Case insensitive string as HashMap key

Question

I would like to use case insensitive string as a HashMap key for the following reasons    During initialization  my program creates HashMap with user defined String While processing an event  network traffic in my case   I might received String in a different case but I should be able to locate the  lt key  value gt  from HashMap ignoring the case I received from traffic    I ve followed this approach  CaseInsensitiveString java      public final class CaseInsensitiveString               private String s               public CaseInsensitiveString String s                                if  s    null                              throw new NullPointerException                                this s   s                             public boolean equals Object o                                return o instanceof CaseInsensitiveString  amp  amp                                CaseInsensitiveString o  s equalsIgnoreCase s                              private volatile int hashCode   0               public int hashCode                                 if  hashCode    0                              hashCode   s toUpperCase   hashCode                                 return hashCode                             public String toString                                 return s                        LookupCode java      node   nodeMap get new CaseInsensitiveString stringFromEvent toString        Because of this  I m creating a new object of CaseInsensitiveString for every event  So  it might hit performance    Is there any other way to solve this issue

User · Answer

As suggested by Guido Garc  a in their answer here   import java util HashMap   public class CaseInsensitiveMap extends HashMap lt String  String gt          Override     public String put String key  String value           return super put key toLowerCase    value                 not  Override because that would require the key parameter to be of type Object     public String get String key           return super get key toLowerCase               Or  https   commons apache org proper commons-collections apidocs org apache commons collections4 map CaseInsensitiveMap html

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Two choices come to my mind    You could use directly the s toUpperCase   hashCode    as the key of the Map  You could use a TreeMap lt String gt  with a custom Comparator that ignore the case    Otherwise  if you prefer your solution  instead of defining a new kind of String  I would rather implement a new Map with the required case insensibility functionality

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Because of this  I m creating a new object of CaseInsensitiveString for every event  So  it might hit performance    Creating wrappers or converting key to lower case before lookup both create new objects  Writing your own java util Map implementation is the only way to avoid this  It s not too hard  and IMO is worth it  I found the following hash function to work pretty well  up to few hundred keys   static int ciHashCode String string           length and the low 5 bits of hashCode   are case insensitive     return  string hashCode    amp  0x1f  33   string length

