Best implementation for hashCode method for a collection

Question

How do we decide on the best implementation of hashCode   method for a collection  assuming that equals method has been overridden correctly

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It is better to use the functionality provided by Eclipse which does a pretty good job and you can put your efforts and energy in developing the business logic

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any hashing method that evenly distributes the hash value over the possible range is a good implementation  See effective java   http   books google com au books id ZZOiqZQIbRMC amp dq effective java amp pg PP1 amp ots UZMZ2siN25 amp sig kR0n73DHJOn-D77qGj0wOxAxiZw amp hl en amp sa X amp oi book result amp resnum 1 amp ct result     there is a good tip in there for hashcode implementation  item 9 i think

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There s a good implementation of the Effective Java s hashcode   and equals   logic in Apache Commons Lang  Checkout HashCodeBuilder and EqualsBuilder

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If you re happy with the Effective Java implementation recommended by dmeister  you can use a library call instead of rolling your own    Override public int hashCode         return Objects hashCode this firstName  this lastName       This requires either Guava  com google common base Objects hashCode  or the standard library in Java 7  java util Objects hash  but works the same way

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It is better to use the functionality provided by Eclipse which does a pretty good job and you can put your efforts and energy in developing the business logic

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about8 blogspot com  you said      if equals   returns true for two objects  then hashCode   should return the same value  If equals   returns false  then hashCode   should return different values   I cannot agree with you  If two objects have the same hashcode it doesn t have to mean that they are equal    If A equals B then A hashcode must be equal to B hascode  but  if A hashcode equals B hascode it does not mean that A must equals B

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If you use eclipse  you can generate equals   and hashCode   using      Source -  Generate hashCode   and equals       Using this function you can decide which fields you want to use for equality and hash code calculation  and Eclipse generates the corresponding methods

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Here is another JDK 1 7  approach demonstration with superclass logics accounted  I see it as pretty convinient with Object class hashCode   accounted  pure JDK dependency and no extra manual work  Please note Objects hash   is null tolerant   I have not include any equals   implementation but in reality you will of course need it   import java util Objects   public class Demo        public static class A            private final String param1           public A final String param1                this param1   param1                      Override         public int hashCode                 return Objects hash                  super hashCode                    this param1                         public static class B extends A            private final String param2          private final String param3           public B              final String param1              final String param2              final String param3                 super param1               this param2   param2              this param3   param3                      Override         public final int hashCode                 return Objects hash                  super hashCode                    this param2                  this param3                        public static void main String    args             A a   new A  A            B b   new B  A    B    C             System out println  A      a hashCode             System out println  B      b hashCode

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As you specifically asked for collections  I d like to add an aspect that the other answers haven t mentioned yet  A HashMap doesn t expect their keys to change their hashcode once they are added to the collection  Would defeat the whole purpose

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The standard implementation is weak and using it leads to unnecessary collisions  Imagine a  class ListPair       List lt Integer gt  first      List lt Integer gt  second       ListPair List lt Integer gt  first  List lt Integer gt  second            this first   first          this second   second             public int hashCode             return Objects hashCode first  second                      Now   new ListPair List of a   List of b  c     and  new ListPair List of b   List of a  c     have the same hashCode  namely 31  a b    c as the multiplier used for List hashCode gets reused here  Obviously  collisions are unavoidable  but producing needless collisions is just    needless   There s nothing substantially smart about using 31  The multiplier must be odd in order to avoid losing information  any even multiplier loses at least the most significant bit  multiples of four lose two  etc    Any odd multiplier is usable  Small multipliers may lead to faster computation  the JIT can use shifts and additions   but given that multiplication has latency of only three cycles on modern Intel AMD  this hardly matters  Small multipliers also leads to more collision for small inputs  which may be a problem sometimes   Using a prime is pointless as primes have no meaning in the ring Z  2  32    So  I d recommend using a randomly chosen big odd number  feel free to take a prime   As i86 amd64 CPUs can use a shorter instruction for operands fitting in a single signed byte  there is a tiny speed advantage for multipliers like 109  For minimizing collisions  take something like 0x58a54cf5   Using different multipliers in different places is helpful  but probably not enough to justify the additional work

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First make sure that equals is implemented correctly  From an IBM DeveloperWorks article         Symmetry  For two references  a and b  a equals b  if and only if b equals a    Reflexivity  For all non-null references  a equals a    Transitivity  If a equals b  and b equals c   then a equals c        Then make sure that their relation with hashCode respects the contact  from the same article          Consistency with hashCode    Two equal objects must have the same hashCode   value      Finally a good hash function should strive to approach the ideal hash function

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For a simple class it is often easiest to implement hashCode   based on the class fields which are checked by the equals   implementation   public class Zam       private String foo      private String bar      private String somethingElse       public boolean equals Object obj            if  this    obj                return true                     if  obj    null                return false                     if  getClass      obj getClass                  return false                     Zam otherObj    Zam obj           if   getFoo      null  amp  amp  otherObj getFoo      null      getFoo      null  amp  amp  getFoo   equals otherObj getFoo                    if   getBar      null  amp  amp  otherObj  getBar      null      getBar      null  amp  amp  getBar   equals otherObj  getBar                        return true                                   return false             public int hashCode             return  getFoo     getBar    hashCode               public String getFoo             return foo             public String getBar             return bar            The most important thing is to keep hashCode   and equals   consistent  if equals   returns true for two objects  then hashCode   should return the same value  If equals   returns false  then hashCode   should return different values

