Why do I need to override the equals and hashCode methods in Java

Question

Recently I read through this  Developer Works Document    The document is all about defining hashCode   and equals   effectively and correctly  however I am not able to figure out why we need to override these two methods    How can I take the decision to implement these methods efficiently

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In order to use our own class objects as keys in collections like HashMap  Hashtable etc     we should override both methods   hashCode   and equals     by having an awareness on internal working of collection  Otherwise  it leads to wrong results which we are not expected

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It is useful when using Value Objects  The following is an excerpt from the Portland Pattern Repository      Examples of value objects are things   like numbers  dates  monies and   strings  Usually  they are small   objects which are used quite widely    Their identity is based on their state   rather than on their object identity    This way  you can have multiple copies   of the same conceptual value object       So I can have multiple copies of an   object that represents the date 16 Jan   1998  Any of these copies will be equal to each other  For a small   object such as this  it is often   easier to create new ones and move   them around rather than rely on a   single object to represent the date       A value object should always override    equals   in Java  or   in Smalltalk      Remember to override  hashCode   as   well

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class A       int i         Hashing Algorithm     if even number return 0 else return 1        Equals Algorithm      if i   this i return true else false      put  key   value   will calculate the hash value using hashCode   to determine the bucket and uses equals   method to find whether the value is already present in the Bucket  If not it will added else it will be replaced with current value get  key   will use hashCode   to find the Entry  bucket  first and equals   to find the value in Entry   if Both are overridden   Map lt A    Map Entry 1 -- gt  1 3 5     Map Entry 2 -- gt  2 4 6       if equals is not overridden  Map lt A    Map Entry 1 -- gt  1 3 5     1 3 5        Duplicate values as equals not overridden Map Entry 2 -- gt  2 4 6     2 4      If hashCode is not overridden  Map lt A    Map Entry 1 -- gt  1 Map Entry 2 -- gt  2 Map Entry 3 -- gt  3 Map Entry 4 -- gt  1 Map Entry 5 -- gt  2 Map Entry 6 -- gt  3    Same values are Stored in different hasCodes violates Contract 1 So on      HashCode Equal Contract   Two keys equal according to equal method should generate same hashCode Two Keys generating same hashCode need not be equal  In above example all even numbers generate same hash Code

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Ok  Let me explain the concept in very simple words   Firstly from a broader perspective we have collections and hashmap is one of the datastructure in the collections   To understand why we have to override the both equals and hashcode method  if need to first understand what is hashmap and what is does   A hashmap is a datastructure which stores key value pairs of data in array fashion  Lets say a    where each element in  a  is a key value pair   Also each index in the above array can be linked list thereby having more than one values at one index   Now why is a hashmap used  If we have to search among  a large array then searching through each if them will not be efficient  so what hash technique tells us that lets pre process the array with some logic and group the elements based on that logic i e  Hashing  eg  we have array 1 2 3 4 5 6 7 8 9 10 11 and we apply a hash function mod 10 so 1 11 will be grouped in together  So if we had to search for 11 in previous array then we would have to iterate the complete array but when we group it we limit our scope of iteration thereby improving speed  That datastructure used to store all the above information can be thought of as a 2d array for simplicity  Now apart from the above hashmap also tells that it wont add any Duplicates in it  And this is the main reason why we have to override the equals and hashcode  So when its said that explain the internal working of hashmap   we need to find what methods the hashmap has and how does it follow the above rules which i explained above  so the hashmap has method called as put K V    and according to hashmap it should follow the above rules of efficiently distributing the array and not adding any duplicates  so what put does is that it will first generate the hashcode for the given key to decide which index the value should go in if nothing is present at that index then the new value will be added over there  if something is already present over there then the new value should be added after the end of the linked list at that index  but remember no duplicates should be added as per the desired behavior of the hashmap  so lets say you have two Integer objects aa 11 bb 11  as every object derived from the object class  the default implementation for comparing two objects is that it compares the reference and not values inside the object  So in the above case both though semantically equal will fail the equality test  and possibility that two objects which same hashcode and same values will exists thereby creating duplicates  If we override then we could avoid adding duplicates  You could also refer to Detail working  import java util HashMap    public class Employee    String name  String mobile  public Employee String name String mobile        this name name      this mobile mobile      Override public int hashCode         System out println  calling hascode method of Employee        String str this name      Integer sum 0      for int i 0 i lt str length   i             sum sum str charAt i             return sum      Override public boolean equals Object obj           TODO Auto-generated method stub     System out println  calling equals method of Employee        Employee emp  Employee obj      if this mobile equalsIgnoreCase emp mobile             System out println  returning true            return true       else          System out println  returning false            return false             public static void main String   args           TODO Auto-generated method stub      Employee emp new Employee  abc    hhh        Employee emp2 new Employee  abc    hhh        HashMap lt Employee  Employee gt  h new HashMap lt  gt           for  int i 0 i lt 5 i             h put emp  emp           h put emp2  emp2                 System out println  ----------------        System out println  size of hashmap    h size