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This is an adapter for HashMaps which I implemented for a recent project  Works in a way similart to what  SandyR does  but encapsulates conversion logic so you don t manually convert strings to a wrapper object   I used Java 8 features but with a few changes  you can adapt it to previous versions  I tested it for most common scenarios  except new Java 8 stream functions    Basically it wraps a HashMap  directs all functions to it while converting strings to from a wrapper object  But I had to also adapt KeySet and EntrySet because they forward some functions to the map itself  So I return two new Sets for keys and entries which actually wrap the original keySet   and entrySet       One note  Java 8 has changed the implementation of putAll method which I could not find an easy way to override  So current implementation may have degraded performance especially if you use putAll   for a large data set   Please let me know if you find a bug or have suggestions to improve the code   package webbit collections   import java util    import java util function    import java util stream Collectors  import java util stream Stream  import java util stream StreamSupport    public class CaseInsensitiveMapAdapter lt T gt  implements Map lt String T gt        private Map lt CaseInsensitiveMapKey T gt  map      private KeySet keySet      private EntrySet entrySet        public CaseInsensitiveMapAdapter                    public CaseInsensitiveMapAdapter Map lt String  T gt  map                this map   getMapImplementation            this putAll map               Override     public int size                 return getMap   size                Override     public boolean isEmpty                 return getMap   isEmpty                Override     public boolean containsKey Object key                return getMap   containsKey lookupKey key                Override     public boolean containsValue Object value                return getMap   containsValue value               Override     public T get Object key                return getMap   get lookupKey key                Override     public T put String key  T value                return getMap   put lookupKey key   value               Override     public T remove Object key                return getMap   remove lookupKey key                           I completely ignore Java 8 implementation and put one by one This will be slower               Override     public void putAll Map lt   extends String    extends T gt  m                for  String key   m keySet                  getMap   put lookupKey key  m get key                          Override     public void clear                 getMap   clear                Override     public Set lt String gt  keySet                 if  keySet    null              keySet   new KeySet getMap   keySet             return keySet              Override     public Collection lt T gt  values                 return getMap   values                Override     public Set lt Entry lt String  T gt  gt  entrySet                 if  entrySet    null              entrySet   new EntrySet getMap   entrySet             return entrySet              Override     public boolean equals Object o                return getMap   equals o               Override     public int hashCode                 return getMap   hashCode                Override     public T getOrDefault Object key  T defaultValue                return getMap   getOrDefault lookupKey key   defaultValue               Override     public void forEach final BiConsumer lt   super String    super T gt  action                getMap   forEach new BiConsumer lt CaseInsensitiveMapKey  T gt                           Override             public void accept CaseInsensitiveMapKey lookupKey  T t                                action accept lookupKey key t                                         Override     public void replaceAll final BiFunction lt   super String    super T    extends T gt  function                getMap   replaceAll new BiFunction lt CaseInsensitiveMapKey  T  T gt                           Override             public T apply CaseInsensitiveMapKey lookupKey  T t                                return function apply lookupKey key t                                         Override     public T putIfAbsent String key  T value                return getMap   putIfAbsent lookupKey key   value               Override     public boolean remove Object key  Object value                return getMap   remove lookupKey key   value               Override     public boolean replace String key  T oldValue  T newValue                return getMap   replace lookupKey key   oldValue  newValue               Override     public T replace String key  T value                return getMap   replace lookupKey key   value               Override     public T computeIfAbsent String key  final Function lt   super String    extends T gt  mappingFunction                return getMap   computeIfAbsent lookupKey key   new Function lt CaseInsensitiveMapKey  T gt                           Override             public T apply CaseInsensitiveMapKey lookupKey                                return mappingFunction apply lookupKey key                                         Override     public T computeIfPresent String key  final BiFunction lt   super String    super T    extends T gt  remappingFunction                return getMap   computeIfPresent lookupKey key   new BiFunction lt CaseInsensitiveMapKey  T  T gt                           Override             public T apply CaseInsensitiveMapKey lookupKey  T t                                return remappingFunction apply lookupKey key  t                                         Override     public T compute String key  final BiFunction lt   super String    super T    extends T gt  remappingFunction                return getMap   compute lookupKey key   new BiFunction lt CaseInsensitiveMapKey  T  T gt                           Override             public T apply CaseInsensitiveMapKey lookupKey  T t                                return remappingFunction apply lookupKey key t                                         Override     public T merge String key  T value  BiFunction lt   super T    super T    extends T gt  remappingFunction                return getMap   merge lookupKey key   value  remappingFunction              protected  Map lt CaseInsensitiveMapKey T gt  getMapImplementation             return new HashMap lt  gt                private Map lt CaseInsensitiveMapKey T gt  getMap             if  map    null              map   getMapImplementation            return map             private CaseInsensitiveMapKey lookupKey Object key                return new CaseInsensitiveMapKey  String key              public class CaseInsensitiveMapKey           private String key          private String lookupKey           public CaseInsensitiveMapKey String key                        this key   key              this lookupKey   key toUpperCase                        Override         public boolean equals Object o                        if  this    o  return true              if  o    null    getClass      o getClass    return false               CaseInsensitiveMapKey that    CaseInsensitiveMapKey  o               return lookupKey equals that lookupKey                        Override         public int hashCode                         return lookupKey hashCode                         private class KeySet implements Set lt String gt             private Set lt CaseInsensitiveMapKey gt  wrapped           public KeySet Set lt CaseInsensitiveMapKey gt  wrapped                        this wrapped   wrapped                      private List lt String gt  keyList                 return stream   collect Collectors toList                        private Collection lt CaseInsensitiveMapKey gt  mapCollection Collection lt   gt  c                return c stream   map it - gt  lookupKey it   collect Collectors toList                         Override         public int size                         return wrapped size                        Override         public boolean isEmpty                         return wrapped isEmpty                        Override         public boolean contains Object o                        return wrapped contains lookupKey o                        Override         public Iterator lt String gt  iterator                         return keyList   iterator                        Override         public Object   toArray                         return keyList   toArray                        Override         public  lt T gt  T   toArray T   a                        return keyList   toArray a                       Override         public boolean add String s                        return wrapped add lookupKey s                        Override         