Should a retrieval method return null or throw an exception when it can t produce the return value

Question

I am using java language I have a method that is supposed to return an object if it is found  If it is not found  should I   return null throw an exception other  Which is the best practise or idiom

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Use the null object pattern or throw an exception

User · Answer

Generally it should return null  The code calling the method should decide whether to throw an exception or to attempt something else

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In some functions I add a parameter        bool verify   true    True means throw  false means return some error return value  This way  whoever uses this function has both options  The default should be true  for the benefit of those who forget about error handling

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Only throw an exception if it is truly an error  If it is expected behavior for the object to not exist  return the null   Otherwise it is a matter of preference

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If it s important for client code to know the difference between found and not found and this is supposed to be a routine behavior  then it s best to return null   Client code can then decide what to do

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If you are using a library or another class which throws an exception  you should rethrow it  Here is an example  Example2 java is like library and Example java uses it s object  Main java is an example to handle this Exception  You should show a meaningful message and  if needed  stack trace to the user in the calling side   Main java  public class Main   public static void main String   args        Example example   new Example         try           Example2 obj   example doExample             if obj    null               System out println  Hey object is null                     catch  Exception e            System out println  Congratulations  you caught the exception             System out println  Here is stack trace             e printStackTrace                Example java         Example java     author Seval     date 10 22 2014     public class Example                  Returns Example2 object        If there is no Example2 object  throws exception                 return obj Example2         throws Exception             public Example2 doExample   throws Exception           try                  Get the object             Example2 obj   new Example2                 return obj             catch  Exception e                   Log the exception and rethrow                Log logException e               throw e                       Example2 java          Example2 java     author Seval        public class Example2                  Constructor of Example2         throws Exception             public Example2   throws Exception          throw new Exception  Please set the   obj

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Generally it should return null  The code calling the method should decide whether to throw an exception or to attempt something else

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As a general rule  if the method should always return an object  then go with the exception  If you anticipate the occasional null and want to handle it in a certain way  go with the null    Whatever you do  I highly advise against the third option  Returning a string that says  WTF

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Return a null instead of throwing an exception and clearly document the possibility of a null return value in the API documentation  If the calling code doesn t honor the API and check for the null case  it will most probably result in some sort of  null pointer exception  anyway     In C    I can think of 3 different flavors of setting up a method that finds an object   Option A  Object  findObject Key  amp key     Return null when an object can t be found  Nice and simple  I d go with this one  The alternative approaches below are for people who don t hate out-params   Option B  void findObject Key  amp key  Object  amp found     Pass in a reference to variable that will be receiving the object  The method thrown an exception when an object can t be found  This convention is probably more suitable if it s not really expected for an object not to be found -- hence you throw an exception to signify that it s an unexpected case   Option C  bool findObject Key  amp key  Object  amp found     The method returns false when an object can t be found  The advantage of this over option A is that you can check for the error case in one clear step   if   findObject myKey  myObj

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It depends on the nature of the method and how it will be used   If it is normal behavior that the object may not be found  then return null   If it is normal behavior that the object is always found  throw an exception   As a rule of thumb  use exceptions only for when something exceptional occurs   Don t write the code in such a way that exception throwing and catching is part of its normal operation

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That really depends on if you expect to find the object  or not  If you follow the school of thought that exceptions should be used for indicating something  well  err  exceptional has occured then    Object found  return object Object not-found  throw exception   Otherwise  return null

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Only throw an exception if it is truly an error  If it is expected behavior for the object to not exist  return the null   Otherwise it is a matter of preference

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Here are a couple more suggestions   If returning a collection  avoid returning null  return an empty collection which makes enumeration easier to deal with without a null check first   Several  NET API s use the pattern of a thrownOnError parameter which gives the caller the choice as whether it is really an exceptional situation or not if the object is not found   Type GetType is an example of this   Another common pattern with BCL is the TryGet pattern where a boolean is returned and the value is passed via an output parameter   You could also consider the Null Object pattern in some circumstances which can either be a default or a version with no behaviour   The key is avoid null checks throughout the code base   See here for more information http   geekswithblogs net dsellers archive 2006 09 08 90656 aspx

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Here are a couple more suggestions   If returning a collection  avoid returning null  return an empty collection which makes enumeration easier to deal with without a null check first   Several  NET API s use the pattern of a thrownOnError parameter which gives the caller the choice as whether it is really an exceptional situation or not if the object is not found   Type GetType is an example of this   Another common pattern with BCL is the TryGet pattern where a boolean is returned and the value is passed via an output parameter   You could also consider the Null Object pattern in some circumstances which can either be a default or a version with no behaviour   The key is avoid null checks throughout the code base   See here for more information http   geekswithblogs net dsellers archive 2006 09 08 90656 aspx

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I just wanted to recapitulate the options mentioned before  throwing some new ones in    return null throw an Exception use the null object pattern provide a boolean parameter to you method  so the caller can chose if he wants you to throw an exception provide an extra parameter  so the caller can set a value which he gets back if no value is found   Or you might combine these options   Provide several overloaded versions of your getter  so the caller can decide which way to go  In most cases  only the first one has an implementation of the search algorithm  and the other ones just wrap around the first one   Object findObjectOrNull String key   Object findObjectOrThrow String key  throws SomeException  Object findObjectOrCreate String key  SomeClass dataNeededToCreateNewObject   Object findObjectOrDefault String key  Object defaultReturnValue     Even if you choose to provide only one implementation  you might want to use a naming convention like that to clarify your contract  and it helps you should you ever decide to add other implementations as well   You should not overuse it  but it may be helpfull  espeacially when writing a helper Class which you will use in hundreds of different applications with many different error handling conventions

