Overriding the java equals method - not working

Question

I ran into an interesting  and very frustrating  issue with the equals   method today which caused what I thought to be a well tested class to crash and cause a bug that took me a very long time to track down    Just for completeness  I wasn t using an IDE or debugger - just good old fashioned text editor and System out s  Time was very limited and it was a school project   Anyhow -   I was developing a basic shopping cart which could contain an ArrayList of Book objects  In order to implement the addBook    removeBook    and hasBook   methods of the Cart  I wanted to check if the Book already existed in the Cart  So off I go -  public boolean equals Book b               More code here - null checks     if  b getID      this getID    return true      else return false      All works fine in testing  I create 6 objects and fill them with data  Do many adds  removes  has   operations on the Cart and everything works fine  I read that you can either have equals TYPE var  or equals Object o     CAST  var   but assumed that since it was working  it didn t matter too much   Then I ran into a problem - I needed to create a Book object with only the ID in it from within the Book class  No other data would be entered into it  Basically the following   public boolean hasBook int i        Book b   new Book i       return hasBook b      public boolean hasBook Book b              more code here     return this books contains b       All of a sudden  the equals Book b  method no longer works  This took a VERY long time to track down without a good debugger and assuming the Cart class was properly tested and correct  After swaapping the equals   method to the following   public boolean equals Object o        Book b    Book  o             The rest goes here        Everything began to work again  Is there a reason the method decided not to take the Book parameter even though it clearly  was  a Book object  The only difference seemed to be it was instantiated from within the same class  and only filled with one data member  I m very very confused  Please  shed some light

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If you use eclipse just go to the top menu      Source --  Generate equals   and   hashCode

User · Answer

Consider   Object obj   new Book    obj equals  hi       Oh noes  What happens now  Can t call it with a String that isn t a Book

User · Answer

Slightly off-topic to your question  but it s probably worth mentioning anyway   Commons Lang has got some excellent methods you can use in overriding equals and hashcode  Check out EqualsBuilder reflectionEquals      and HashCodeBuilder reflectionHashCode       Saved me plenty of headache in the past - although of course if you just want to do  equals  on ID it may not fit your circumstances   I also agree that you should use the  Override annotation whenever you re overriding equals  or any other method

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in Android Studio is alt   insert ---  equals and hashCode  Example        Override public boolean equals Object o        if  this    o  return true      if  o    null    getClass      o getClass    return false       Proveedor proveedor    Proveedor  o       return getId      proveedor getId         Override public int hashCode         return getId

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recordId  is property of the  object   Override     public boolean equals Object obj            if  this    obj              return true          if  obj    null              return false          if  getClass      obj getClass                return false          Nai record other    Nai record  obj          if  recordId    null                if  other recordId    null                  return false            else if   recordId equals other recordId               return false          return true

User · Answer

Slightly off-topic to your question  but it s probably worth mentioning anyway   Commons Lang has got some excellent methods you can use in overriding equals and hashcode  Check out EqualsBuilder reflectionEquals      and HashCodeBuilder reflectionHashCode       Saved me plenty of headache in the past - although of course if you just want to do  equals  on ID it may not fit your circumstances   I also agree that you should use the  Override annotation whenever you re overriding equals  or any other method

User · Answer

In Java  the equals   method that is inherited from Object is   public boolean equals Object other     In other words  the parameter must be of type Object  This is called overriding  your method public boolean equals Book other  does what is called overloading to the equals   method   The ArrayList uses overridden equals   methods to compare contents  e g  for its contains   and equals   methods   not overloaded ones  In most of your code  calling the one that didn t properly override Object s equals was fine  but not compatible with ArrayList   So  not overriding the method correctly can cause problems   I override equals the following everytime    Override public boolean equals Object other       if  other    null  return false      if  other    this  return true      if    other instanceof MyClass   return false      MyClass otherMyClass    MyClass other         test other properties here        The use of the  Override annotation can help a ton with silly mistakes   Use it whenever you think you are overriding a super class  or interface s method  That way  if you do it the wrong way  you will get a compile error

User · Answer

If you use eclipse just go to the top menu      Source --  Generate equals   and   hashCode

User · Answer

In Java  the equals   method that is inherited from Object is   public boolean equals Object other     In other words  the parameter must be of type Object  This is called overriding  your method public boolean equals Book other  does what is called overloading to the equals   method   The ArrayList uses overridden equals   methods to compare contents  e g  for its contains   and equals   methods   not overloaded ones  In most of your code  calling the one that didn t properly override Object s equals was fine  but not compatible with ArrayList   So  not overriding the method correctly can cause problems   I override equals the following everytime    Override public boolean equals Object other       if  other    null  return false      if  other    this  return true      if    other instanceof MyClass   return false      MyClass otherMyClass    MyClass other         test other properties here        The use of the  Override annotation can help a ton with silly mistakes   Use it whenever you think you are overriding a super class  or interface s method  That way  if you do it the wrong way  you will get a compile error

User · Answer

the instanceOf statement is often used in implementation of equals   This is a popular pitfall    The problem is that using instanceOf violates the rule of symmetry    object1 equals object2     true  if and only if  object2 equals object1    if the first equals is true  and object2 is an instance of a subclass of the class where obj1 belongs to  then the second equals will return false   if the regarded class where ob1 belongs to is declared as final  then this problem can not arise  but in general  you should test as follows   this getClass      otherObject getClass    if not  return false  otherwise test the fields to compare for equality