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hashCode   method is used to get a unique integer for given object  This integer is used for determining the bucket location  when this object needs to be stored in some HashTable  HashMap like data structure  By default  Object   s hashCode   method returns and integer representation of memory address where object is stored   The hashCode   method of objects is used when we insert them into a HashTable  HashMap or HashSet  More about HashTables on Wikipedia org for reference   To insert any entry in map data structure  we need both key and value  If both key and values are user define data types  the hashCode   of the key will be determine where to store the object internally  When require to lookup the object from the map also  the hash code of the key will be determine where to search for the object   The hash code only points to a certain  area   or list  bucket etc  internally  Since different key objects could potentially have the same hash code  the hash code itself is no guarantee that the right key is found  The HashTable then iterates this area  all keys with the same hash code  and uses the key s equals   method to find the right key  Once the right key is found  the object stored for that key is returned   So  as we can see  a combination of the hashCode   and equals   methods are used when storing and when looking up objects in a HashTable   NOTES    Always use same attributes of an object to generate hashCode   and equals   both  As in our case  we have used employee id  equals   must be consistent  if the objects are not modified  then it must keep returning the same value   Whenever a equals b   then a hashCode   must be same as b hashCode    If you override one  then you should override the other    http   parameshk blogspot in 2014 10 examples-of-comparable-comporator html

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When you want to store and retrieve your custom object as a key in Map  then you should always override equals and hashCode in your custom Object   Eg   Person p1   new Person  A  23   Person p2   new Person  A  23   HashMap map   new HashMap    map put p1  value 1    map put p2  value 2      Here p1  amp  p2 will consider as only one object and map size will be only 1 because they are equal

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Bah -  You must override hashCode   in every class that overrides equals       from Effective Java  by Joshua Bloch    Isn t this the wrong way round   Overriding hashCode likely implies you re writing a hash-key class  but overriding equals certainly does not  There are many classes that are not used as hash-keys  but do want a logical-equality-testing method for some other reason  If you choose  equals  for it  you may then be mandated to write a hashCode implementation by overzealous application of this rule  All that achieves is adding untested code in the codebase  an evil waiting to trip someone up in the future  Also writing code you don t need is anti-agile  It s just wrong  and an ide generated one will probably be incompatible with your hand-crafted equals     Surely they should have mandated an Interface on objects written to be used as keys  Regardless  Object should never have provided default hashCode   and equals   imho  It s probably encouraged many broken hash collections   But anyway  I think the  rule  is written back to front  In the meantime  I ll keep avoiding using  equals  for equality testing methods  -

