HashMap to return default value for non-found keys

Question

Is it possible to have a HashMap return a default value for all keys that are not found in the set

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Not directly  but you can extend the class to modify its get method  Here is a ready to use example  http   www java2s com Code Java Collections-Data-Structure ExtendedVersionofjavautilHashMapthatprovidesanextendedgetmethodaccpetingadefaultvalue htm

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It does this by default  It returns null

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public final Map lt String  List lt String gt  gt  stringMap   new ConcurrentHashMap lt String  List lt String gt  gt               Nullable          Override         public List lt String gt  get  NonNull Object key                return computeIfAbsent  String  key  s - gt  new ArrayList lt String gt                        HashMap cause dead loop  so use ConcurrentHashMap instead of HashMap

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Use   myHashMap getOrDefault key  defaultValue

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I needed to read the results returned from a server in JSON where I couldn t guarantee the fields would be present   I m using class org json simple JSONObject which is derived from HashMap   Here are some helper functions I employed   public static String getString  final JSONObject response                                   final String key      return getString  response  key            public static String getString  final JSONObject response                                   final String key  final String defVal      return response containsKey  key      String response get  key     defVal     public static long getLong  final JSONObject response                               final String key      return getLong  response  key  0       public static long getLong  final JSONObject response                               final String key  final long defVal      return response containsKey  key      long response get  key     defVal     public static float getFloat  final JSONObject response                                 final String key      return getFloat  response  key  0 0f       public static float getFloat  final JSONObject response                                 final String key  final float defVal      return response containsKey  key      float response get  key     defVal     public static List lt JSONObject gt  getList  final JSONObject response                                           final String key      return getList  response  key  new ArrayList lt JSONObject gt            public static List lt JSONObject gt  getList  final JSONObject response                                           final String key  final List lt JSONObject gt  defVal          try   return response containsKey  key      List lt JSONObject gt   response get  key     defVal        catch  ClassCastException e     return defVal

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Extension of TreeMap to provide default value getter creator         NOTE  This class performs no null key or value checking          author N David Brown        param  lt K gt    Key type     param  lt V gt    Value type     public abstract class Hash lt K  V gt  extends TreeMap lt K  V gt         private static final long serialVersionUID   1905150272531272505L                  Same as   link  get Object   but first stores result of          link  create Object   under given key if key doesn t exist                  param k         return             public V getOrCreate final K k            V v   get k           if  v    null                v   create k               put k  v                     return v                        Same as   link  get Object   but returns specified default value        if key doesn t exist  Note that default value isn t automatically        stored under the given key                  param k         param  default         return             public V getDefault final K k  final V  default            V v   get k           return v    null    default   v                        Creates a default value for the specified key                  param k         return             abstract protected V create final K k       Example Usage   protected class HashList extends Hash lt String  ArrayList lt String gt  gt        private static final long serialVersionUID   6658900478219817746L        Override         public ArrayList lt Short gt  create Short key                return new ArrayList lt Short gt                  final HashList haystack   new HashList    final String needle    hide and   haystack getOrCreate needle  add  seek   System out println haystack get needle  get 0

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I found the LazyMap quite helpful       When the get Object  method is called with a key that does not exist in the map  the factory is used to create the object  The created object will be added to the map using the requested key    This allows you to do something like this       Map lt String  AtomicInteger gt  map   LazyMap lazyMap new HashMap lt  gt       - gt new AtomicInteger 0        map get notExistingKey  incrementAndGet      The call to get creates a default value for the given key  You specify how to create the default value with the factory argument to LazyMap lazyMap map  factory   In the example above  the map is initialized to a new AtomicInteger with value 0

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On java 8   Map getOrDefault Object key V defaultValue

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Can t you just create a static method that does exactly this   private static  lt K  V gt  V getOrDefault Map lt K V gt  map  K key  V defaultValue        return map containsKey key    map get key    defaultValue

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Update   As noted by other answers and commenters  as of Java 8 you can simply call Map getOrDefault         Original   There s no Map implementation that does this exactly but it would be trivial to implement your own by extending HashMap   public class DefaultHashMap lt K V gt  extends HashMap lt K V gt      protected V defaultValue    public DefaultHashMap V defaultValue        this defaultValue   defaultValue         Override   public V get Object k        return containsKey k    super get k    defaultValue

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You can simply create a new class that inherits HashMap and add getDefault method  Here is a sample code   public class DefaultHashMap lt K V gt  extends HashMap lt K V gt        public V getDefault K key  V defaultValue            if  containsKey key                 return get key                      return defaultValue            I think that you should not override get K key  method in your implementation  because of the reasons specified by Ed Staub in his comment and because you will break the contract of Map interface  this can potentially lead to some hard-to-find bugs

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In mixed Java Kotlin projects also consider Kotlin s Map withDefault

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In Java 8  use Map getOrDefault  It takes the key  and the value to return if no matching key is found

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Java 8 introduced a nice computeIfAbsent default method to Map interface which stores lazy-computed value and so doesn t break map contract   Map lt Key  Graph gt  map   new HashMap lt  gt     map computeIfAbsent aKey  key - gt  createExpensiveGraph key      Origin  http   blog javabien net 2014 02 20 loadingcache-in-java-8-without-guava   Disclamer   This answer doesn t match exactly what OP asked but may be handy in some cases matching question s title when keys number is limited and caching of different values would be profitable  It shouldn t be used in opposite case with plenty of keys and same default value as this would needlessly waste memory

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Use Commons  DefaultedMap if you don t feel like reinventing the wheel  e g    Map lt String  String gt  map   new DefaultedMap lt  gt    NO ENTRY FOUND     String surname   map get  Surname        surname      NO ENTRY FOUND     You can also pass in an existing map if you re not in charge of creating the map in the first place

[java] HashMap to return default value for non-found keys?

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