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Advantages of throwing an exception    Cleaner control flow in your calling code   Checking for null injects a conditional branch which is natively handled by try catch   Checking for null doesn t indicate what it is you re checking for - are you checking for null because you re looking for an error you re expecting  or are you checking for null so you don t pass it further on downchain  Removes ambiguity of what  null  means   Is null representative of an error or is null what is actually stored in the value   Hard to say when you only have one thing to base that determination off of  Improved consistency between method behavior in an application   Exceptions are typically exposed in method signatures  so you re more able to understand what edge cases the methods in an application account for  and what information your application can react to in a predictable manner    For more explanation with examples  see  http   metatations com 2011 11 17 returning-null-vs-throwing-an-exception

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Advantages of throwing an exception    Cleaner control flow in your calling code   Checking for null injects a conditional branch which is natively handled by try catch   Checking for null doesn t indicate what it is you re checking for - are you checking for null because you re looking for an error you re expecting  or are you checking for null so you don t pass it further on downchain  Removes ambiguity of what  null  means   Is null representative of an error or is null what is actually stored in the value   Hard to say when you only have one thing to base that determination off of  Improved consistency between method behavior in an application   Exceptions are typically exposed in method signatures  so you re more able to understand what edge cases the methods in an application account for  and what information your application can react to in a predictable manner    For more explanation with examples  see  http   metatations com 2011 11 17 returning-null-vs-throwing-an-exception

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Prefer returning null --  If the caller uses it without checking  the exception happens right there anyway   If the caller doesn t really use it  don t tax him a try catch block

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Don t think anyone mentioned the overhead in exception handling - takes additional resources to load up and process the exception so unless its a true app killing or process stopping event  going forward would cause more harm than good  I would opt for passing back a value the calling environment could interpret as it sees fit

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The  other  option could be to let the find method take an additional parameter with a default object that would be returned if the sought for object cannot be found   Otherwise I d just return null unless it really is an exceptional case when the object isn t found

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That depends on your method  If you method is supposed to always return a valid object and none is found  throwing an exception is the way to go  If the method is just ment to return an object which might or might not be there  like perhaps a image on a contact person  you shouldn t raise an error   You might also want to expose a method returning a boolean true false if this method actually will return an object so you don t have to either a  check for null or b  catch an exception

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I prefer to just return a null  and rely on the caller to handle it appropriately  The  for lack of a better word  exception is if I am absolutely  certain  this method will return an object  In that case a failure is an exceptional should and should throw

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I agree with what seems to be the consensus here  return null if  not found  is a normal possible outcome  or throw an exception if the semantics of the situation require that the object always be found    There is  however  a third possibility that might make sense depending on your particular situation  Your method could return a default object of some sort in the  not found  condition  allowing calling code to be assured that it will always receive a valid object without the need for null checking or exception catching

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Don t think anyone mentioned the overhead in exception handling - takes additional resources to load up and process the exception so unless its a true app killing or process stopping event  going forward would cause more harm than good  I would opt for passing back a value the calling environment could interpret as it sees fit

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That really depends on if you expect to find the object  or not  If you follow the school of thought that exceptions should be used for indicating something  well  err  exceptional has occured then    Object found  return object Object not-found  throw exception   Otherwise  return null

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If not finding it is an exceptional event  i e  it should be there under normal circumstances   then throw  Otherwise  return a  not found  value  can be null  but does not have to   or even have the method return a boolean for found notfound and an out parameter for the actual object

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Be consistent with the API s  you re using

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Return a null instead of throwing an exception and clearly document the possibility of a null return value in the API documentation  If the calling code doesn t honor the API and check for the null case  it will most probably result in some sort of  null pointer exception  anyway     In C    I can think of 3 different flavors of setting up a method that finds an object   Option A  Object  findObject Key  amp key     Return null when an object can t be found  Nice and simple  I d go with this one  The alternative approaches below are for people who don t hate out-params   Option B  void findObject Key  amp key  Object  amp found     Pass in a reference to variable that will be receiving the object  The method thrown an exception when an object can t be found  This convention is probably more suitable if it s not really expected for an object not to be found -- hence you throw an exception to signify that it s an unexpected case   Option C  bool findObject Key  amp key  Object  amp found     The method returns false when an object can t be found  The advantage of this over option A is that you can check for the error case in one clear step   if   findObject myKey  myObj

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I just wanted to recapitulate the options mentioned before  throwing some new ones in    return null throw an Exception use the null object pattern provide a boolean parameter to you method  so the caller can chose if he wants you to throw an exception provide an extra parameter  so the caller can set a value which he gets back if no value is found   Or you might combine these options   Provide several overloaded versions of your getter  so the caller can decide which way to go  In most cases  only the first one has an implementation of the search algorithm  and the other ones just wrap around the first one   Object findObjectOrNull String key   Object findObjectOrThrow String key  throws SomeException  Object findObjectOrCreate String key  SomeClass dataNeededToCreateNewObject   Object findObjectOrDefault String key  Object defaultReturnValue     Even if you choose to provide only one implementation  you might want to use a naming convention like that to clarify your contract  and it helps you should you ever decide to add other implementations as well   You should not overuse it  but it may be helpfull  espeacially when writing a helper Class which you will use in hundreds of different applications with many different error handling conventions

