Storing and Retrieving ArrayList values from hashmap

Question

I have a hashmap of the following type   HashMap lt String ArrayList lt Integer gt  gt  map new HashMap lt String ArrayList lt Integer gt  gt           The values stored are like this    mango   0 4 8 9 12 apple   2 3 grapes  1 7 peach   5 6 11   I want to store as well as fetch those Integers using Iterator or any other way with minimum lines of code How can I do it   EDIT 1  The numbers are added at random  not together  as key is matched to the appropriate line   EDIT 2  How can I point to the arraylist while adding     I am getting error while adding a new number 18 in the line map put string number

User · Answer

for  Map Entry lt String  ArrayList lt Integer gt  gt  entry   map entrySet       System out println  entry getKey           System out println  entry getValue      Returns the list of values

User · Answer

Our variable   Map lt String  List lt Integer gt  gt  map   new HashMap lt String  List lt Integer gt  gt       To store   map put  mango   new ArrayList lt Integer gt  Arrays asList 0  4  8  9  12       To add numbers one and one  you can do something like this   String key    mango   int number   42  if  map get key     null        map put key  new ArrayList lt Integer gt        map get key  add number     In Java 8 you can use putIfAbsent to add the list if it did not exist already   map putIfAbsent key  new ArrayList lt Integer gt      map get key  add number       Use the map entrySet   method to iterate on   for  Entry lt String  List lt Integer gt  gt  ee   map entrySet          String key   ee getKey        List lt Integer gt  values   ee getValue           TODO  Do something

User · Answer

You can use like this Though the random number generator logic is not upto the mark    public class WorkSheet       HashMap lt String ArrayList lt Integer gt  gt  map   new HashMap lt String ArrayList lt Integer gt  gt      public static void main String args          WorkSheet test   new WorkSheet        test inputData  mango   5       test inputData  apple   2       test inputData  grapes   2       test inputData  peach   3       test displayData       public void displayData        for  Entry lt String  ArrayList lt Integer gt  gt  entry   map entrySet              System out print entry getKey                   for int fruitNo   entry getValue                 System out print fruitNo                         System out println            public void inputData String name  int number        Random rndData   new Random        ArrayList lt Integer gt  fruit   new ArrayList lt Integer gt         for int i 0   i lt number   i             fruit add rndData nextInt 10              map put name  fruit         OUTPUT  grapes   7 5  apple   9 5  peach   5 5 8  mango   4 7 1 5 5

User · Answer

The modern way  as of 2020  to add entries to a multimap  a map of lists  in Java is  map computeIfAbsent  quot apple quot   k - gt  new ArrayList lt  gt     add 2   map computeIfAbsent  quot apple quot   k - gt  new ArrayList lt  gt     add 3    According to Map computeIfAbsent docs   If the specified key is not already associated with a value  or is mapped to null   attempts to compute its value using the given mapping function and enters it into this map unless null  Returns  the current  existing or computed  value associated with the specified key  or null if the computed value is null  The most idiomatic way to iterate a map of lists is using Map forEach and Iterable forEach  map forEach  k  l  - gt  l forEach v - gt     use k and v here        Or  as shown in other answers  a traditional for loop  for  Map Entry lt String  List lt Integer gt  gt  e   map entrySet          String k   e getKey        for  Integer v   e getValue                 use k and v here

User · Answer

Iterator it   map entrySet   iterator    while  it hasNext           Map Entry pairs    Map Entry it next          if pairs getKey   equals  mango                   map put pairs getKey    pairs getValue   add 18                 else if  map containsKey  mango                   List lt Integer gt  ints   new ArrayList lt Integer gt             ints add 18           map put  mango  ints                it remove       avoids a ConcurrentModificationException     EDIT  So inside the while try this   map put pairs getKey    pairs getValue   add number     You are getting the error because you are trying to put an integer to the values  whereas it is expected an ArrayList    EDIT 2  Then put the following inside your while loop   if pairs getKey   equals  mango          map put pairs getKey    pairs getValue   add 18       else if  map containsKey  mango           List lt Integer gt  ints   new ArrayList lt Integer gt          ints add 18        map put  mango  ints        EDIT 3  By reading your requirements  I come to think you may not need a loop  You may want to only check if the map contains the key mango  and if it does add 18  else create a new entry in the map with key mango and value 18   So all you may need is the following  without the loop   if map containsKey  mango          map put  mango   map get  mango  add 18       else       List lt Integer gt  ints   new ArrayList lt Integer gt         ints add 18       map put  mango   ints

User · Answer

Fetch all at once    List lt Integer gt  list   null   if map   null        list   new ArrayList lt Integer gt  map values          For Storing     if map   null        list   map get keyString       if list    null                 list   new ArrayList lt Integer gt             list add value     map put keyString list

User · Answer

You could try using MultiMap instead of HashMap  Initialising it will require fewer lines of codes  Adding and retrieving the values will also make it shorter   Map lt String  List lt Integer gt  gt  map   new HashMap lt String  List lt Integer gt  gt       would become   Multimap lt String  Integer gt  multiMap   ArrayListMultimap create      You can check this link  http   java dzone com articles hashmap- E2 80 93-single-key-and

[java] Storing and Retrieving ArrayList values from hashmap

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