What is the use of hashCode in Java

Question

In Java  obj hashCode   returns some value  What is the use of this hash code in programming

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hashCode  Whenever you override equals    you are also expected to override hashCode    The hash code is used when storing the object as a key in a map  A hash code is a number that puts instances of a class into a finite number of categories  Imagine that I gave you a deck of cards  and I told you that I was going to ask you for specific cards and I want to get the right card back quickly  You have as long as you want to prepare  but I   m in a big hurry when I start asking for cards  You might make 13 piles of cards  All of the aces in one pile  all the twos in another pile  and so forth  That way  when I ask for the five of hearts  you can just pull the right card out of the four cards in the pile with fives  It is certainly faster than going through the whole deck of 52 cards  You could even make 52 piles if you had enough space on the table  reference   OCP Oracle Certified Professional Java SE 8 Programmer II

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From the Javadoc   Returns a hash code value for the object  This method is supported for the benefit of hashtables such as those provided by java util Hashtable  The general contract of hashCode is   Whenever it is invoked on the same object more than once during an execution of a Java application  the hashCode method must consistently return the same integer  provided no information used in equals comparisons on the object is modified  This integer need not remain consistent from one execution of an application to another execution of the same application   If two objects are equal according to the equals Object  method  then calling the hashCode method on each of the two objects must produce the same integer result   It is not required that if two objects are unequal according to the equals java lang Object  method  then calling the hashCode method on each of the two objects must produce distinct integer results  However  the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables    As much as is reasonably practical  the hashCode method defined by class Object does return distinct integers for distinct objects   This is typically implemented by converting the internal address of the object into an integer  but this implementation technique is not required by the Java programming language

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Although hashcode does nothing with your business logic  we have to take care of it in most cases  Because when your object is put into a hash based container HashSet  HashMap      the container puts gets the element s hashcode

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The value returned by hashCode   is the object s hash code  which is the object s memory address in hexadecimal  By definition  if two objects are equal  their hash code must also be equal  If you override the equals   method  you change the way two objects are equated and Object s implementation of hashCode   is no longer valid  Therefore  if you override the equals   method  you must also override the hashCode   method as well   This  answer is from the java SE 8 official tutorial documentation

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One of the uses of hashCode   is building a Catching mechanism  Look at this example           class Point             public int x  y         public Point int x  int y                  this x   x          this y   y                  Override       public boolean equals Object o                  if  this    o  return true          if  o    null    getClass      o getClass    return false           Point point    Point  o           if  x    point x  return false          return y    point y                  Override       public int hashCode                   int result   x          result   31   result   y          return result           class Line     public Point start  end     public Line Point start  Point end          this start   start      this end   end          Override   public boolean equals Object o          if  this    o  return true      if  o    null    getClass      o getClass    return false       Line line    Line  o       if   start equals line start   return false      return end equals line end           Override   public int hashCode           int result   start hashCode        result   31   result   end hashCode        return result        class LineToPointAdapter implements Iterable lt Point gt      private static int count   0    private static Map lt Integer  List lt Point gt  gt  cache   new HashMap lt  gt       private int hash     public LineToPointAdapter Line line          hash   line hashCode        if  cache get hash     null  return     we already have it      System out println        String format   d  Generating points for line   d  d -  d  d   no caching              count  line start x  line start y  line end x  line end y

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hashCode   is used for bucketing in Hash implementations like HashMap  HashTable  HashSet  etc   The value received from hashCode   is used as the bucket number for storing elements of the set map  This bucket number is the address of the element inside the set map   When you do contains   it will take the hash code of the element  then look for the bucket where hash code points to  If more than 1 element is found in the same bucket  multiple objects can have the same hash code   then it uses the equals   method to evaluate if the objects are equal  and then decide if contains   is true or false  or decide if element could be added in the set or not

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A hashcode is a number generated from any object    This is what allows objects to be stored retrieved quickly in a Hashtable   Imagine the following simple example   On the table in front of you  you have nine boxes  each marked with a number 1 to 9  You also have a pile of wildly different objects to store in these boxes  but once they are in there you need to be able to find them as quickly as possible   What you need is a way of instantly deciding which box you have put each object in  It works like an index  you decide to find the cabbage so you look up which box the cabbage is in  then go straight to that box to get it   Now imagine that you don t want to bother with the index  you want to be able to find out immediately from the object which box it lives in   In the example  let s use a really simple way of doing this - the number of letters in the name of the object  So the cabbage goes in box 7  the pea goes in box 3  the rocket in box 6  the banjo in box 5 and so on   What about the rhinoceros  though  It has 10 characters  so we ll change our algorithm a little and  wrap around  so that 10-letter objects go in box 1  11 letters in box 2 and so on  That should cover any object   Sometimes a box will have more than one object in it  but if you are looking for a rocket  it s still much quicker to compare a peanut and a rocket  than to check a whole pile of cabbages  peas  banjos  and rhinoceroses   That s a hash code  A way of getting a number from an object so it can be stored in a Hashtable  In Java  a hash code can be any integer  and each object type is responsible for generating its own  Lookup the  hashCode  method of Object   Source - here

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hashCode   is a function that takes an object and outputs a numeric value  The hashcode for an object is always the same if the object doesn t change   Functions like HashMap  HashTable  HashSet  etc  that need to store objects will use a hashCode modulo the size of their internal array to choose in what  memory position   i e  array position  to store the object   There are some cases where collisions may occur  two objects end up with the same hashcode   and that  of course  needs to be solved carefully

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hashCode   is a unique code which is generated by the JVM for every object creation   We use hashCode   to perform some operation on hashing related algorithm like Hashtable  Hashmap etc    The advantages of hashCode   make searching operation easy because when we search for an object that has unique code  it helps to find out that object   But we can t say hashCode   is the address of an object  It is a unique code generated by JVM for every object   That is why nowadays hashing algorithm is the most popular search algorithm

[java] What is the use of hashCode in Java?

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