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Unfortunately JDK is inconsistent  if you trying access non existing key in resource bundle  you get not found exception and when you request value from map you get null if it doesn t exists  So I would change winner answer to the following  if found value can be null  then raise exception when it isn t found  otherwise return null  So follow to the rule with one exception  if you need to know why value isn t found then always raise exception  or

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Use the null object pattern or throw an exception

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It not containing the object can happen during normal operations and should be dealt with by the caller return NULL   If not containing the object indicates a bug by the calling code or internal state then do an assert   If not containing the object indicates an infrequent event   Like someone deleted an item from the store while you were checking out the item at the same time   Then throw an exception

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That depends on your method  If you method is supposed to always return a valid object and none is found  throwing an exception is the way to go  If the method is just ment to return an object which might or might not be there  like perhaps a image on a contact person  you shouldn t raise an error   You might also want to expose a method returning a boolean true false if this method actually will return an object so you don t have to either a  check for null or b  catch an exception

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As a general rule  if the method should always return an object  then go with the exception  If you anticipate the occasional null and want to handle it in a certain way  go with the null    Whatever you do  I highly advise against the third option  Returning a string that says  WTF

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I agree with what seems to be the consensus here  return null if  not found  is a normal possible outcome  or throw an exception if the semantics of the situation require that the object always be found    There is  however  a third possibility that might make sense depending on your particular situation  Your method could return a default object of some sort in the  not found  condition  allowing calling code to be assured that it will always receive a valid object without the need for null checking or exception catching

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If null never indicates an error then just return null   If null is always an error then throw an exception   If null is sometimes an exception then code two routines  One routine throws an exception and the other is a boolean test routine that returns the object in an output parameter and the routine returns a false if the object was not found   It s hard to misuse a Try routine  It s real easy to forget to check for null   So when null is an error you just write  object o   FindObject      When the null isn t an error you can code something like  if  TryFindObject out object o       Do something with o else      o was not found

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Be consistent with the API s  you re using

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In data layer code  I some times use the following code  allowing the caller to decide if  object not found  means an error has occured    DataType GetObject DBConnection conn  string id  bool throwOnNotFound        DataType retval          find object in database     if  retval    null      throwOnNotFound            return retval        else           throw new NoRowsFoundException  DataType object with id  id  not found in database             DataType GetObject DBConnection conn  string id        return GetObject conn  id  true

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If null never indicates an error then just return null   If null is always an error then throw an exception   If null is sometimes an exception then code two routines  One routine throws an exception and the other is a boolean test routine that returns the object in an output parameter and the routine returns a false if the object was not found   It s hard to misuse a Try routine  It s real easy to forget to check for null   So when null is an error you just write  object o   FindObject      When the null isn t an error you can code something like  if  TryFindObject out object o       Do something with o else      o was not found

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Or return an Option  An option is basically a container class that forces the client to handle booth cases  Scala has this concept  look up it s API   Then you have methods like T getOrElse T valueIfNull  on this object thet either return the found object  or an allternative the client specifieces

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It not containing the object can happen during normal operations and should be dealt with by the caller return NULL   If not containing the object indicates a bug by the calling code or internal state then do an assert   If not containing the object indicates an infrequent event   Like someone deleted an item from the store while you were checking out the item at the same time   Then throw an exception

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The  other  option could be to let the find method take an additional parameter with a default object that would be returned if the sought for object cannot be found   Otherwise I d just return null unless it really is an exceptional case when the object isn t found

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Depends on what it means that the object is not found   If it s a normal state of affairs  then return null  This is just something that might happen once in an while  and the callers should check for it    If it s an error  then throw an exception  the callers should decide what to do with the error condition of missing object    Ultimately either would work  although most people generally consider it good practice to only use Exceptions when something  well  Exceptional has happened

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Return a null  exceptions are exactly that   something your code does that isn t expected

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Here are a couple more suggestions   If returning a collection  avoid returning null  return an empty collection which makes enumeration easier to deal with without a null check first   Several  NET API s use the pattern of a thrownOnError parameter which gives the caller the choice as whether it is really an exceptional situation or not if the object is not found   Type GetType is an example of this   Another common pattern with BCL is the TryGet pattern where a boolean is returned and the value is passed via an output parameter   You could also consider the Null Object pattern in some circumstances which can either be a default or a version with no behaviour   The key is avoid null checks throughout the code base   See here for more information http   geekswithblogs net dsellers archive 2006 09 08 90656 aspx

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Return a null  exceptions are exactly that   something your code does that isn t expected

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Return a null  exceptions are exactly that   something your code does that isn t expected