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Joshua Bloch says on Effective Java  You must override hashCode   in every class that overrides equals    Failure to do so will result in a violation of the general contract for Object hashCode    which will prevent your class from functioning properly in conjunction with all hash-based collections  including HashMap  HashSet  and Hashtable   Let s try to understand it with an example of what would happen if we override equals   without overriding hashCode   and attempt to use a Map  Say we have a class like this and that two objects of MyClass are equal if their importantField is equal  with hashCode   and equals   generated by eclipse  public class MyClass       private final String importantField      private final String anotherField       public MyClass final String equalField  final String anotherField            this importantField   equalField          this anotherField   anotherField              Override     public int hashCode             final int prime   31          int result   1          result   prime   result                     importantField    null    0   importantField hashCode             return result              Override     public boolean equals final Object obj            if  this    obj              return true          if  obj    null              return false          if  getClass      obj getClass                return false          final MyClass other    MyClass  obj          if  importantField    null                if  other importantField    null                  return false            else if   importantField equals other importantField               return false          return true            Imagine you have this MyClass first   new MyClass  quot a quot   quot first quot    MyClass second   new MyClass  quot a quot   quot second quot     Override only equals If only equals is overriden  then when you call myMap put first someValue  first will hash to some bucket and when you call myMap put second someOtherValue  it will hash to some other bucket  as they have a different hashCode   So  although they are equal  as they don t hash to the same bucket  the map can t realize it and both of them stay in the map   Although it is not necessary to override equals   if we override hashCode    let s see what would happen in this particular case where we know that two objects of MyClass are equal if their importantField is equal but we do not override equals    Override only hashCode If you only override hashCode then when you call myMap put first someValue  it takes first  calculates its hashCode and stores it in a given bucket  Then when you call myMap put second someOtherValue  it should replace first with second  as per the Map Documentation because they are equal  according to the business requirement   But the problem is that equals was not redefined  so when the map hashes second and iterates through the bucket looking if there is an object k such that second equals k  is true it won t find any as second equals first  will be false  Hope it was clear

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Simply put  the equals-method in Object check for reference equality  where as two instances of your class could still be semantically equal when the properties are equal  This is for instance important when putting your objects into a container that utilizes equals and hashcode  like HashMap and Set  Let s say we have a class like   public class Foo       String id      String whatevs       Foo String id  String whatevs            this id   id          this whatevs   whatevs            We create two instances with the same id   Foo a   new Foo  id    something    Foo b   new Foo  id    something else      Without overriding equals we are getting    a equals b  is false because they are two different instances a equals a  is true since it s the same instance b equals b  is true since it s the same instance   Correct  Well maybe  if this is what you want  But let s say we want objects with the same id to be the same object  regardless if it s two different instances  We override the equals  and hashcode    public class Foo       String id      String whatevs       Foo String id  String whatevs            this id   id          this whatevs   whatevs              Override     public boolean equals Object other            if  other instanceof Foo                return   Foo other  id equals this id                            Override     public int hashCode             return this id hashCode              As for implementing equals and hashcode I can recommend using Guava s helper methods

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String class and wrapper classes have different implementation of equals   and hashCode   methods than Object class  equals   method of Object class compares the references of the objects  not the contents  hashCode   method of Object class returns distinct hashcode for every single object whether the contents are same   It leads problem when you use Map collection and the key is of Persistent type  StringBuffer builder type  Since they don t override equals   and hashCode   unlike String class  equals   will return false when you compare two different objects even though both have same contents  It will make the hashMap storing same content keys  Storing same content keys means it is violating the rule of Map because Map doesnt allow duplicate keys at all  Therefore you override equals   as well as hashCode   methods in your class and provide the implementation IDE can generate these methods  so that they work same as String s equals   and hashCode   and prevent same content keys    You have to override hashCode   method along with equals   because equals   work according hashcode    Moreover overriding hashCode   method along with equals   helps to intact the equals  -hashCode   contract   If two objects are equal  then they must have the same hash code    When do you need to write custom implementation for hashCode     As we know that internal working of HashMap is on principle of Hashing  There are certain buckets where entrysets get stored  You customize the hashCode   implementation according your requirement so that same category objects can be stored into same index   when you store the values into Map collection using put k v method  the internal implementation of put   is   put k  v   hash k   index hash  amp   n-1       Means  it generates index and the index is generated based on the hashcode of particular key object  So make this method generate hashcode according your requirement because same hashcode entrysets will be stored into same bucket or index    That s it