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I prefer using utility methods fromm Google Collections lib from class Objects that helps me to keep my code clean  Very often equals and hashcode methods are made from IDE s template  so their are not clean to read

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about8   there is a pretty serious bug there     Zam obj1   new Zam  foo    bar    baz    Zam obj2   new Zam  fo    obar    baz      same hashcode  you probably want something like  public int hashCode         return  getFoo   hashCode     getBar   hashCode    toString   hashCode       can you get hashCode directly from int in Java these days  I think it does some autocasting   if that s the case  skip the toString  it s ugly

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I use a tiny wrapper around Arrays deepHashCode      because it handles arrays supplied as parameters correctly  public static int hash final Object    objects        return Arrays deepHashCode objects

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Just a quick note for completing other more detailed answer  in term of code    If I consider the question how-do-i-create-a-hash-table-in-java and especially the jGuru FAQ entry  I believe some other criteria upon which a hash code could be judged are    synchronization  does the algo support concurrent access or not    fail safe iteration  does the algo detect a collection which changes during iteration  null value  does the hash code support null value in the collection

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Although this is linked to Android documentation  Wayback Machine  and My own code on Github  it will work for Java in general  My answer is an extension of dmeister s Answer with just code that is much easier to read and understand    Override  public int hashCode             Start with a non-zero constant  Prime is preferred     int result   17          Include a hash for each field          Primatives      result   31   result    booleanField   1   0                        1 bit      32-bit      result   31   result   byteField                                    8 bits     32-bit      result   31   result   charField                                    16 bits    32-bit     result   31   result   shortField                                   16 bits    32-bit     result   31   result   intField                                     32 bits    32-bit      result   31   result    int  longField    longField  gt  gt  gt  32          64 bits    32-bit      result   31   result   Float floatToIntBits floatField              32 bits    32-bit      long doubleFieldBits   Double doubleToLongBits doubleField          64 bits  double     64-bit  long     32-bit  int      result   31   result    int  doubleFieldBits    doubleFieldBits  gt  gt  gt  32            Objects      result   31   result   Arrays hashCode arrayField                   var bits    32-bit      result   31   result   referenceField hashCode                      var bits    32-bit  non-nullable         result   31   result                                                var bits    32-bit  nullable              nullableReferenceField    null               0               nullableReferenceField hashCode          return result       EDIT  Typically  when you override hashcode       you also want to override equals       So for those that will or has already implemented equals  here is a good reference from my Github      Override public boolean equals Object o            Optimization  not required       if  this    o            return true                Return false if the other object has the wrong type  interface  or is null      if    o instanceof MyType             return false             MyType lhs    MyType  o     lhs means  left hand side                  Primitive fields     return     booleanField    lhs booleanField              amp  amp  byteField       lhs byteField              amp  amp  charField       lhs charField              amp  amp  shortField      lhs shortField              amp  amp  intField        lhs intField              amp  amp  longField       lhs longField              amp  amp  floatField      lhs floatField              amp  amp  doubleField     lhs doubleField                 Arrays               amp  amp  Arrays equals arrayField  lhs arrayField                  Objects               amp  amp  referenceField equals lhs referenceField               amp  amp   nullableReferenceField    null                           lhs nullableReferenceField    null                           nullableReferenceField equals lhs nullableReferenceField

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Use the reflection methods on Apache Commons EqualsBuilder and HashCodeBuilder

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The best implementation  That is a hard question because it depends on the usage pattern   A for nearly all cases reasonable good implementation was proposed in Josh Bloch s  Effective Java in Item 8  second edition   The best thing is to look it up there because the author explains there why the approach is good   A short version   Create a int result and assign a non-zero value  For every field f tested in the equals   method  calculate a hash code c by    If the field f is a boolean   calculate  f   0   1   If the field f is a byte  char  short or int  calculate  int f  If the field f is a long  calculate  int  f    f  gt  gt  gt  32    If the field f is a float  calculate Float floatToIntBits f   If the field f is a double  calculate Double doubleToLongBits f  and handle the return value like every long value  If the field f is an object  Use the result of the hashCode   method or 0 if f    null  If the field f is an array  see every field as separate element and calculate the hash value in a recursive fashion and combine the values as described next   Combine the hash value c with result   result   37   result   c  Return result   This should result in a proper distribution of hash values for most use situations

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If I understand your question correctly  you have a custom collection class  i e  a new class that extends from the Collection interface  and you want to implement the hashCode   method   If your collection class extends AbstractList  then you don t have to worry about it  there is already an implementation of equals   and hashCode   that works by iterating through all the objects and adding their hashCodes   together      public int hashCode           int hashCode   1        Iterator i   iterator          while  i hasNext              Object obj   i next            hashCode   31 hashCode    obj  null   0   obj hashCode               return hashCode         Now if what you want is the best way to calculate the hash code for a specific class  I normally use the    bitwise exclusive or  operator to process all fields that I use in the equals method   public int hashCode       return intMember    stringField    null   stringField hashCode     0

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When combining hash values  I usually use the combining method that s used in the boost c   library  namely   seed    hasher v    0x9e3779b9    seed lt  lt 6     seed gt  gt 2     This does a fairly good job of ensuring an even distribution   For some discussion of how this formula works  see the StackOverflow post  Magic number in boost  hash combine  There s a good discussion of different hash functions at  http   burtleburtle net bob hash doobs html

[java] Best implementation for hashCode method for a collection

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