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If the method returns a collection  then return an empty collection  like sayed above   But please not Collections EMPTY LIST or such   in case of Java   If the method retrives a single object  then You have some options    If the method should always find the result and it s a real exception case not to find the object  then you should throw an exception  in Java  please an unchecked Exception   Java only  If you can tolerate that the method throws a checked exception  throw a project specific ObjectNotFoundException or the like  In this case the compiler says you if you forget to handle the exception   This is my preferred handling of not found things in Java   If you say it s really ok  if the object is not found and your Method name is like findBookForAuthorOrReturnNull      then you can return null  In this case it is strongly recomminded to use some sort of static check or compiler check  wich prevents dereferencing of the result without a null check  In case of Java it can be eg  FindBugs  see DefaultAnnotation at http   findbugs sourceforge net manual annotations html  or IntelliJ-Checking    Be careful  if you decide to return a null  If you are not the only programmer in project you will get NullPointerExceptions  in Java or whatever in other Languages  at run time  So don t return nulls which are not checked at compile time

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It not containing the object can happen during normal operations and should be dealt with by the caller return NULL   If not containing the object indicates a bug by the calling code or internal state then do an assert   If not containing the object indicates an infrequent event   Like someone deleted an item from the store while you were checking out the item at the same time   Then throw an exception

User · Answer

If you are always expecting to find a value then throw the exception if it is missing   The exception would mean that there was a problem   If the value can be missing or present and both are valid for the application logic then return a null   More important  What do you do other places in the code   Consistency is important

User · Answer

Unfortunately JDK is inconsistent  if you trying access non existing key in resource bundle  you get not found exception and when you request value from map you get null if it doesn t exists  So I would change winner answer to the following  if found value can be null  then raise exception when it isn t found  otherwise return null  So follow to the rule with one exception  if you need to know why value isn t found then always raise exception  or

User · Answer

It depends on the nature of the method and how it will be used   If it is normal behavior that the object may not be found  then return null   If it is normal behavior that the object is always found  throw an exception   As a rule of thumb  use exceptions only for when something exceptional occurs   Don t write the code in such a way that exception throwing and catching is part of its normal operation

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If the method returns a collection  then return an empty collection  like sayed above   But please not Collections EMPTY LIST or such   in case of Java   If the method retrives a single object  then You have some options    If the method should always find the result and it s a real exception case not to find the object  then you should throw an exception  in Java  please an unchecked Exception   Java only  If you can tolerate that the method throws a checked exception  throw a project specific ObjectNotFoundException or the like  In this case the compiler says you if you forget to handle the exception   This is my preferred handling of not found things in Java   If you say it s really ok  if the object is not found and your Method name is like findBookForAuthorOrReturnNull      then you can return null  In this case it is strongly recomminded to use some sort of static check or compiler check  wich prevents dereferencing of the result without a null check  In case of Java it can be eg  FindBugs  see DefaultAnnotation at http   findbugs sourceforge net manual annotations html  or IntelliJ-Checking    Be careful  if you decide to return a null  If you are not the only programmer in project you will get NullPointerExceptions  in Java or whatever in other Languages  at run time  So don t return nulls which are not checked at compile time

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In some functions I add a parameter        bool verify   true    True means throw  false means return some error return value  This way  whoever uses this function has both options  The default should be true  for the benefit of those who forget about error handling

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Generally it should return null  The code calling the method should decide whether to throw an exception or to attempt something else

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That really depends on if you expect to find the object  or not  If you follow the school of thought that exceptions should be used for indicating something  well  err  exceptional has occured then    Object found  return object Object not-found  throw exception   Otherwise  return null

User · Answer

Depends on what it means that the object is not found   If it s a normal state of affairs  then return null  This is just something that might happen once in an while  and the callers should check for it    If it s an error  then throw an exception  the callers should decide what to do with the error condition of missing object    Ultimately either would work  although most people generally consider it good practice to only use Exceptions when something  well  Exceptional has happened

User · Answer

Or return an Option  An option is basically a container class that forces the client to handle booth cases  Scala has this concept  look up it s API   Then you have methods like T getOrElse T valueIfNull  on this object thet either return the found object  or an allternative the client specifieces

User · Answer

As a general rule  if the method should always return an object  then go with the exception  If you anticipate the occasional null and want to handle it in a certain way  go with the null    Whatever you do  I highly advise against the third option  Returning a string that says  WTF

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As long as it s supposed to return a reference to the object  returning a NULL should be good    However  if it s returning the whole bloody thing  like in C   if you do   return blah   rather than  return  blah    or  blah  is a pointer   then you can t return a NULL  because it s not of type  object   In that case  throwing an exception  or returning a blank object that doesn t have a success flag set is how I would approach the problem

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Don t think anyone mentioned the overhead in exception handling - takes additional resources to load up and process the exception so unless its a true app killing or process stopping event  going forward would cause more harm than good  I would opt for passing back a value the calling environment could interpret as it sees fit

User · Answer

The  other  option could be to let the find method take an additional parameter with a default object that would be returned if the sought for object cannot be found   Otherwise I d just return null unless it really is an exceptional case when the object isn t found

