -1505711364  new ArrayList lt  gt  Arrays asList  4      map put 294357273  new ArrayList lt  gt  Arrays asList  15    71              To add a new key value pair in this map   public void add Integer key  String newValue        List lt String gt  currentValue   map get key       if  currentValue    null            currentValue   new ArrayList lt String gt             map put key  currentValue             currentValue add newValue

User · Answer

Map does not supports duplicate keys  you can use collection as value against same key   Associates the specified value with the specified key in this map  optional operation   If the map previously contained a mapping for the key  the old value is replaced by the specified value   Documentation you can use any kind of  List or Set implementation according to your requirement  If your values might be also duplicate you can go with ArrayList or LinkedList  in case values are unique you can use HashSet or TreeSet etc   Also In google guava collection library Multimap is available  it is a collection that maps keys to values  similar to Map  but in which each key may be associated with multiple values  You can visualize the contents of a multimap either as a map from keys to nonempty collections of values  a   1  2 b   3    Example - ListMultimap lt String  String gt  multimap   ArrayListMultimap create    multimap put  quot a quot    quot 1 quot    multimap put  quot a quot    quot 2 quot    multimap put  quot c quot    quot 3 quot

User · Answer

hashMaps can t have duplicate keys  That said  you can create a map with list values   Map lt Integer  List lt String gt  gt    However  using this approach will have performance implications

[java] How to include duplicate keys in HashMap?

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