adding multiple entries to a HashMap at once in one statement

Question

I need to initialize a constant HashMap and would like to do it in one line statement  Avoiding sth like this     hashMap put  One   new Integer 1       adding value into HashMap   hashMap put  Two   new Integer 2            hashMap put  Three   new Integer 3      similar to this in objective C    NSDictionary dictionaryWithObjectsAndKeys    w   NSNumber numberWithInt 1     K   NSNumber numberWithInt 2     e   NSNumber numberWithInt 4     z   NSNumber numberWithInt 5     l   NSNumber numberWithInt 6   nil     I have not found any example that shows how to do this having looked at so many

User · Answer

boolean x      for  x   false           map put  One   new Integer 1             map put  Two   new Integer 2                  map put  Three   new Integer 3    x      Ignoring the declaration of x  which is necessary to avoid an  unreachable statement  diagnostic   technically it s only one statement

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You can use the Double Brace Initialization as shown below  Map lt String  Integer gt  hashMap   new HashMap lt String  Integer gt            put  quot One quot   1        put  quot Two quot   2        put  quot Three quot   3        As a piece of warning  please refer to the thread Efficiency of Java    Double Brace Initialization quot  for the performance implications that it might have

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Based on solution  presented by  Dakusan  the class defining to extend the HashMap   I did it this way    public static HashMap lt String String gt  SetHash String   pairs         HashMap lt String String gt  rtn   new HashMap lt String String gt  pairs length 2        for   int n 0  n  lt  pairs length  n  2   rtn put pairs n   pairs n   1        return rtn           and using it this way  HashMap lt String String gt  hm   SetHash   quot one quot   quot aa quot    quot two quot   quot bb quot    quot tree quot   quot cc quot      Not sure if there is any disadvantages in that way  I am not a java developer  just has to do some task in java   but it works and seems to me comfortable

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Here s a simple class that will accomplish what you want  import java util HashMap   public class QuickHash extends HashMap lt String String gt        public QuickHash String   KeyValuePairs            super KeyValuePairs length 2           for int i 0 i lt KeyValuePairs length i  2              put KeyValuePairs i   KeyValuePairs i 1              And then to use it  Map lt String  String gt  Foo QuickHash       a    1        b    2       This yields  a 1  b 2

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You can use Google Guava s ImmutableMap   This works as long as you don t care about modifying the Map later  you can t call  put   on the map after constructing it using this method    import com google common collect ImmutableMap      For up to five entries  use  of   Map lt String  Integer gt  littleMap   ImmutableMap of       One   Integer valueOf 1        Two   Integer valueOf 2        Three   Integer valueOf 3         For more than five entries  use  builder   Map lt String  Integer gt  bigMap   ImmutableMap  lt String  Integer gt builder        put  One   Integer valueOf 1        put  Two   Integer valueOf 2        put  Three   Integer valueOf 3        put  Four   Integer valueOf 4        put  Five   Integer valueOf 5        put  Six   Integer valueOf 6        build      See also  http   docs guava-libraries googlecode com git javadoc com google common collect ImmutableMap html  A somewhat related question  ImmutableMap of   workaround for HashMap in Maps

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Another approach may be writing special function to extract all elements values from one string by regular-expression   import java util HashMap  import java util regex Matcher  import java util regex Pattern   public class Example       public static void main  String   args           HashMap lt String Integer gt  hashMapStringInteger   createHashMapStringIntegerInOneStat   one    gt   1    two    gt   2     three   gt  3                System out println hashMapStringInteger       one 1  two 2  three 3             private static HashMap lt String  Integer gt  createHashMapStringIntegerInOneStat String str            HashMap lt String  Integer gt  returnVar   new HashMap lt String  Integer gt              String currentStr   str          Pattern pattern1   Pattern compile     s            s    s  gt   s            s     s                      Parse all elements in the given string          boolean thereIsMore   true          while  thereIsMore               Matcher matcher   pattern1 matcher currentStr               if  matcher find                      returnVar put matcher group 1  Integer valueOf matcher group 2                     currentStr   matcher group 3                             else                  thereIsMore   false                                      Validate that all elements in the given string were parsed properly         if  currentStr length    gt  0               System out println  WARNING  Problematic string format  given String      str                      return returnVar

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Since Java 9  it is possible to use Map of       like so   Map lt String  Integer gt  immutableMap   Map of  One   1                                               Two   2                                               Three   3     This map is immutable  If you want the map to be mutable  you have to add   Map lt String  Integer gt  hashMap   new HashMap lt  gt  immutableMap     If you can t use Java 9  you re stuck with writing a similar helper method yourself or using a third-party library  like Guava  to add that functionality for you

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You could add this utility function to a utility class   public static  lt K  V gt  Map lt K  V gt  mapOf Object    keyValues        Map lt K  V gt  map   new HashMap lt  gt          for  int index   0  index  lt  keyValues length   2  index              map put  K keyValues index   2    V keyValues index   2   1               return map     Map lt Integer  String gt  map1   YourClass mapOf 1   value1   2   value2    Map lt String  String gt  map2   YourClass mapOf  key1    value1    key2    value2      Note  in Java 9 you can use Map of

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Maps have also had factory methods added in Java 9  For up to 10 entries Maps have overloaded constructors that take pairs of keys and values  For example we could build a map of various cities and their populations  according to google in October 2016  as follow   Map lt String  Integer gt  cities   Map of  Brussels   1 139000   Cardiff   341 000     The var-args case for Map is a little bit harder  you need to have both keys and values  but in Java  methods can   t have two var-args parameters  So the general case is handled by taking a var-args method of Map Entry lt K  V gt  objects and adding a static entry   method that constructs them  For example   Map lt String  Integer gt  cities   Map ofEntries      entry  Brussels   1139000        entry  Cardiff   341000       Collection Factory Methods in Java 9

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Java has no map literal  so there s no nice way to do exactly what you re asking   If you need that type of syntax  consider some Groovy  which is Java-compatible and lets you do   def map    name  Gromit   likes  cheese   id 1234

[java] adding multiple entries to a HashMap at once in one statement

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