User · Answer

Return a null instead of throwing an exception and clearly document the possibility of a null return value in the API documentation  If the calling code doesn t honor the API and check for the null case  it will most probably result in some sort of  null pointer exception  anyway     In C    I can think of 3 different flavors of setting up a method that finds an object   Option A  Object  findObject Key  amp key     Return null when an object can t be found  Nice and simple  I d go with this one  The alternative approaches below are for people who don t hate out-params   Option B  void findObject Key  amp key  Object  amp found     Pass in a reference to variable that will be receiving the object  The method thrown an exception when an object can t be found  This convention is probably more suitable if it s not really expected for an object not to be found -- hence you throw an exception to signify that it s an unexpected case   Option C  bool findObject Key  amp key  Object  amp found     The method returns false when an object can t be found  The advantage of this over option A is that you can check for the error case in one clear step   if   findObject myKey  myObj

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Exceptions are related to Design by Contract    The interface of an objects is actually a contract between two objects  the caller must meet the contract or else the receiver may just fail with an exception  There are two possible contracts  1  all input the method is valid  in which case you must return null when the object is not found   2  only some input is valid  ie that which results in a found object  In which case you MUST offer a second method that allows the caller to determine if its input will be correct  For example  is present key  find key  throws Exception   IF and ONLY IF you provide both methods of the 2nd contract  you are allowed to throw an exception is nothing is found

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I agree with what seems to be the consensus here  return null if  not found  is a normal possible outcome  or throw an exception if the semantics of the situation require that the object always be found    There is  however  a third possibility that might make sense depending on your particular situation  Your method could return a default object of some sort in the  not found  condition  allowing calling code to be assured that it will always receive a valid object without the need for null checking or exception catching

User · Answer

In some functions I add a parameter        bool verify   true    True means throw  false means return some error return value  This way  whoever uses this function has both options  The default should be true  for the benefit of those who forget about error handling

User · Answer

If you are always expecting to find a value then throw the exception if it is missing   The exception would mean that there was a problem   If the value can be missing or present and both are valid for the application logic then return a null   More important  What do you do other places in the code   Consistency is important

User · Answer

Be consistent with the API s  you re using

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Depends on what it means that the object is not found   If it s a normal state of affairs  then return null  This is just something that might happen once in an while  and the callers should check for it    If it s an error  then throw an exception  the callers should decide what to do with the error condition of missing object    Ultimately either would work  although most people generally consider it good practice to only use Exceptions when something  well  Exceptional has happened

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Just ask yourself    is it an exceptional case that the object is not found    If it is expected to happen in the normal course of your program  you probably should not raise an exception  since it is not exceptional behavior    Short version   use exceptions to handle exceptional behavior  not to handle normal flow of control in your program   -Alan

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Depends on what it means that the object is not found   If it s a normal state of affairs  then return null  This is just something that might happen once in an while  and the callers should check for it    If it s an error  then throw an exception  the callers should decide what to do with the error condition of missing object    Ultimately either would work  although most people generally consider it good practice to only use Exceptions when something  well  Exceptional has happened

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Exceptions should be exceptional  Return null if it is valid to return a null

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If not finding it is an exceptional event  i e  it should be there under normal circumstances   then throw  Otherwise  return a  not found  value  can be null  but does not have to   or even have the method return a boolean for found notfound and an out parameter for the actual object

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Only throw an exception if it is truly an error  If it is expected behavior for the object to not exist  return the null   Otherwise it is a matter of preference

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If you are using a library or another class which throws an exception  you should rethrow it  Here is an example  Example2 java is like library and Example java uses it s object  Main java is an example to handle this Exception  You should show a meaningful message and  if needed  stack trace to the user in the calling side   Main java  public class Main   public static void main String   args        Example example   new Example         try           Example2 obj   example doExample             if obj    null               System out println  Hey object is null                     catch  Exception e            System out println  Congratulations  you caught the exception             System out println  Here is stack trace             e printStackTrace                Example java         Example java     author Seval     date 10 22 2014     public class Example                  Returns Example2 object        If there is no Example2 object  throws exception                 return obj Example2         throws Exception             public Example2 doExample   throws Exception           try                  Get the object             Example2 obj   new Example2                 return obj             catch  Exception e                   Log the exception and rethrow                Log logException e               throw e                       Example2 java          Example2 java     author Seval        public class Example2                  Constructor of Example2         throws Exception             public Example2   throws Exception          throw new Exception  Please set the   obj

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I prefer to just return a null  and rely on the caller to handle it appropriately  The  for lack of a better word  exception is if I am absolutely  certain  this method will return an object  In that case a failure is an exceptional should and should throw

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it depends if your language and code promotes   LBYL  look before you leap  or  EAFP  easier to ask forgiveness than permission   LBYL says you should check for values  so return a null  EAFP says to just try the operation and see if it fails  throw an exception   though I agree with above   exceptions should be used for exceptional error conditions  and returning a null is best when using checks     EAFP vs  LBYL in Python   http   mail python org pipermail python-list 2003-May 205182 html   Web Archive

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If you are always expecting to find a value then throw the exception if it is missing   The exception would mean that there was a problem   If the value can be missing or present and both are valid for the application logic then return a null   More important  What do you do other places in the code   Consistency is important

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As long as it s supposed to return a reference to the object  returning a NULL should be good    However  if it s returning the whole bloody thing  like in C   if you do   return blah   rather than  return  blah    or  blah  is a pointer   then you can t return a NULL  because it s not of type  object   In that case  throwing an exception  or returning a blank object that doesn t have a success flag set is how I would approach the problem