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Because if you do not override them you will be use the default implentation in Object   Given that instance equality and hascode values generally require knowledge of what makes up an object they generally will need to be redefined in your class to have any tangible meaning

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The methods equals and hashcode are defined in the object class  By default if the equals method returns true  then the system will go further and check the value of the hash code  If the hash code of the 2 objects is also same only then the objects will be considered as same  So if you override only equals method  then even though the overridden equals method indicates 2 objects to be equal   the system defined hashcode may not indicate that the 2 objects are equal  So we need to override hash code as well

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If you override equals   and not hashcode    you will not find any problem unless you or someone else uses that class type in a hashed collection like HashSet  People before me have clearly explained the documented theory multiple times  I am just here to provide a very simple example   Consider a class whose equals   need to mean something customized  -      public class Rishav            private String rshv           public Rishav String rshv                this rshv   rshv                                    return the rshv                    public String getRshv                 return rshv                                    param rshv the rshv to set                    public void setRshv String rshv                this rshv   rshv                      Override         public boolean equals Object obj                if  obj instanceof Rishav                    obj    Rishav  obj                  if  this rshv equals   Rishav  obj  getRshv                           return true                    else                       return false                                  else                   return false                                    Override         public int hashCode                 return rshv hashCode                       Now consider this main class  -      import java util HashSet      import java util Set       public class TestRishav            public static void main String   args                Rishav rA   new Rishav  rishav                Rishav rB   new Rishav  rishav                System out println rA equals rB                System out println  -----------------------------------                 Set lt Rishav gt  hashed   new HashSet lt  gt                 hashed add rA               System out println hashed contains rB                System out println  -----------------------------------                 hashed add rB               System out println hashed size                        This will yield the following output  -      true     -----------------------------------     true     -----------------------------------     1   I am happy with the results  But if I have not overridden hashCode    it will cause nightmare as objects of Rishav with same member content will no longer be treated as unique as the hashCode will be different  as generated by default behavior  here s the would be output  -      true     -----------------------------------     false     -----------------------------------     2

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Adding to  Lombo  s answer  When will you need to override equals      The default implementation of Object s equals   is  public boolean equals Object obj            return  this    obj       which means two objects will be considered equal only if they have the same memory address which will be true only if you are comparing an object with itself   But you might want to consider two objects the same if they have the same value for one or more of their properties  Refer the example given in  Lombo  s answer     So you will override equals   in these situations and you would give your own conditions for equality   I have successfully implemented equals   and it is working great So why are they asking to override hashCode   as well   Well As long as you don t use  Hash  based Collections on your user-defined class it is fine  But some time in the future you might want to use HashMap or HashSet and if you don t override and  correctly implement  hashCode    these Hash based collection won t work as intended   Override only equals  Addition to  Lombo  s answer   myMap put first someValue  myMap contains second   -- gt  But it should be the same since the key are the same But returns false    How    First of all HashMap checks if the hashCode of second is the same as first  Only if the values are the same it will proceed to check the equality in the same bucket   But here the hashCode is different for these 2 objects  because they have different memory address-from default implementation   Hence it will not even care to check for equality   If you have a breakpoint inside your overridden equals   method it wouldn t step in if they have different hashCodes  contains   checks hashCode   and only if they are the same it would call your equals   method   Why can t we make the HashMap check for equality in all the buckets  So there is no necessity for me to override hashCode       Then you are missing the point of Hash based Collections  Consider the following    Your hashCode   implementation   intObject 9    The following are the keys stored in the form of buckets   Bucket 1   1 10 19      in thousands  Bucket 2   2 20 29    Bucket 3   3 21 30           Say you want to know if the map contains the key 10  Would you want to search all the buckets  or Would you want to search only one bucket   Based on the hashCode you would identify that if 10 is present it must be present in Bucket 1  So only Bucket 1 will be searched

