HashMap One Key multiple Values

Question

How I can get the third value for the first key in this map  Is this possible

User · Answer

This is what i found in a similar question's answer

Map<String, List<String>> hm = new HashMap<String, List<String>>();
List<String> values = new ArrayList<String>();
values.add("Value 1");
values.add("Value 2");
hm.put("Key1", values);

// to get the arraylist
System.out.println(hm.get("key1"));

RESULT: [Value 1, Value 2]

User · Answer

A standard Java HashMap cannot store multiple values per key  any new entry you add will overwrite the previous one

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HashMap     Single Key and Multiple Values Using List  Map lt String  List lt String gt  gt  map   new HashMap lt String  List lt String gt  gt           create list one and store values      List lt String gt  One   new ArrayList lt String gt         One add  Apple        One add  Aeroplane            create list two and store values     List lt String gt  Two   new ArrayList lt String gt         Two add  Bat        Two add  Banana            put values into map     map put  A   One       map put  B   Two       map put  C   Three

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Here is the code how to get extract the hashmap into arrays  hashmap that contains arraylist Map lt String  List lt String gt  gt  country hashmap   new HashMap lt String  List lt String gt  gt        Creating two lists and inserting some data in it List lt String gt  list 1   new ArrayList lt String gt     list 1 add  quot 16873538 webp quot    list 1 add  quot 16873539 webp quot     List lt String gt  list 2   new ArrayList lt String gt     list 2 add  quot 16873540 webp quot    list 2 add  quot 16873541 webp quot       Inserting both the lists and key to the Map  country hashmap put  quot Malaysia quot   list 1   country hashmap put  quot Japanese quot   list 2    for Map Entry lt String  List lt String gt  gt  hashmap data   country hashmap entrySet           String key   hashmap data getKey       contains the keys       List lt String gt  val   hashmap data getValue       contains arraylists          print all the key and values in the hashmap       System out println key    quot    quot   val                   using interator to get the specific values arraylists       Iterator lt String gt  itr   val iterator          int i   0        String   data   new String val size             while  itr hasNext                 String array   itr next                data i    array              System out println data i       GET THE VALUE             i

User · Answer

Libraries exist to do this  but the simplest plain Java way is to create a Map of List like this   Map lt Object ArrayList lt Object gt  gt  multiMap   new HashMap lt  gt

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Write a new class that holds all the values that you need and use the new class s object as the value in your HashMap  HashMap lt String  MyObject gt   class MyObject   public String value1  public int value2  public List lt String gt  value3

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Try using collections to store the values of a key   Map lt Key  Collection lt Value gt  gt    you have to maintain the value list yourself

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For example   Map lt Object Pair lt Integer String gt  gt  multiMap   new HashMap lt Object Pair lt Integer String gt  gt       where the Pair is a parametric class  public class Pair lt A  B gt        A first   null      B second   null       Pair A first  B second            this first   first          this second   second             public A getFirst             return first             public void setFirst A first            this first   first             public B getSecond             return second             public void setSecond B second            this second   second

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You can do something like this  add access modifiers as required    Map lt String Map lt String String gt  gt  complexMap new HashMap lt String Map lt String String gt  gt       You can insert data like this        Map lt String String gt  componentMap   new HashMap lt String String gt         componentMap put  foo   bar        componentMap put  secondFoo   secondBar        complexMap put  superFoo  componentMap     The Generated Data Structure would be    superFoo  secondFoo secondBar  foo bar     This way each value for the key should have a unique identifier  Also gives O 1  for fetches if keys are known

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Thinking about a Map with 2 keys immediately compelled me to use a user-defined key  and that would probably be a Class  Following is the key Class   public class MapKey       private Object key1      private Object key2       public Object getKey1             return key1             public void setKey1 Object key1            this key1   key1             public Object getKey2             return key2             public void setKey2 Object key2            this key2   key2               Create first map entry with key  lt A B gt           MapKey mapKey1   new MapKey            mapKey1 setKey1  A            mapKey1 setKey2  B

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Have you got something like this   HashMap lt String  ArrayList lt String gt  gt    If so  you can iterate through your ArrayList and get the item you like with arrayList get i

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It sounds like you re looking for a multimap  Guava has various Multimap implementations  usually created via the Multimaps class  I would suggest that using that implementation is likely to be simpler than rolling your own  working out what the API should look like  carefully checking for an existing list when adding a value etc  If your situation has a particular aversion to third party libraries it may be worth doing that  but otherwise Guava is a fabulous library which will probably help you with other code too

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I found the blog on random search  i think this will help for doing this  http   tomjefferys blogspot com tr 2011 09 multimaps-google-guava html      public class MutliMapTest   public static void main String    args       Multimap lt String  String gt  myMultimap   ArrayListMultimap create           Adding some key value    myMultimap put  Fruits    Bannana       myMultimap put  Fruits    Apple       myMultimap put  Fruits    Pear       myMultimap put  Vegetables    Carrot           Getting the size    int size   myMultimap size       System out println size       4        Getting values    Collection lt String gt  fruits   myMultimap get  Fruits       System out println fruits       Bannana  Apple  Pear      Collection lt string gt  vegetables   myMultimap get  Vegetables       System out println vegetables       Carrot         Iterating over entire Mutlimap    for String value   myMultimap values            System out println value               Removing a single value    myMultimap remove  Fruits   Pear       System out println myMultimap get  Fruits         Bannana  Pear         Remove all values for a key    myMultimap removeAll  Fruits       System out println myMultimap get  Fruits            Empty Collection

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Apache Commons collection classes is the solution      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 multiMap get  quot fruit quot           Mango Orange Blueberry  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

[java] HashMap: One Key, multiple Values

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