public boolean remove Object o                        return wrapped remove lookupKey o                        Override         public boolean containsAll Collection lt   gt  c                        return keyList   containsAll c                       Override         public boolean addAll Collection lt   extends String gt  c                        return wrapped addAll mapCollection c                        Override         public boolean retainAll Collection lt   gt  c                        return wrapped retainAll mapCollection c                        Override         public boolean removeAll Collection lt   gt  c                        return wrapped removeAll mapCollection c                        Override         public void clear                         wrapped clear                        Override         public boolean equals Object o                        return wrapped equals lookupKey o                        Override         public int hashCode                         return wrapped hashCode                        Override         public Spliterator lt String gt  spliterator                         return keyList   spliterator                        Override         public boolean removeIf Predicate lt   super String gt  filter                        return wrapped removeIf new Predicate lt CaseInsensitiveMapKey gt                                   Override                 public boolean test CaseInsensitiveMapKey lookupKey                                        return filter test lookupKey key                                                         Override         public Stream lt String gt  stream                         return wrapped stream   map it - gt  it key                       Override         public Stream lt String gt  parallelStream                         return wrapped stream   map it - gt  it key  parallel                        Override         public void forEach Consumer lt   super String gt  action                        wrapped forEach new Consumer lt CaseInsensitiveMapKey gt                                   Override                 public void accept CaseInsensitiveMapKey lookupKey                                        action accept lookupKey key                                                          private class EntrySet implements Set lt Map Entry lt String T gt  gt             private Set lt Entry lt CaseInsensitiveMapKey T gt  gt  wrapped           public EntrySet Set lt Entry lt CaseInsensitiveMapKey T gt  gt  wrapped                        this wrapped   wrapped                      private List lt Map Entry lt String T gt  gt  keyList                 return stream   collect Collectors toList                        private Collection lt Entry lt CaseInsensitiveMapKey T gt  gt  mapCollection Collection lt   gt  c                return c stream   map it - gt  new CaseInsensitiveEntryAdapter  Entry lt String T gt  it   collect Collectors toList                         Override         public int size                         return wrapped size                        Override         public boolean isEmpty                         return wrapped isEmpty                        Override         public boolean contains Object o                        return wrapped contains lookupKey o                        Override         public Iterator lt Map Entry lt String T gt  gt  iterator                         return keyList   iterator                        Override         public Object   toArray                         return keyList   toArray                        Override         public  lt T gt  T   toArray T   a                        return keyList   toArray a                       Override         public boolean add Entry lt String T gt  s                        return wrapped add null                        Override         public boolean remove Object o                        return wrapped remove lookupKey o                        Override         public boolean containsAll Collection lt   gt  c                        return keyList   containsAll c                       Override         public boolean addAll Collection lt   extends Entry lt String T gt  gt  c                        return wrapped addAll mapCollection c                        Override         public boolean retainAll Collection lt   gt  c                        return wrapped retainAll mapCollection c                        Override         public boolean removeAll Collection lt   gt  c                        return wrapped removeAll mapCollection c                        Override         public void clear                         wrapped clear                        Override         public boolean equals Object o                        return wrapped equals lookupKey o                        Override         public int hashCode                         return wrapped hashCode                        Override         public Spliterator lt Entry lt String T gt  gt  spliterator                         return keyList   spliterator                        Override         public boolean removeIf Predicate lt   super Entry lt String  T gt  gt  filter                        return wrapped removeIf new Predicate lt Entry lt CaseInsensitiveMapKey  T gt  gt                                   Override                 public boolean test Entry lt CaseInsensitiveMapKey  T gt  entry                                        return filter test new FromCaseInsensitiveEntryAdapter entry                                                          Override         public Stream lt Entry lt String T gt  gt  stream                         return wrapped stream   map it - gt  new Entry lt String  T gt                                   Override                 public String getKey                                         return it getKey   key                                      Override                 public T getValue                                         return it getValue                                        Override                 public T setValue T value                                        return it setValue value                                                         Override         public Stream lt Map Entry lt String T gt  gt  parallelStream                         return StreamSupport stream spliterator    true                       Override         public void forEach Consumer lt   super Entry lt String  T gt  gt  action                        wrapped forEach new Consumer lt Entry lt CaseInsensitiveMapKey  T gt  gt                                   Override                 public void accept Entry lt CaseInsensitiveMapKey  T gt  entry                                        action accept new FromCaseInsensitiveEntryAdapter entry                                                           private class EntryAdapter implements Map Entry lt String T gt            private Entry lt String T gt  wrapped           public EntryAdapter Entry lt String  T gt  wrapped                        this wrapped   wrapped                      Override         public String getKey                         return wrapped getKey                        Override         public T getValue                         return wrapped getValue                        Override         public T setValue T value                        return wrapped setValue value                       Override         public boolean equals Object o                        return wrapped equals o                       Override         public int hashCode                         return wrapped hashCode                           private class CaseInsensitiveEntryAdapter implements Map Entry lt CaseInsensitiveMapKey T gt             private Entry lt String T gt  wrapped           public CaseInsensitiveEntryAdapter Entry lt String  T gt  wrapped                        this wrapped   wrapped                      Override         public CaseInsensitiveMapKey getKey                         return lookupKey wrapped getKey                         Override         public T getValue                         return wrapped getValue                        Override         public T setValue T value                        return wrapped setValue value                        private class FromCaseInsensitiveEntryAdapter implements Map Entry lt String T gt             private Entry lt CaseInsensitiveMapKey T gt  wrapped           public FromCaseInsensitiveEntryAdapter Entry lt CaseInsensitiveMapKey  T gt  wrapped                        this wrapped   wrapped                      Override         public String getKey                         return wrapped getKey   key                      Override         public T getValue                         return wrapped getValue                        Override         public T setValue T value                        return wrapped setValue value

