How to override equals method in Java

Question

I am trying to override equals method in Java  I have a class People which basically has 2 data fields name and age  Now I want to override equals method so that I can check between 2 People objects    My code is as follows  public boolean equals People other       boolean result      if  other    null      getClass      other getClass              result   false           end if     else          People otherPeople    People other          result   name equals other name   amp  amp   age equals other age            end else      return result       end equals   But when I write age equals other age  it gives me error as equals method can only compare String and age is Integer    Solution  I used    operator as suggested and my problem is solved

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Here is the solution that I recently used  public class Test       public String a      public long b      public Date c      public String d            Override     public boolean equals Object obj            if  this    obj                return true                    if    obj instanceof Test                 return false                    Test testOther    Test  obj          return  a    null   a equals testOther a    testOther a    null                   amp  amp   b    testOther b                   amp  amp   c    null   c equals testOther c    testOther c    null                   amp  amp   d    null   d equals testOther d    testOther d    null

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Introducing a new method signature that changes the parameter types is called overloading   public boolean equals People other     Here People is different than Object   When a method signature remains the identical to that of its superclass  it is called overriding and the  Override annotation helps distinguish the two at compile-time    Override public boolean equals Object other     Without seeing the actual declaration of age  it is difficult to say why the error appears

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When comparing objects in Java  you make a semantic check  comparing the type and identifying state of the objects to    itself  same instance  itself  clone  or reconstructed copy  other objects of different types other objects of the same type null   Rules    Symmetry  a equals b     b equals a  equals   always yields true or false  but never a NullpointerException  ClassCastException or any other throwable   Comparison    Type check  both instances need to be of the same type  meaning you have to compare the actual classes for equality  This is often not correctly implemented  when developers use instanceof for type comparison  which only works as long as there are no subclasses  and violates the symmetry rule when A extends B - gt  a instanceof b    b instanceof a    Semantic check of identifying state  Make sure you understand by which state the instances are identified  Persons may be identified by their social security number  but not by hair color  can be dyed   name  can be changed  or age  changes all the time   Only with value objects should you compare the full state  all non-transient fields   otherwise check only what identifies the instance     For your Person class   public boolean equals Object obj            same instance     if  obj    this            return true               null     if  obj    null            return false               type     if   getClass   equals obj getClass               return false               cast and compare state     Person other    Person  obj      return Objects equals name  other name   amp  amp  Objects equals age  other age       Reusable  generic utility class   public final class Equals        private Equals                private constructor  no instances allowed                       Convenience equals implementation  does the object equality  null and type checking  and comparison of the identifying state                param instance       object instance  where the equals   is implemented          param other          other instance to compare to         param stateAccessors stateAccessors for state to compare  optional         param  lt T gt             instance type         return true when equals  false otherwise             public static  lt T gt  boolean as T instance  Object other  Function lt   super T  Object gt     stateAccessors            if  instance    null                return other    null                    if  instance    other                return true                    if  other    null                return false                    if   instance getClass   equals other getClass                   return false                    if  stateAccessors    null                return true                    return Stream of stateAccessors  allMatch s - gt  Objects equals s apply instance   s apply  T  other               For your Person class  using this utility class   public boolean equals Object obj        return Equals as this  obj  t - gt  t name  t - gt  t age

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Written by K stackoverflow public class Main                    param args the command line arguments             public static void main String   args               TODO code application logic here         ArrayList lt Person gt  people   new ArrayList lt Person gt             people add new Person  quot Subash Adhikari quot   28            people add new Person  quot K quot   28            people add new Person  quot StackOverflow quot   4            people add new Person  quot Subash Adhikari quot   28             for  int i   0  i  lt  people size   - 1  i                  for  int y   i   1  y  lt   people size   - 1  y                      boolean check   people get i  equals people get y                     System out println  quot --  quot    people get i  getName      quot  - VS -  quot    people get y  getName                     System out println check                                      written by K stackoverflow public class Person       private String name      private int age       public Person String name  int age           this name   name          this age   age              Override     public boolean equals Object obj            if  obj    null                return false                     if  obj getClass      this getClass                  return false                     final Person other    Person  obj          if   this name    null     other name    null     this name equals other name                 return false                     if  this age    other age                return false                     return true              Override     public int hashCode             int hash   3          hash   53   hash    this name    null   this name hashCode     0           hash   53   hash   this age          return hash             public int getAge             return age             public void setAge int age            this age   age             public String getName             return name             public void setName String name            this name   name           Output   run  -- Subash Adhikari - VS - K false -- Subash Adhikari - VS - StackOverflow false -- Subash Adhikari - VS - Subash Adhikari true -- K - VS - StackOverflow false -- K - VS - Subash Adhikari false -- StackOverflow - VS - Subash Adhikari false -- BUILD SUCCESSFUL  total time  0 seconds

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tl dr record Person   String name   int age        if       new Person   quot Carol quot    27                   Compiler auto-generates implicitly the constructor       equals                                    Compiler auto-generates implicitly the  equals  method          new Person   quot Carol quot    42                                                          Returns  false   as the name matches but the age differs           Details While your specific problem is solved  using    for equality test between int primitive values   there is an alternative that eliminates the need to write that code  record Java 16 brings the record feature  A record is a brief way to write a class whose main purpose is to transparently and immutably carry data  The compiler implicitly creates the constructor  getters  equals  amp  hashCode  and toString  equals method provided automatically The default implicit equals method compares each and every member field that you declared for the record  The members can be objects or primitives  both types are automatically compared in the default equals method  For example  if you have a Person record carrying two fields  name  amp  age  both of those fields are automatically compared to determine equality between a pair of Person objects  public record Person   String name   int age       Try it  Person alice   new Person   quot Alice quot    23     Person alice2   new Person   quot Alice quot    23     Person bob   new Person   quot Bob quot    19      boolean samePerson1   alice equals  alice2         true  boolean samePerson2   alice equals  bob         false   You can override the equals method on a record  if you want a behavior other than the default  But if you do override equals  be sure to override hashCode for consistent logic  as you would for a conventional Java class  And  think twice  Whenever adding methods to a record  reconsider if a record structure is really appropriate to that problem domain  Tip  A record can be defined within another class  and even locally within a method