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Or return an Option  An option is basically a container class that forces the client to handle booth cases  Scala has this concept  look up it s API   Then you have methods like T getOrElse T valueIfNull  on this object thet either return the found object  or an allternative the client specifieces

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Generally it should return null  The code calling the method should decide whether to throw an exception or to attempt something else

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Exceptions are related to Design by Contract    The interface of an objects is actually a contract between two objects  the caller must meet the contract or else the receiver may just fail with an exception  There are two possible contracts  1  all input the method is valid  in which case you must return null when the object is not found   2  only some input is valid  ie that which results in a found object  In which case you MUST offer a second method that allows the caller to determine if its input will be correct  For example  is present key  find key  throws Exception   IF and ONLY IF you provide both methods of the 2nd contract  you are allowed to throw an exception is nothing is found

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If null never indicates an error then just return null   If null is always an error then throw an exception   If null is sometimes an exception then code two routines  One routine throws an exception and the other is a boolean test routine that returns the object in an output parameter and the routine returns a false if the object was not found   It s hard to misuse a Try routine  It s real easy to forget to check for null   So when null is an error you just write  object o   FindObject      When the null isn t an error you can code something like  if  TryFindObject out object o       Do something with o else      o was not found

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It depends on the nature of the method and how it will be used   If it is normal behavior that the object may not be found  then return null   If it is normal behavior that the object is always found  throw an exception   As a rule of thumb  use exceptions only for when something exceptional occurs   Don t write the code in such a way that exception throwing and catching is part of its normal operation

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Just ask yourself    is it an exceptional case that the object is not found    If it is expected to happen in the normal course of your program  you probably should not raise an exception  since it is not exceptional behavior    Short version   use exceptions to handle exceptional behavior  not to handle normal flow of control in your program   -Alan

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As long as it s supposed to return a reference to the object  returning a NULL should be good    However  if it s returning the whole bloody thing  like in C   if you do   return blah   rather than  return  blah    or  blah  is a pointer   then you can t return a NULL  because it s not of type  object   In that case  throwing an exception  or returning a blank object that doesn t have a success flag set is how I would approach the problem

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That depends on your method  If you method is supposed to always return a valid object and none is found  throwing an exception is the way to go  If the method is just ment to return an object which might or might not be there  like perhaps a image on a contact person  you shouldn t raise an error   You might also want to expose a method returning a boolean true false if this method actually will return an object so you don t have to either a  check for null or b  catch an exception

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referring only to the case where null is not considered an exceptional behavior i am definitely for the try method  it is clear  no need to  read the book  or  look before you leap  as was said here  so basically    bool TryFindObject RequestParam request  out ResponseParam response    and this means that the user s code will also be clear      if TryFindObject request  out response      handleSuccess response    else     handleFailure

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That depends on your method  If you method is supposed to always return a valid object and none is found  throwing an exception is the way to go  If the method is just ment to return an object which might or might not be there  like perhaps a image on a contact person  you shouldn t raise an error   You might also want to expose a method returning a boolean true false if this method actually will return an object so you don t have to either a  check for null or b  catch an exception

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I just wanted to recapitulate the options mentioned before  throwing some new ones in    return null throw an Exception use the null object pattern provide a boolean parameter to you method  so the caller can chose if he wants you to throw an exception provide an extra parameter  so the caller can set a value which he gets back if no value is found   Or you might combine these options   Provide several overloaded versions of your getter  so the caller can decide which way to go  In most cases  only the first one has an implementation of the search algorithm  and the other ones just wrap around the first one   Object findObjectOrNull String key   Object findObjectOrThrow String key  throws SomeException  Object findObjectOrCreate String key  SomeClass dataNeededToCreateNewObject   Object findObjectOrDefault String key  Object defaultReturnValue     Even if you choose to provide only one implementation  you might want to use a naming convention like that to clarify your contract  and it helps you should you ever decide to add other implementations as well   You should not overuse it  but it may be helpfull  espeacially when writing a helper Class which you will use in hundreds of different applications with many different error handling conventions

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Just ask yourself    is it an exceptional case that the object is not found    If it is expected to happen in the normal course of your program  you probably should not raise an exception  since it is not exceptional behavior    Short version   use exceptions to handle exceptional behavior  not to handle normal flow of control in your program   -Alan

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Or return an Option  An option is basically a container class that forces the client to handle booth cases  Scala has this concept  look up it s API   Then you have methods like T getOrElse T valueIfNull  on this object thet either return the found object  or an allternative the client specifieces

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In data layer code  I some times use the following code  allowing the caller to decide if  object not found  means an error has occured    DataType GetObject DBConnection conn  string id  bool throwOnNotFound        DataType retval          find object in database     if  retval    null      throwOnNotFound            return retval        else           throw new NoRowsFoundException  DataType object with id  id  not found in database             DataType GetObject DBConnection conn  string id        return GetObject conn  id  true

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Be consistent with the API s  you re using

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it depends if your language and code promotes   LBYL  look before you leap  or  EAFP  easier to ask forgiveness than permission   LBYL says you should check for values  so return a null  EAFP says to just try the operation and see if it fails  throw an exception   though I agree with above   exceptions should be used for exceptional error conditions  and returning a null is best when using checks     EAFP vs  LBYL in Python   http   mail python org pipermail python-list 2003-May 205182 html   Web Archive