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in Android Studio is alt   insert ---  equals and hashCode  Example        Override public boolean equals Object o        if  this    o  return true      if  o    null    getClass      o getClass    return false       Proveedor proveedor    Proveedor  o       return getId      proveedor getId         Override public int hashCode         return getId

User · Answer

the instanceOf statement is often used in implementation of equals   This is a popular pitfall    The problem is that using instanceOf violates the rule of symmetry    object1 equals object2     true  if and only if  object2 equals object1    if the first equals is true  and object2 is an instance of a subclass of the class where obj1 belongs to  then the second equals will return false   if the regarded class where ob1 belongs to is declared as final  then this problem can not arise  but in general  you should test as follows   this getClass      otherObject getClass    if not  return false  otherwise test the fields to compare for equality

User · Answer

Another fast solution that saves boilerplate code is Lombok EqualsAndHashCode annotation  It is easy  elegant and customizable  And does not depends on the IDE  For example   import lombok EqualsAndHashCode    EqualsAndHashCode of   errorNumber   messageCode       Will only use this fields to generate equals  public class ErrorMessage       private long        errorNumber      private int         numberOfParameters      private Level       loggingLevel      private String      messageCode    See the options avaliable to customize which fields to use in the equals  Lombok is avalaible in maven  Just add it with provided scope    lt dependency gt       lt groupId gt org projectlombok lt  groupId gt       lt artifactId gt lombok lt  artifactId gt       lt version gt 1 14 8 lt  version gt       lt scope gt provided lt  scope gt   lt  dependency gt

User · Answer

Another fast solution that saves boilerplate code is Lombok EqualsAndHashCode annotation  It is easy  elegant and customizable  And does not depends on the IDE  For example   import lombok EqualsAndHashCode    EqualsAndHashCode of   errorNumber   messageCode       Will only use this fields to generate equals  public class ErrorMessage       private long        errorNumber      private int         numberOfParameters      private Level       loggingLevel      private String      messageCode    See the options avaliable to customize which fields to use in the equals  Lombok is avalaible in maven  Just add it with provided scope    lt dependency gt       lt groupId gt org projectlombok lt  groupId gt       lt artifactId gt lombok lt  artifactId gt       lt version gt 1 14 8 lt  version gt       lt scope gt provided lt  scope gt   lt  dependency gt

User · Answer

In Java  the equals   method that is inherited from Object is   public boolean equals Object other     In other words  the parameter must be of type Object  This is called overriding  your method public boolean equals Book other  does what is called overloading to the equals   method   The ArrayList uses overridden equals   methods to compare contents  e g  for its contains   and equals   methods   not overloaded ones  In most of your code  calling the one that didn t properly override Object s equals was fine  but not compatible with ArrayList   So  not overriding the method correctly can cause problems   I override equals the following everytime    Override public boolean equals Object other       if  other    null  return false      if  other    this  return true      if    other instanceof MyClass   return false      MyClass otherMyClass    MyClass other         test other properties here        The use of the  Override annotation can help a ton with silly mistakes   Use it whenever you think you are overriding a super class  or interface s method  That way  if you do it the wrong way  you will get a compile error

User · Answer

Consider   Object obj   new Book    obj equals  hi       Oh noes  What happens now  Can t call it with a String that isn t a Book

User · Answer

recordId  is property of the  object   Override     public boolean equals Object obj            if  this    obj              return true          if  obj    null              return false          if  getClass      obj getClass                return false          Nai record other    Nai record  obj          if  recordId    null                if  other recordId    null                  return false            else if   recordId equals other recordId               return false          return true

User · Answer

In Java  the equals   method that is inherited from Object is   public boolean equals Object other     In other words  the parameter must be of type Object  This is called overriding  your method public boolean equals Book other  does what is called overloading to the equals   method   The ArrayList uses overridden equals   methods to compare contents  e g  for its contains   and equals   methods   not overloaded ones  In most of your code  calling the one that didn t properly override Object s equals was fine  but not compatible with ArrayList   So  not overriding the method correctly can cause problems   I override equals the following everytime    Override public boolean equals Object other       if  other    null  return false      if  other    this  return true      if    other instanceof MyClass   return false      MyClass otherMyClass    MyClass other         test other properties here        The use of the  Override annotation can help a ton with silly mistakes   Use it whenever you think you are overriding a super class  or interface s method  That way  if you do it the wrong way  you will get a compile error

User · Answer

Slightly off-topic to your question  but it s probably worth mentioning anyway   Commons Lang has got some excellent methods you can use in overriding equals and hashcode  Check out EqualsBuilder reflectionEquals      and HashCodeBuilder reflectionHashCode       Saved me plenty of headache in the past - although of course if you just want to do  equals  on ID it may not fit your circumstances   I also agree that you should use the  Override annotation whenever you re overriding equals  or any other method

User · Answer

Slightly off-topic to your question  but it s probably worth mentioning anyway   Commons Lang has got some excellent methods you can use in overriding equals and hashcode  Check out EqualsBuilder reflectionEquals      and HashCodeBuilder reflectionHashCode       Saved me plenty of headache in the past - although of course if you just want to do  equals  on ID it may not fit your circumstances   I also agree that you should use the  Override annotation whenever you re overriding equals  or any other method

[java] Overriding the java equals() method - not working?

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