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if age is int you should use    if it is Integer object then you can use equals    You also need to implement hashcode method if you override equals  Details of the contract is available in the javadoc of Object and also at various pages in web

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I m not sure of the details as you haven t posted the whole code  but    remember to override hashCode   as well the equals method should have Object  not People as its argument type  At the moment you are overloading  not overriding  the equals method  which probably isn t what you want  especially given that you check its type later  you can use instanceof to check it is a People object e g  if    other instanceof People     result   false   equals is used for all objects  but not primitives  I think you mean age is an int  primitive   in which case just use     Note that an Integer  with a capital  I   is an Object which should be compared with equals    See What issues should be considered when overriding equals and hashCode in Java  for more details

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Since I m guessing age is of type int   public boolean equals Object other       boolean result      if  other    null      getClass      other getClass              result   false           end if     else          People otherPeople    People other          result   name equals otherPeople name   amp  amp   age    otherPeople age           end else      return result       end equals

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Override public boolean equals Object that     if this    that  return true   if both of them points the same address in memory    if   that instanceof People   return false     if  that  is not a People or a childclass    People thatPeople    People that     than we can cast it to People safely    return this name equals thatPeople name   amp  amp  this age    thatPeople age    if they have the same name and same age  then the 2 objects are equal unless they re pointing to different memory adresses

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Item 10  Obey the general contract when overriding equals     According to Effective Java  Overriding the equals method seems simple  but there are many ways to get it wrong  and consequences can be dire  The easiest way to avoid problems is not to override the equals method  in which case each instance of the class is equal only to itself  This is the right thing to do if any of the following conditions apply     Each instance of the class is inherently unique  This is true for classes such as Thread that represent active entities rather than values  The equals implementation provided by Object has exactly the right behavior for these classes  There is no need for the class to provide a    logical equality    test  For example  java util regex Pattern could have overridden equals to check whether two Pattern instances represented exactly the same regular expression  but the designers didn   t think that clients would need or want this functionality  Under these circumstances  the equals implementation inherited from Object is ideal  A superclass has already overridden equals  and the superclass behavior is appropriate for this class  For example  most Set implementations inherit their equals implementation from AbstractSet  List implementations from AbstractList  and Map implementations from AbstractMap  The class is private or package-private  and you are certain that its equals method will never be invoked  If you are extremely risk-averse  you can override the equals method to ensure that it isn   t invoked accidentally    The equals method implements an equivalence relation  It has these properties    Reflexive  For any non-null reference value x  x equals x  must return true  Symmetric  For any non-null reference values x and y  x equals y  must return true if and only if y equals x  returns true  Transitive  For any non-null reference values x  y  z  if x equals y  returns true and y equals z  returns true  then x equals z  must return true  Consistent  For any non-null reference values x and y  multiple invocations of x equals y  must consistently return true or consistently return false  provided no information used in equals comparisons is modified  For any non-null reference value x  x equals null  must return false    Here   s a recipe for a high-quality equals method    Use the    operator to check if the argument is a reference to this object  If so  return true  This is just a performance optimization but one that is worth doing if the comparison is potentially expensive  Use the instanceof operator to check if the argument has the correct type  If not  return false  Typically  the correct type is the class in which the method occurs  Occasionally  it is some interface implemented by this class  Use an interface if the class implements an interface that refines the equals contract to permit comparisons across classes that implement the interface  Collection interfaces such as Set  List  Map  and Map Entry have this property  Cast the argument to the correct type  Because this cast was preceded by an instanceof test  it is guaranteed to succeed  For each    significant    field in the class  check if that field of the argument matches the corresponding field of this object  If all these tests succeed  return true  otherwise  return false  If the type in Step 2 is an interface  you must access the argument   s fields via interface methods  if the type is a class  you may be able to access the fields directly  depending on their accessibility  For primitive fields whose type is not float or double  use the    operator for comparisons  for object reference fields  call the equals method recursively  for float fields  use the static Float compare float  float  method  and for double fields  use Double compare double  double   The special treatment of float and double fields is made necessary by the existence of Float NaN  -0 0f and the analogous double values  While you could compare float and double fields with the static methods Float equals and Double equals  this would entail autoboxing on every comparison  which would have poor performance  For array fields  apply these guidelines to each element  If every element in an array field is significant  use one of the Arrays equals methods  Some object reference fields may legitimately contain null  To avoid the possibility of a NullPointerException  check such fields for equality using the static method Objects equals Object  Object       Class with a typical equals method  public final class PhoneNumber        private final short areaCode  prefix  lineNum       public PhoneNumber int areaCode  int prefix  int lineNum             this areaCode   rangeCheck areaCode   999   area code             this prefix     rangeCheck prefix     999   prefix             this lineNum    rangeCheck lineNum   9999   line num                private static short rangeCheck int val  int max  String arg             if  val  lt  0    val  gt  max              throw new IllegalArgumentException arg          val            return  short  val               Override public boolean equals Object o            if  o    this              return true          if    o instanceof PhoneNumber               return false          PhoneNumber pn    PhoneNumber o          return pn lineNum    lineNum  amp  amp  pn prefix    prefix                  amp  amp  pn areaCode    areaCode                   Remainder omitted

[java] How to override equals method in Java

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