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As long as it s supposed to return a reference to the object  returning a NULL should be good    However  if it s returning the whole bloody thing  like in C   if you do   return blah   rather than  return  blah    or  blah  is a pointer   then you can t return a NULL  because it s not of type  object   In that case  throwing an exception  or returning a blank object that doesn t have a success flag set is how I would approach the problem

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Don t think anyone mentioned the overhead in exception handling - takes additional resources to load up and process the exception so unless its a true app killing or process stopping event  going forward would cause more harm than good  I would opt for passing back a value the calling environment could interpret as it sees fit

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Just ask yourself    is it an exceptional case that the object is not found    If it is expected to happen in the normal course of your program  you probably should not raise an exception  since it is not exceptional behavior    Short version   use exceptions to handle exceptional behavior  not to handle normal flow of control in your program   -Alan

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In some functions I add a parameter        bool verify   true    True means throw  false means return some error return value  This way  whoever uses this function has both options  The default should be true  for the benefit of those who forget about error handling

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Exceptions should be exceptional  Return null if it is valid to return a null

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Use the null object pattern or throw an exception

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As a general rule  if the method should always return an object  then go with the exception  If you anticipate the occasional null and want to handle it in a certain way  go with the null    Whatever you do  I highly advise against the third option  Returning a string that says  WTF

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Exceptions are related to Design by Contract    The interface of an objects is actually a contract between two objects  the caller must meet the contract or else the receiver may just fail with an exception  There are two possible contracts  1  all input the method is valid  in which case you must return null when the object is not found   2  only some input is valid  ie that which results in a found object  In which case you MUST offer a second method that allows the caller to determine if its input will be correct  For example  is present key  find key  throws Exception   IF and ONLY IF you provide both methods of the 2nd contract  you are allowed to throw an exception is nothing is found

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The  other  option could be to let the find method take an additional parameter with a default object that would be returned if the sought for object cannot be found   Otherwise I d just return null unless it really is an exceptional case when the object isn t found

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I just wanted to recapitulate the options mentioned before  throwing some new ones in    return null throw an Exception use the null object pattern provide a boolean parameter to you method  so the caller can chose if he wants you to throw an exception provide an extra parameter  so the caller can set a value which he gets back if no value is found   Or you might combine these options   Provide several overloaded versions of your getter  so the caller can decide which way to go  In most cases  only the first one has an implementation of the search algorithm  and the other ones just wrap around the first one   Object findObjectOrNull String key   Object findObjectOrThrow String key  throws SomeException  Object findObjectOrCreate String key  SomeClass dataNeededToCreateNewObject   Object findObjectOrDefault String key  Object defaultReturnValue     Even if you choose to provide only one implementation  you might want to use a naming convention like that to clarify your contract  and it helps you should you ever decide to add other implementations as well   You should not overuse it  but it may be helpfull  espeacially when writing a helper Class which you will use in hundreds of different applications with many different error handling conventions

User · Answer

Use the null object pattern or throw an exception

User · Answer

In data layer code  I some times use the following code  allowing the caller to decide if  object not found  means an error has occured    DataType GetObject DBConnection conn  string id  bool throwOnNotFound        DataType retval          find object in database     if  retval    null      throwOnNotFound            return retval        else           throw new NoRowsFoundException  DataType object with id  id  not found in database             DataType GetObject DBConnection conn  string id        return GetObject conn  id  true

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Exceptions should be exceptional  Return null if it is valid to return a null

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If it s important for client code to know the difference between found and not found and this is supposed to be a routine behavior  then it s best to return null   Client code can then decide what to do

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Here are a couple more suggestions   If returning a collection  avoid returning null  return an empty collection which makes enumeration easier to deal with without a null check first   Several  NET API s use the pattern of a thrownOnError parameter which gives the caller the choice as whether it is really an exceptional situation or not if the object is not found   Type GetType is an example of this   Another common pattern with BCL is the TryGet pattern where a boolean is returned and the value is passed via an output parameter   You could also consider the Null Object pattern in some circumstances which can either be a default or a version with no behaviour   The key is avoid null checks throughout the code base   See here for more information http   geekswithblogs net dsellers archive 2006 09 08 90656 aspx

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it depends if your language and code promotes   LBYL  look before you leap  or  EAFP  easier to ask forgiveness than permission   LBYL says you should check for values  so return a null  EAFP says to just try the operation and see if it fails  throw an exception   though I agree with above   exceptions should be used for exceptional error conditions  and returning a null is best when using checks     EAFP vs  LBYL in Python   http   mail python org pipermail python-list 2003-May 205182 html   Web Archive

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If you are always expecting to find a value then throw the exception if it is missing   The exception would mean that there was a problem   If the value can be missing or present and both are valid for the application logic then return a null   More important  What do you do other places in the code   Consistency is important

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Only throw an exception if it is truly an error  If it is expected behavior for the object to not exist  return the null   Otherwise it is a matter of preference

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That really depends on if you expect to find the object  or not  If you follow the school of thought that exceptions should be used for indicating something  well  err  exceptional has occured then    Object found  return object Object not-found  throw exception   Otherwise  return null

