HashMap with multiple values under the same key

Question

Is it possible for us to implement a HashMap with one key and two values  Just as HashMap    Please do help me  also by telling  if there is no way  any other way to implement the storage of three values with one as the key

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Yes  this is frequently called a multimap   See  http   google-collections googlecode com svn trunk javadoc index html com google common collect Multimap html

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Using Java Collectors     Group employees by department Map lt Department  List lt Employee gt  gt  byDept   employees stream                        collect Collectors groupingBy Employee  getDepartment      where Department is your key

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I use Map lt KeyType  Object   gt  for associating multiple values with a key in a Map  This way  I can store multiple values of different types associated with a key  You have to take care by maintaining proper order of inserting and retrieving from Object     Example  Consider  we want to store Student information  Key is id  while we would like to store name  address and email associated to the student            To make entry into Map         Map lt Integer  String   gt  studenMap   new HashMap lt Integer  String   gt             String   studentInformationArray   new String    name    address    email            int studenId   1          studenMap put studenId  studentInformationArray              To retrieve values from Map         String name   studenMap get studenId  1           String address   studenMap get studenId  2           String email   studenMap get studenId  3

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You can do it implicitly      Create the map  There is no restriction to the size that the array String can have HashMap lt Integer  String   gt  map   new HashMap lt Integer  String   gt        initialize a key chosing the array of String you want for your values map put 1  new String      name1    name2         edit value of a key map get 1  0     othername     This is very simple and effective  If you want values of diferent classes instead  you can do the following   HashMap lt Integer  Object   gt  map   new HashMap lt Integer  Object   gt

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The easiest way would be to use a google collection library   import com google common collect ArrayListMultimap  import com google common collect Multimap   public class Test        public static void main final String   args                multimap can handle one key with a list of values         final Multimap lt String  String gt  cars   ArrayListMultimap create            cars put  Nissan    Qashqai            cars put  Nissan    Juke            cars put  Bmw    M3            cars put  Bmw    330E            cars put  Bmw    X6            cars put  Bmw    X5             cars get  Bmw   forEach System out  println               It will print the             M3            330E            X6            X5            maven link  https   mvnrepository com artifact com google collections google-collections 1 0-rc2  more on this  http   tomjefferys blogspot be 2011 09 multimaps-google-guava html

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Try LinkedHashMap  sample  Map lt String String gt  map   new LinkedHashMap lt String String gt         map put  1   linked   map put  1   hash        map put  2   map   map put  3   java        output   keys  1 1 2 3 values  linked hash  map  java

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If you use Spring Framework  There is  org springframework util MultiValueMap   To create unmodifiable multi value map   Map lt String List lt String gt  gt  map       MultiValueMap lt String  String gt  multiValueMap   CollectionUtils toMultiValueMap map     Or use org springframework util LinkedMultiValueMap

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Apache Commons collection classes can implement multiple values under same key  MultiMap multiMapDemo   new MultiValueMap     multiMapDemo  put  quot fruit quot    quot Mango quot    multiMapDemo  put  quot fruit quot    quot Orange quot    multiMapDemo put  quot fruit quot    quot Blueberry quot     System out println multiMapDemo get  quot fruit quot      Maven Dependency  lt  -- https   mvnrepository com artifact org apache commons commons-collections4 -- gt   lt dependency gt      lt groupId gt org apache commons lt  groupId gt      lt artifactId gt commons-collections4 lt  artifactId gt      lt version gt 4 4 lt  version gt   lt  dependency gt

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You could    Use a map that has a list as the value  Map lt KeyType  List lt ValueType gt  gt   Create a new wrapper class and place instances of this wrapper in the map  Map lt KeyType  WrapperType gt   Use a tuple like class  saves creating lots of wrappers   Map lt KeyType  Tuple lt Value1Type  Value2Type gt  gt   Use mulitple maps side-by-side      Examples  1  Map with list as the value     create our map Map lt String  List lt Person gt  gt  peopleByForename   new HashMap lt  gt             populate it List lt Person gt  people   new ArrayList lt  gt     people add new Person  Bob Smith     people add new Person  Bob Jones     peopleByForename put  Bob   people       read from it List lt Person gt  bobs   peopleByForename  Bob    Person bob1   bobs 0   Person bob2   bobs 1     The disadvantage with this approach is that the list is not bound to exactly two values   2  Using wrapper class     define our wrapper class Wrapper       public Wrapper Person person1  Person person2           this person1   person1         this person2   person2             public Person getPerson1   return this person1        public Person getPerson2   return this person2         private Person person1      private Person person2        create our map Map lt String  Wrapper gt  peopleByForename   new HashMap lt  gt         populate it Wrapper people   new Wrapper    peopleByForename put  Bob   new Wrapper new Person  Bob Smith                                            new Person  Bob Jones         read from it Wrapper bobs   peopleByForename get  Bob    Person bob1   bobs getPerson1  Person bob2   bobs getPerson2    The disadvantage to this approach is that you have to write a lot of boiler-plate code for all of these very simple container classes   3  Using a tuple     you ll have to write or download a Tuple class in Java    NET ships with one      create our map Map lt String  Tuple2 lt Person  Person gt  peopleByForename   new HashMap lt  gt         populate it peopleByForename put  Bob   new Tuple2 new Person  Bob Smith                                          new Person  Bob Jones         read from it Tuple lt Person  Person gt  bobs   peopleByForename  Bob    Person bob1   bobs Item1  Person bob2   bobs Item2    This is the best solution in my opinion   4  Multiple maps     create our maps Map lt String  Person gt  firstPersonByForename   new HashMap lt  gt     Map lt String  Person gt  secondPersonByForename   new HashMap lt  gt         populate them firstPersonByForename put  Bob   new Person  Bob Smith     secondPersonByForename put  Bob   new Person  Bob Jones         read from them Person bob1   firstPersonByForename  Bob    Person bob2   secondPersonByForename  Bob      The disadvantage of this solution is that it s not obvious that the two maps are related  a programmatic error could see the two maps get out of sync