User · Answer

Instead of creating your own class to validate and store case insensitive string as a HashMap key  you can use   LinkedCaseInsensitiveMap wraps a LinkedHashMap  which is a Map based on a hash table and a linked list  Unlike LinkedHashMap  it doesn t allow null key inserting  LinkedCaseInsensitiveMap preserves the original order as well as the original casing of keys while allowing calling functions like get and remove with any case   Eg  Map lt String  Integer gt  linkedHashMap   new LinkedCaseInsensitiveMap lt  gt     linkedHashMap put  quot abc quot   1   linkedHashMap put  quot AbC quot   2    System out println linkedHashMap     Output   AbC 2   Mvn Dependency  Spring Core is a Spring Framework module that also provides utility classes  including LinkedCaseInsensitiveMap   lt dependency gt       lt groupId gt org springframework lt  groupId gt       lt artifactId gt spring-core lt  artifactId gt       lt version gt 5 2 5 RELEASE lt  version gt   lt  dependency gt    CaseInsensitiveMap is a hash-based Map  which converts keys to lower case before they are being added or retrieved  Unlike TreeMap  CaseInsensitiveMap allows null key inserting   Eg  Map lt String  Integer gt  commonsHashMap   new CaseInsensitiveMap lt  gt     commonsHashMap put  quot ABC quot   1   commonsHashMap put  quot abc quot   2   commonsHashMap put  quot aBc quot   3    System out println commonsHashMap     Output   abc 3   Dependency   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    TreeMap is an implementation of NavigableMap  which means that it always sorts the entries after inserting  based on a given Comparator  Also  TreeMap uses a Comparator to find if an inserted key is a duplicate or a new one   Therefore  if we provide a case-insensitive String Comparator  we ll get a case-insensitive TreeMap  Eg  Map lt String  Integer gt  treeMap   new TreeMap lt  gt  String CASE INSENSITIVE ORDER   treeMap put  quot ABC quot   1   treeMap put  quot ABc quot   2   treeMap put  quot cde quot   1            System out println treeMap     Output   ABC 2  cde 1