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IMHO  it s as per the rule says - If two objects are equal then they should have same hash  i e   equal objects should produce equal hash values   Given above  default equals   in Object is    which does comparison on the address  hashCode   returns the address in integer hash on actual address  which is again distinct for distinct Object   If you need to use the custom Objects in the Hash based collections  you need to override both equals   and hashCode    example If I want to maintain the HashSet of the Employee Objects  if I don t use stronger hashCode and equals I may endup overriding the two different Employee Objects  this happen when I use the age as the hashCode    however I should be using the unique value which can be the Employee ID

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Identity is not equality    equals operator    test identity  equals Object obj  method compares equality test i e  we need to tell equality by overriding the method       Why do I need to override the equals and hashCode methods in Java       First we have to understand the use of equals method   In order to identity differences between two objects we need to override equals method      For example   Customer customer1 new Customer  peter    Customer customer2 customer1  customer1 equals customer2      returns true by JVM  i e  both are refering same Object ------------------------------ Customer customer1 new Customer  peter    Customer customer2 new Customer  peter    customer1 equals customer2     return false by JVM i e  we have two different peter customers   ------------------------------ Now I have overriden Customer class equals method as follows    Override     public boolean equals Object obj            if  this    obj       it checks references             return true          if  obj    null     checks null             return false          if  getClass      obj getClass       both object are instances of same class or not             return false          Customer other    Customer  obj          if  name    null                if  other name    null                  return false            else if   name equals other name      it again using bulit in String object equals to identify the difference              return false          return true         Customer customer1 new Customer  peter    Customer customer2 new Customer  peter    Insteady identify the Object equality by JVM  we can do it by overring equals method  customer1 equals customer2       returns true by our own logic   Now hashCode method can understand easily   hashCode produces integer in order to store object in data structures like HashMap  HashSet   Assume we have override equals method of Customer as above    customer1 equals customer2       returns true by our own logic   While working with data structure when we store object in buckets bucket is a fancy name for folder   If we use built-in hash technique  for above two customers it generates two different hashcode  So we are storing the same identical object in two different places  To avoid this kind of issues we should override the hashCode method also based on the following principles      un-equal instances may have same hashcode  equal instances should return same hashcode

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Java puts a rule that       If two objects are equal using Object class equals method  then the hashcode method should give the same value for these two objects     So  if in our class we override equals   we should override hashcode   method also to follow this rule  Both methods  equals   and hashcode    are used in Hashtable  for example  to store values as key-value pairs  If we override one and not the other  there is a possibility that the Hashtable may not work as we want  if we use such object as a key

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Equals and Hashcode methods in Java  They are methods of java lang Object class which is the super class of all the classes  custom classes as well and others defined in java API    Implementation      public boolean equals Object obj       public int hashCode       public boolean equals Object obj   This method simply checks if two object references x and y refer to the same object  i e  It checks if x    y   It is reflexive  for any reference value x  x equals x  should return true   It is symmetric  for any reference values x and y  x equals y  should return true if and only if y equals x  returns true   It is transitive  for any reference values x  y  and z  if x equals y  returns true and y equals z  returns true  then x equals z  should return true   It is consistent  for any reference values x and y  multiple invocations of x equals y  consistently return true or consistently return false  provided no information used in equals comparisons on the object is modified      For any non-null reference value x  x equals null  should return   false    public int hashCode    This method returns the hash code value for the object on which this method is invoked  This method returns the hash code value as an integer and is supported for the benefit of hashing based collection classes such as Hashtable  HashMap  HashSet etc  This method must be overridden in every class that overrides the equals method   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      Equal objects must produce the same hash code as long as they are   equal  however unequal objects need not produce distinct hash codes    Resources    JavaRanch  Picture

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You must override hashCode   in every class that overrides equals    Failure to do so will result in a violation of the general contract for Object hashCode    which will prevent your class from functioning properly in conjunction with all hash-based collections  including HashMap  HashSet  and Hashtable        from Effective Java  by Joshua Bloch  By defining equals   and hashCode   consistently  you can improve the usability of your classes as keys in hash-based collections  As the API doc for hashCode explains   quot This method is supported for the benefit of hashtables such as those provided by java util Hashtable  quot  The best answer to your question about how to implement these methods efficiently is suggesting you to read Chapter 3 of Effective Java