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Yes and no  The solution is to build a Wrapper clas for your values that contains the 2  3  or more  values that correspond to your key

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Take a look at Multimap from the guava-libraries and its implementation - HashMultimap     A collection similar to a Map  but which may associate multiple values with a single key  If you call put K  V  twice  with the same key but different values  the multimap contains mappings from the key to both values

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Just for the record  the pure JDK8 solution would be to use Map  compute method   map compute key   s  strings  - gt  strings    null   new ArrayList lt  gt      strings  add value     Such as  public static void main String   args        Map lt String  List lt String gt  gt  map   new HashMap lt  gt          put map   first    hello        put map   first    foo        put map   bar    foo        put map   first    hello         map forEach  s  strings  - gt            System out print s                  System out println strings stream   collect Collectors joining                     private static  lt KEY  VALUE gt  void put Map lt KEY  List lt VALUE gt  gt  map  KEY key  VALUE value        map compute key   s  strings  - gt  strings    null   new ArrayList lt  gt      strings  add value       with output   bar  foo first  hello  foo  hello   Note that to ensure consistency in case multiple threads access this data structure  ConcurrentHashMap and CopyOnWriteArrayList for instance need to be used

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Can be done using an identityHashMap  subjected to the condition that the keys comparison will be done by    operator and not equals

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I prefer the following to store any number of variables without having to create a separate class    final public static Map lt String  Map lt String  Float gt  gt  myMap      new HashMap lt String  Map lt String  Float gt  gt

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I am so used to just doing this with a Data Dictionary in Objective C   It was harder to get a similar result in Java for Android   I ended up creating a custom class  and then just doing a hashmap of my custom class   public class Test1    Override protected void onCreate Bundle savedInstanceState        super onCreate savedInstanceState       setContentView R layout addview      create the datastring     HashMap lt Integer  myClass gt  hm   new HashMap lt Integer  myClass gt         hm put 1  new myClass  Car    Small   3000        hm put 2  new myClass  Truck    Large   4000        hm put 3  new myClass  Motorcycle    Small   1000       pull the datastring back for a specific item    also can edit the data using the set methods   this just shows getting it for display      myClass test1   hm get 1       String testitem   test1 getItem        int testprice   test1 getPrice        Log i  Class Info Example  testitem Integer toString testprice           custom class   You could make it public to use on several activities  or just include in the activity if using only here class myClass      private String item      private String type      private int price       public myClass String itm  String ty  int pr           this item   itm          this price   pr          this type   ty             public String getItem             return item             public void setItem String item            this item   item             public String getType             return item             public void setType String type            this type   type             public int getPrice             return price             public void setPrice int price            this price   price

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import java io     import java util      import com google common collect      class finTech  public static void main String args            Multimap lt String  String gt  multimap   ArrayListMultimap create           multimap put  1   11           multimap put  1   14           multimap put  1   12           multimap put  1   13           multimap put  11   111           multimap put  12   121            System out println multimap           System out println multimap get  11                                                                                                                                                                               Output           1    11   12   13   14    11    111    12    121             111     This is Google-Guava library for utility functionalities  This is the required solution

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String key   services servicename   ArrayList lt String gt  data   for int i   0  i lessthen data size    i          HashMap lt String  String gt  servicesNameHashmap   new HashMap lt String  String gt         servicesNameHashmap put key data get i  getServiceName         mServiceNameArray add i servicesNameHashmap       I have got the Best Results   You just have to create new HashMap like   HashMap lt String  String gt  servicesNameHashmap   new HashMap lt String  String gt       in your for loop  It will have same effect like same key and multiple values

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HashMap lt Integer ArrayList lt String gt  gt  map   new    HashMap lt Integer ArrayList lt String gt  gt      ArrayList lt String gt  list   new ArrayList lt String gt     list add  abc    list add  xyz    map put 100 list

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We can create a class to have multiple keys or values and the object of this class can be used as a parameter in map  You can refer to https   stackoverflow com a 44181931 8065321

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No  not just as a HashMap  You d basically need a HashMap from a key to a collection of values   If you re happy to use external libraries  Guava has exactly this concept in Multimap with implementations such as ArrayListMultimap and HashMultimap

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I could not post a reply on Paul s comment so I am creating new comment for Vidhya here   Wrapper will be a SuperClass for the two classes which we want to store as a value   and inside wrapper class  we can put the associations as the instance variable objects for the two class objects   e g   class MyWrapper     Class1 class1obj   new Class1     Class2 class2obj   new Class2            and in HashMap we can put in this way   Map lt KeyObject  WrapperObject gt     WrapperObj will have class variables  class1Obj  class2Obj

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Another nice choice is to use MultiValuedMap from Apache Commons  Take a look at the All Known Implementing Classes at the top of the page for specialized implementations   Example   HashMap lt K  ArrayList lt String gt  gt  map   new HashMap lt K  ArrayList lt String gt  gt      could be replaced with  MultiValuedMap lt K  String gt  map   new MultiValuedHashMap lt K  String gt       So   map put key   A    map put key   B    map put key   C     Collection lt String gt  coll   map get key     would result in collection coll containing  A    B   and  C

[java] HashMap with multiple values under the same key

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