Hashmap does not work with int char

Question

Possible Duplicate    Storing primitive values in a Java collection        In java when I use the following  -  public HashMap lt char  int gt  buildMap String letters        HashMap lt char  int gt  checkSum   new HashMap lt char  int gt          for   int i   0  i  lt  letters length      i                 checkSum put letters charAt i   primes i               return checkSum      I get errors related to inappropriate types  I solved my problem by using Character and Integer instead of char and int respectively  However  I m having trouble figuring out why HashMap fails to be able to deal with primitive data types

User · Answer

Generics only support object types  not primitives   Unlike C   templates  generics don t involve code generatation and there is only one HashMap code regardless of the number of generic types of it you use   Trove4J gets around this by pre-generating selected collections to use primitives and supports TCharIntHashMap which to can wrap to support the Map lt Character  Integer gt  if you need to      TCharIntHashMap  An open addressed Map implementation for char keys and int values

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Hashmaps can only use classes  not primitives   This page from programmerinterview com might be of use in guiding you to finding the answer   To be honest  I haven t figured out the answer to this problem in detail myself

User · Answer

Generic Collection classes cant be used with primitives  Use the Character and Integer wrapper classes instead   Map lt Character   Integer  gt  checkSum   new HashMap lt Character  Integer gt

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Generics can be defined using Wrapper classes only  If you don t want to define using Wrapper types  you may use the Raw definition as below   SuppressWarnings  rawtypes   public HashMap buildMap String letters        HashMap checkSum   new HashMap         for   int i   0  i  lt  letters length      i                checkSum put letters charAt i   primes i              return checkSum      Or define the HashMap using wrapper types  and store the primitive types  The primitive values will be promoted to their wrapper types   public HashMap lt Character  Integer gt  buildMap String letters      HashMap lt Character  Integer gt  checkSum   new HashMap lt Character  Integer gt        for   int i   0  i  lt  letters length      i           checkSum put letters charAt i   primes i          return checkSum

User · Answer

Generic parameters can only bind to reference types  not primitive types  so you need to use the corresponding wrapper types   Try HashMap lt Character  Integer gt  instead      However  I m having trouble figuring out why HashMap fails to be able to deal with primitive data types    This is due to type erasure   Java didn t have generics from the beginning so a HashMap lt Character  Integer gt  is really a HashMap lt Object  Object gt    The compiler does a bunch of additional checks and implicit casts to make sure you don t put the wrong type of value in or get the wrong type out  but at runtime there is only one HashMap class and it stores objects   Other languages  specialize  types so in C    a vector lt bool gt  is very different from a vector lt my class gt  internally and they share no common vector lt   gt  super-type   Java defines things though so that a List lt T gt  is a List regardless of what T is for backwards compatibility with pre-generic code   This backwards-compatibility requirement that there has to be a single implementation class for all parameterizations of a generic type prevents the kind of template specialization which would allow generic parameters to bind to primitives

User · Answer

You cannot put primitive types into collections  However  you can declare them using their corresponding object wrappers and still add the primitive values  as long as the boxing allows you

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Generics can t use primitive types in the form of keywords   Use  public HashMap lt Character  Integer gt  buildMap String letters        HashMap lt Character  Integer gt  checkSum   new HashMap lt Character  Integer gt          for   int i   0  i  lt  letters length      i                 checkSum put letters charAt i   primes i               return checkSum      Updated  With Java 7 and later  you can use the diamond operator   HashMap lt Character  Integer gt  checkSum   new HashMap lt  gt

[java] Hashmap does not work with int, char

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