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In the example below  if you comment out the override for equals or hashcode in the Person class  this code will fail to look up Tom s order  Using the default implementation of hashcode can cause failures in hashtable lookups   What I have below is a simplified code that pulls up people s order by Person  Person is being used as a key in the hashtable   public class Person       String name      int age      String socialSecurityNumber       public Person String name  int age  String socialSecurityNumber            this name   name          this age   age          this socialSecurityNumber   socialSecurityNumber              Override     public boolean equals Object p              Person is same if social security number is same          if   p instanceof Person   amp  amp  this socialSecurityNumber equals   Person  p  socialSecurityNumber                 return true            else               return false                         Override     public int hashCode              I am using a hashing function in String java instead of writing my own          return socialSecurityNumber hashCode              public class Order       String    items       public void insertOrder String    items                this items items              import java util Hashtable   public class Main        public static void main String   args            Person p1 new Person  Tom  32  548-56-4412            Person p2 new Person  Jerry  60  456-74-4125            Person p3 new Person  Sherry  38  418-55-1235             Order order1 new Order            order1 insertOrder new String    mouse   car charger              Order order2 new Order            order2 insertOrder new String    Multi vitamin              Order order3 new Order            order3 insertOrder new String    handbag    iPod              Hashtable lt Person Order gt  hashtable new Hashtable lt Person Order gt             hashtable put p1 order1           hashtable put p2 order2           hashtable put p3 order3             The line below will fail if Person class does not override hashCode          Order tomOrder  hashtable get new Person  Tom   32   548-56-4412             for String item tomOrder items                        System out println item

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I was looking into the explanation   If you only override hashCode then when you call myMap put first someValue  it takes first  calculates its hashCode and stores it in a given bucket  Then when you call myMap put first someOtherValue  it should replace first with second as per the Map Documentation because they are equal  according to our definition       I think 2nd time when we are adding in myMap then it should be the  second  object like myMap put second someOtherValue

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public class Employee        private int empId      private String empName       public Employee int empId  String empName            super            this empId   empId          this empName   empName             public int getEmpId             return empId             public void setEmpId int empId            this empId   empId             public String getEmpName             return empName             public void setEmpName String empName            this empName   empName              Override     public String toString             return  Employee  empId     empId      empName     empName                    Override     public int hashCode             return empId   empName hashCode                Override     public boolean equals Object obj             if  this    obj                return true                    if    this instanceof Employee                 return false                    Employee emp    Employee  obj          return this getEmpId      emp getEmpId    amp  amp  this getEmpName   equals emp getEmpName                Test Class   public class Test        public static void main String   args            Employee emp1   new Employee 101  Manash            Employee emp2   new Employee 101  Manash            Employee emp3   new Employee 103  Ranjan            System out println emp1 hashCode             System out println emp2 hashCode             System out println emp1 equals emp2            System out println emp1 equals emp3               In Object Class equals Object obj  is used to compare address comparesion thats why when in Test class if you compare two objects then equals method giving false but when we override hashcode   the it can compare content and give proper result

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To help you check for duplicate Objects  we need a custom equals and hashCode   Since hashcode always returns a number its always fast to retrieve an object using a number rather than an alphabetic key  How will it do  Assume we created a new object by passing some value which is already available in some other object  Now the new object will return the same hash value as of another object because the value passed is same  Once the same hash value is returned  JVM will go to the same memory address every time and if in case there are more than one objects present for the same hash value it will use equals   method to identify the correct object

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hashCode     If you only override the hash-code method nothing happens  because it always returns a new hashCode for each object as an Object class  equals     If you only override the equals method  if a equals b  is true it means the hashCode of a and b must be the same but that does not happen since you did not override the hashCode method  Note    hashCode   method of Object class always returns a new hashCode for each object  So when you need to use your object in the hashing based collection  you must override both equals   and hashCode