User · Answer

Map lt String  String gt  nodeMap        new TreeMap lt  gt  String CASE INSENSITIVE ORDER     That s really all you need

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I like using ICU4J   s CaseInsensitiveString wrap of the Map key because it takes care of the hash equals and issue and it works for unicode i18n  HashMap lt CaseInsensitiveString  String gt  caseInsensitiveMap   new HashMap lt  gt     caseInsensitiveMap put  quot tsch     quot    quot bye quot    caseInsensitiveMap containsKey  quot TSCH  SS quot      true

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Wouldn t it be better to  wrap  the String in order to memorize the hashCode  In the normal String class hashCode   is O N  the first time and then it is O 1  since it is kept for future use   public class HashWrap       private final String value      private final int hash       public String get             return value             public HashWrap String value            this value   value          String lc   value toLowerCase            this hash   lc hashCode                Override     public boolean equals Object o            if  this    o  return true          if  o instanceof HashWrap                HashWrap that    HashWrap  o              return value equalsIgnoreCase that value             else               return false                        Override     public int hashCode             return this hash               might want to implement compare too if you want to use with SortedMaps Sets      This would allow you to use any implementation of Hashtable in java and to have O 1  hasCode

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You can use CollationKey objects instead of strings  Locale locale        Collator collator   Collator getInstance locale   collator setStrength Collator SECONDARY      Case-insensitive  collator setDecomposition Collator FULL DECOMPOSITION    CollationKey collationKey   collator getCollationKey stringKey   hashMap put collationKey  value   hashMap get collationKey    Use Collator PRIMARY to ignore accent differences  The CollationKey API does not guarantee that hashCode   and equals   are implemented  but in practice you ll be using RuleBasedCollationKey  which does implement these  If you re paranoid  you can use a TreeMap instead  which is guaranteed to work at the cost of O log n  time instead of O 1

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You can use a HashingStrategy based Map from Eclipse Collections  HashingStrategy lt String gt  hashingStrategy       HashingStrategies fromFunction String  toUpperCase   MutableMap lt String  String gt  node   HashingStrategyMaps mutable of hashingStrategy     Note  I am a contributor to Eclipse Collections

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Subclass HashMap and create a version that lower-cases the key on put and get  and probably the other key-oriented methods    Or composite a HashMap into the new class and delegate everything to the map  but translate the keys   If you need to keep the original key you could either maintain dual maps  or store the original key along with the value

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Based on other answers  there are basically two approaches  subclassing HashMap or wrapping String  The first one requires a little more work  In fact  if you want to do it correctly  you must override almost all methods  containsKey  entrySet  get  put  putAll and remove     Anyway  it has a problem  If you want to avoid future problems  you must specify a Locale in String case operations  So you would create new methods  get String  Locale         Everything is easier and clearer wrapping String   public final class CaseInsensitiveString        private final String s       public CaseInsensitiveString String s  Locale locale            this s   s toUpperCase locale                 equals  hashCode  amp  toString  no need for memoizing hashCode     And well  about your worries on performance  premature optimization is the root of all evil

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How about using java 8 streams   nodeMap entrySet   stream   filter x- gt x getKey   equalsIgnoreCase stringfromEven toString    collect Collectors toList

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One approach is to create a custom subclass of the Apache Commons AbstractHashedMap class  overriding the hash and isEqualKeys methods to perform case insensitive hashing and comparison of keys    Note - I ve never tried this myself       This avoids the overhead of creating new objects each time you need to do a map lookup or update   And the common Map operations should O 1      just like a regular HashMap   And if you are prepared to accept the implementation choices they have made  the Apache Commons CaseInsensitiveMap does the work of customizing   specializing AbstractHashedMap for you     But if O logN  get and put operations are acceptable  a TreeMap with a case insensitive string comparator is an option  e g  using String CASE INSENSITIVE ORDER   And if you don t mind creating a new temporary String object each time you do a put or get  then Vishal s answer is just fine    Though  I note that you wouldn t be preserving the original case of the keys if you did that

[java] Case insensitive string as HashMap key

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