builder for HashMap

Question

Guava provides us with great factory methods for Java types  such as Maps newHashMap     But are there also builders for java Maps   HashMap lt String Integer gt  m   Maps BuildHashMap      put  a  1       put  b  2       build

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You can use   HashMap lt String Integer gt  m   Maps newHashMap      ImmutableMap of  a  1  b  2       It s not as classy and readable  but does the work

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I had a similar requirement a while back  Its nothing to do with Guava but you can do something like this to be able to cleanly construct a Map using a fluent builder   Create a base class that extends Map   public class FluentHashMap lt K  V gt  extends LinkedHashMap lt K  V gt        private static final long serialVersionUID   4857340227048063855L       public FluentHashMap           public FluentHashMap lt K  V gt  delete Object key            this remove key           return this            Then create the fluent builder with methods that suit your needs   public class ValueMap extends FluentHashMap lt String  Object gt        private static final long serialVersionUID   1L       public ValueMap           public ValueMap withValue String key  String val            super put key  val           return this             Add withXYZ to suit         You can then implement it like this   ValueMap map   new ValueMap          withValue  key 1    value 1          withValue  key 2    value 2          withValue  key 3    value 3

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There is no such thing for HashMaps  but you can create an ImmutableMap with a builder   final Map lt String  Integer gt  m   ImmutableMap  lt String  Integer gt builder          put  a   1         put  b   2         build      And if you need a mutable map  you can just feed that to the HashMap constructor   final Map lt String  Integer gt  m   Maps newHashMap      ImmutableMap  lt String  Integer gt builder            put  a   1           put  b   2           build

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A simple map builder is trivial to write   public class Maps        public static  lt Q W gt  MapWrapper lt Q W gt  map Q q  W w            return new MapWrapper lt Q  W gt  q  w              public static final class MapWrapper lt Q W gt            private final HashMap lt Q W gt  map          public MapWrapper Q q  W w                map   new HashMap lt Q  W gt                 map put q  w                     public MapWrapper lt Q W gt  map Q q  W w                map put q  w               return this                    public Map lt Q W gt  getMap                 return map                       public static void main String   args            Map lt String  Integer gt  map   Maps map  one   1  map  two   2  map  three   3  getMap            for  Map Entry lt String  Integer gt  entry   map entrySet                  System out println entry getKey             entry getValue

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Since Java 9 Map interface contains    Map of k1 v1  k2 v2       Map ofEntries Map entry k1 v1   Map entry k2 v2           Limitations of those factory methods are that they    can t hold nulls as keys and or values  if you need to store nulls take a look at other answers  produce immutable maps     If we need mutable map  like HashMap  we can use its copy-constructor and let it copy content of map created via Map of      Map lt Integer  String gt  map   new HashMap lt  gt   Map of 1  a   2  b   3  c

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This is something I always wanted  especially while setting up test fixtures  Finally  I decided to write a simple fluent builder of my own that could build any Map implementation -  https   gist github com samshu b471f5a2925fa9d9b718795d8bbdfe42 file-mapbuilder-java                  param mapClass Any   link Map  implementation type  e g   HashMap class             public static  lt K  V gt  MapBuilder lt K  V gt  builder  SuppressWarnings  rawtypes   Class lt   extends Map gt  mapClass              throws InstantiationException              IllegalAccessException           return new MapBuilder lt K  V gt  mapClass              public MapBuilder lt K  V gt  put K key  V value            map put key  value           return this             public Map lt K  V gt  build             return map

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Here is a very simple one      public class FluentHashMap lt K  V gt  extends java util HashMap lt K  V gt      public FluentHashMap lt K  V gt  with K key  V value        put key  value       return this         public static  lt K  V gt  FluentHashMap lt K  V gt  map K key  V value        return new FluentHashMap lt K  V gt    with key  value           then   import static FluentHashMap map   HashMap lt String  Integer gt  m   map  a   1  with  b   2     See https   gist github com culmat a3bcc646fa4401641ac6eb01f3719065

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There s ImmutableMap builder   in Guava

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Using java 8   This is a approach of Java-9 Map ofEntries Map entry k1 v1   Map entry k2 v2         public class MapUtil       import static java util stream Collectors toMap       import java util AbstractMap SimpleEntry      import java util Map      import java util Map Entry      import java util stream Stream       private MapUtil            SafeVarargs     public static Map lt String  Object gt  ofEntries SimpleEntry lt String  Object gt     values            return Stream of values  collect toMap Entry  getKey  Entry  getValue               public static SimpleEntry lt String  Object gt  entry String key  Object value            return new SimpleEntry lt String  Object gt  key  value             How to Use   import static your package name MapUtil     import java util Map   Map lt String  Object gt  map   ofEntries          entry  id   1           entry  description    xyz            entry  value   1 05           entry  enable   true

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You can use the fluent API in Eclipse Collections   Map lt String  Integer gt  map   Maps mutable  lt String  Integer gt empty            withKeyValue  a   1           withKeyValue  b   2    Assert assertEquals Maps mutable with  a   1   b   2   map     Here s a blog with more detail and examples   Note  I am a committer for Eclipse Collections

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Not quite a builder  but using an initializer    Map lt String  String gt  map   new HashMap lt String  String gt           put  a    1        put  b    2

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Underscore-java can build hashmap  Map lt String  Object gt  value   U objectBuilder            add  quot firstName quot    quot John quot            add  quot lastName quot    quot Smith quot            add  quot age quot   25           add  quot address quot   U arrayBuilder                add U objectBuilder                    add  quot streetAddress quot    quot 21 2nd Street quot                    add  quot city quot    quot New York quot                    add  quot state quot    quot NY quot                    add  quot postalCode quot    quot 10021 quot              add  quot phoneNumber quot   U arrayBuilder                add U objectBuilder                    add  quot type quot    quot home quot                    add  quot number quot    quot 212 555-1234 quot                 add U objectBuilder                    add  quot type quot    quot fax quot                    add  quot number quot    quot 646 555-4567 quot              build            firstName John  lastName Smith  age 25  address   streetAddress 21 2nd Street         city New York  state NY  postalCode 10021    phoneNumber   type home  number 212 555-1234           type fax  number 646 555-4567

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HashMap is mutable  there s no need for a builder   Map lt String  Integer gt  map   Maps newHashMap    map put  a   1   map put  b   2

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Here s one I wrote  import java util Collections  import java util HashMap  import java util Map  import java util function Supplier   public class MapBuilder lt K  V gt         private final Map lt K  V gt  map                  Create a HashMap builder             public MapBuilder             map   new HashMap lt  gt                           Create a HashMap builder         param initialCapacity             public MapBuilder int initialCapacity            map   new HashMap lt  gt  initialCapacity                         Create a Map builder         param mapFactory             public MapBuilder Supplier lt Map lt K  V gt  gt  mapFactory            map   mapFactory get               public MapBuilder lt K  V gt  put K key  V value            map put key  value           return this             public Map lt K  V gt  build             return map                        Returns an unmodifiable Map  Strictly speaking  the Map is not immutable because any code with a reference to        the builder could mutate it                 return             public Map lt K  V gt  buildUnmodifiable             return Collections unmodifiableMap map             You use it like this   Map lt String  Object gt  map   new MapBuilder lt String  Object gt  LinkedHashMap  new       put  event type   newEvent getType         put  app package name   newEvent getPackageName         put  activity   newEvent getActivity         build

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This is similar to the accepted answer  but a little cleaner  in my view   ImmutableMap of  key1   val1   key2   val2   key3   val3     There are several variations of the above method  and they are great for making static  unchanging  immutable maps

[java] builder for HashMap

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