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Consider collection of balls in a bucket all in black color  Your Job is to color those balls as follows and use it for appropriate game   For Tennis - Yellow  Red  For Cricket - White  Now bucket has balls in three colors Yellow  Red and White  And that now you did the coloring Only you know which color is for which game   Coloring the balls - Hashing  Choosing the ball for game - Equals   If you did the coloring and some one chooses the ball for either cricket or tennis they wont mind the color

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Collections such as HashMap and HashSet use a hashcode value of an object to determine how it should be stored inside a collection  and the hashcode is used again in order to locate the object in its collection   Hashing retrieval is a two-step process    Find the right bucket  using hashCode    Search the bucket for the right element  using equals       Here is a small example on why we should overrride equals   and hashcode     Consider an Employee class which has two fields  age and name   public class Employee        String name      int age       public Employee String name  int age            this name   name          this age   age             public String getName             return name             public void setName String name            this name   name             public int getAge             return age             public void setAge int age            this age   age              Override     public boolean equals Object obj            if  obj    this              return true          if    obj instanceof Employee               return false          Employee employee    Employee  obj          return employee getAge      this getAge                    amp  amp  employee getName      this getName                  commented              Override         public int hashCode                 int result 17              result 31 result age              result 31 result  name  null   name hashCode   0               return result                        Now create a class  insert Employee object into a HashSet and test whether that object is present or not   public class ClientTest       public static void main String   args            Employee employee   new Employee  rajeev   24           Employee employee1   new Employee  rajeev   25           Employee employee2   new Employee  rajeev   24            HashSet lt Employee gt  employees   new HashSet lt Employee gt             employees add employee           System out println employees contains employee2            System out println  employee hashCode         employee hashCode                employee2 hashCode       employee2 hashCode               It will print the following   false employee hashCode     321755204  employee2 hashCode   375890482   Now uncomment hashcode   method   execute the same and the output would be    true employee hashCode     -938387308  employee2 hashCode   -938387308   Now can you see why if two objects are considered equal  their hashcodes must also be equal  Otherwise  you d never be able to find the object since the default hashcode method in class Object virtually always comes up with a unique number for each object  even if the equals   method is overridden in such a way that two or more objects are considered equal  It doesn t matter how equal the objects are if their hashcodes don t reflect that  So one more time  If two objects are equal  their hashcodes must be equal as well

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There s no mention in this answer of testing the equals hashcode contract  I ve found the EqualsVerifier library to be very useful and comprehensive  It is also very easy to use  Also  building equals   and hashCode   methods from scratch involves a lot of boilerplate code  The Apache Commons Lang library provides the EqualsBuilder and HashCodeBuilder classes  These classes greatly simplify implementing equals   and hashCode   methods for complex classes  As an aside  it s worth considering overriding the toString   method to aid debugging  Apache Commons Lang library provides the ToStringBuilder class to help with this

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Both the methods are defined in Object class  And both are in its simplest implementation  So when you need you want add some more implementation to these methods then you have override in  your class    For Ex  equals   method in object only checks its equality on the reference  So if you need compare its state as well then you can override that as it is done in String class

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Assume you have class  A  that aggregates two other  B   C   and you need to store instances of  A  inside hashtable  Default implementation only allows distinguishing of instances  but not by  B  and  C   So two instances of A could be equal  but default wouldn t allow you to compare them in correct way

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1  The common mistake is shown in the example below   public class Car        private String color       public Car String color            this color   color             public boolean equals Object obj            if obj  null  return false          if    obj instanceof Car               return false             if  obj    this              return true          return this color equals   Car  obj  color              public static void main String   args            Car a1   new Car  green            Car a2   new Car  red               hashMap stores Car type and its quantity         HashMap lt Car  Integer gt  m   new HashMap lt Car  Integer gt             m put a1  10           m put a2  20           System out println m get new Car  green                the green Car is not found  2  Problem caused by hashCode    The problem is caused by the un-overridden method hashCode    The contract between equals   and hashCode   is    If two objects are equal  then they must have the same hash code  If two objects have the same hash code  they may or may not be equal   public int hashCode        return this color hashCode

[java] Why do I need to override the equals and hashCode methods in Java?

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