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It not containing the object can happen during normal operations and should be dealt with by the caller return NULL   If not containing the object indicates a bug by the calling code or internal state then do an assert   If not containing the object indicates an infrequent event   Like someone deleted an item from the store while you were checking out the item at the same time   Then throw an exception

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Exceptions should be exceptional  Return null if it is valid to return a null

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I prefer to just return a null  and rely on the caller to handle it appropriately  The  for lack of a better word  exception is if I am absolutely  certain  this method will return an object  In that case a failure is an exceptional should and should throw

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If not finding it is an exceptional event  i e  it should be there under normal circumstances   then throw  Otherwise  return a  not found  value  can be null  but does not have to   or even have the method return a boolean for found notfound and an out parameter for the actual object

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referring only to the case where null is not considered an exceptional behavior i am definitely for the try method  it is clear  no need to  read the book  or  look before you leap  as was said here  so basically    bool TryFindObject RequestParam request  out ResponseParam response    and this means that the user s code will also be clear      if TryFindObject request  out response      handleSuccess response    else     handleFailure

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Prefer returning null --  If the caller uses it without checking  the exception happens right there anyway   If the caller doesn t really use it  don t tax him a try catch block

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I agree with what seems to be the consensus here  return null if  not found  is a normal possible outcome  or throw an exception if the semantics of the situation require that the object always be found    There is  however  a third possibility that might make sense depending on your particular situation  Your method could return a default object of some sort in the  not found  condition  allowing calling code to be assured that it will always receive a valid object without the need for null checking or exception catching

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Return a null  exceptions are exactly that   something your code does that isn t expected

User · Answer

It depends on the nature of the method and how it will be used   If it is normal behavior that the object may not be found  then return null   If it is normal behavior that the object is always found  throw an exception   As a rule of thumb  use exceptions only for when something exceptional occurs   Don t write the code in such a way that exception throwing and catching is part of its normal operation

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If not finding it is an exceptional event  i e  it should be there under normal circumstances   then throw  Otherwise  return a  not found  value  can be null  but does not have to   or even have the method return a boolean for found notfound and an out parameter for the actual object

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Return a null instead of throwing an exception and clearly document the possibility of a null return value in the API documentation  If the calling code doesn t honor the API and check for the null case  it will most probably result in some sort of  null pointer exception  anyway     In C    I can think of 3 different flavors of setting up a method that finds an object   Option A  Object  findObject Key  amp key     Return null when an object can t be found  Nice and simple  I d go with this one  The alternative approaches below are for people who don t hate out-params   Option B  void findObject Key  amp key  Object  amp found     Pass in a reference to variable that will be receiving the object  The method thrown an exception when an object can t be found  This convention is probably more suitable if it s not really expected for an object not to be found -- hence you throw an exception to signify that it s an unexpected case   Option C  bool findObject Key  amp key  Object  amp found     The method returns false when an object can t be found  The advantage of this over option A is that you can check for the error case in one clear step   if   findObject myKey  myObj

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it depends if your language and code promotes   LBYL  look before you leap  or  EAFP  easier to ask forgiveness than permission   LBYL says you should check for values  so return a null  EAFP says to just try the operation and see if it fails  throw an exception   though I agree with above   exceptions should be used for exceptional error conditions  and returning a null is best when using checks     EAFP vs  LBYL in Python   http   mail python org pipermail python-list 2003-May 205182 html   Web Archive

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If it s important for client code to know the difference between found and not found and this is supposed to be a routine behavior  then it s best to return null   Client code can then decide what to do

User · Answer

If null never indicates an error then just return null   If null is always an error then throw an exception   If null is sometimes an exception then code two routines  One routine throws an exception and the other is a boolean test routine that returns the object in an output parameter and the routine returns a false if the object was not found   It s hard to misuse a Try routine  It s real easy to forget to check for null   So when null is an error you just write  object o   FindObject      When the null isn t an error you can code something like  if  TryFindObject out object o       Do something with o else      o was not found

User · Answer

Exceptions are related to Design by Contract    The interface of an objects is actually a contract between two objects  the caller must meet the contract or else the receiver may just fail with an exception  There are two possible contracts  1  all input the method is valid  in which case you must return null when the object is not found   2  only some input is valid  ie that which results in a found object  In which case you MUST offer a second method that allows the caller to determine if its input will be correct  For example  is present key  find key  throws Exception   IF and ONLY IF you provide both methods of the 2nd contract  you are allowed to throw an exception is nothing is found

User · Answer

In data layer code  I some times use the following code  allowing the caller to decide if  object not found  means an error has occured    DataType GetObject DBConnection conn  string id  bool throwOnNotFound        DataType retval          find object in database     if  retval    null      throwOnNotFound            return retval        else           throw new NoRowsFoundException  DataType object with id  id  not found in database             DataType GetObject DBConnection conn  string id        return GetObject conn  id  true

User · Answer

If it s important for client code to know the difference between found and not found and this is supposed to be a routine behavior  then it s best to return null   Client code can then decide what to do

User · Answer

I prefer to just return a null  and rely on the caller to handle it appropriately  The  for lack of a better word  exception is if I am absolutely  certain  this method will return an object  In that case a failure is an exceptional should and should throw

[java] Should a retrieval method return 'null' or throw an exception when it can't produce the return value?

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