IllegalArgumentException or NullPointerException for a null parameter

Question

I have a simple setter method for a property and null is not appropriate for this particular property  I have always been torn in this situation  should I throw an IllegalArgumentException  or a NullPointerException  From the javadocs  both seem appropriate  Is there some kind of an understood standard   Or is this just one of those things that you should do whatever you prefer and both are really correct

User · Answer

The definitions from the links to the two exceptions above are IllegalArgumentException: Thrown to indicate that a method has been passed an illegal or inappropriate argument. NullPointerException: Thrown when an application attempts to use null in a case where an object is required.

The big difference here is the IllegalArgumentException is supposed to be used when checking that an argument to a method is valid. NullPointerException is supposed to be used whenever an object being "used" when it is null.

I hope that helps put the two in perspective.

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If it s a  setter   or somewhere I m getting a member to use later  I tend to use IllegalArgumentException   If it s something I m going to use  dereference  right now in the method  I throw a NullPointerException proactively  I like this better than letting the runtime do it  because I can provide a helpful message  seems like the runtime could do this too  but that s a rant for another day    If I m overriding a method  I use whatever the overridden method uses

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If you choose to throw a NPE and you are using the argument in your method  it might be redundant and expensive to explicitly check for a null  I think the VM already does that for you

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Some collections assume that null is rejected using NullPointerException rather than IllegalArgumentException  For example  if you compare a set containing null to a set that rejects null  the first set will call containsAll on the other and catch its NullPointerException -- but not IllegalArgumentException   I m looking at the implementation of AbstractSet equals    You could reasonably argue that using unchecked exceptions in this way is an antipattern  that comparing collections that contain null to collections that can t contain null is a likely bug that really should produce an exception  or that putting null in a collection at all is a bad idea  Nevertheless  unless you re willing to say that equals should throw an exception in such a case  you re stuck remembering that NullPointerException is required in certain circumstances but not in others    IAE before NPE except after  c

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The standard is to throw the NullPointerException  The generally infallible  Effective Java  discusses this briefly in Item 42  first edition   Item 60  second edition   or Item 72  third edition   Favor the use of standard exceptions        Arguably  all erroneous method   invocations boil down to an illegal   argument or illegal state  but other   exceptions are standardly used for   certain kinds of illegal arguments and   states  If a caller passes null in   some parameter for which null values   are prohibited  convention dictates   that NullPointerException be thrown   rather than IllegalArgumentException

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I tend to follow the design of JDK libraries  especially Collections and Concurrency  Joshua Bloch  Doug Lea  those guys know how to design solid APIs   Anyway  many APIs in the JDK pro-actively throws NullPointerException    For example  the Javadoc for Map containsKey states       throws NullPointerException if the key is null and this map     does not permit null keys  optional     It s perfectly valid to throw your own NPE  The convention is to include the parameter name which was null in the message of the exception    The pattern goes   public void someMethod Object mustNotBeNull          if  mustNotBeNull    null              throw new NullPointerException  mustNotBeNull must not be null                  Whatever you do  don t allow a bad value to get set and throw an exception later when other code attempts to use it  That makes debugging a nightmare  You should always the follow the  fail-fast  principle

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Ideally runtime exceptions should not be thrown  A checked exception business exception  should be created for your scenario  Because if either of these exception is thrown and logged  it misguides the developer while going through the logs  Instead business exceptions do not create that panic and usually ignored while troubleshooting logs

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As a subjective question this should be closed  but as it s still open   This is part of the internal policy used at my previous place of employment and it worked really well  This is all from memory so I can t remember the exact wording  It s worth noting that they did not use checked exceptions  but that is beyond the scope of the question  The unchecked exceptions they did use fell into 3 main categories   NullPointerException  Do not throw intentionally  NPEs are to be thrown only by the VM when dereferencing a null reference  All possible effort is to be made to ensure that these are never thrown   Nullable and  NotNull should be used in conjunction with code analysis tools to find these errors   IllegalArgumentException  Thrown when an argument to a function does not conform to the public documentation  such that the error can be identified and described in terms of the arguments passed in  The OP s situation would fall into this category   IllegalStateException  Thrown when a function is called and its arguments are either unexpected at the time they are passed or incompatible with the state of the object the method is a member of   For example  there were two internal versions of the IndexOutOfBoundsException used in things that had a length  One a sub-class of IllegalStateException  used if the index was larger than the length  The other a subclass of IllegalArgumentException  used if the index was negative  This was because you could add more items to the object and the argument would be valid  while a negative number is never valid   As I said  this system works really well  and it took someone to explain why the distinction is there   Depending on the type of error it is quite straightforward for you to figure out what to do  Even if you can t actually figure out what went wrong you can figure out where to catch that error and create additional debugging information    NullPointerException  Handle the Null case or put in an assertion so that the NPE is not thrown  If you put in an assertion is just one of the other two types  If possible  continue debugging as if the assertion was there in the first place   IllegalArgumentException  you have something wrong at your call site  If the values being passed in are from another function  find out why you are receiving an incorrect value  If you are passing in one of your arguments propagate the error checks up the call stack until you find the function that is not returning what you expect   IllegalStateException  You have not called your functions in the correct order  If you are using one of your arguments  check them and throw an IllegalArgumentException describing the issue  You can then propagate the cheeks up against the stack until you find the issue   Anyway  his point was that you can only copy the IllegalArgumentAssertions up the stack  There is no way for you to propagate the IllegalStateExceptions or NullPointerExceptions up the stack because they had something to do with your function

User · Answer

It seems like an IllegalArgumentException is called for if you don t want null to be an allowed value  and the NullPointerException would be thrown if you were trying to use a variable that turns out to be null

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If it s a setter method and null is being passed to it  I think it would make more sense to throw an IllegalArgumentException  A NullPointerException seems to make more sense in the case where you re attempting to actually use the null   So  if you re using it and it s null  NullPointer  If it s being passed in and it s null  IllegalArgument

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Actually  the question of throwing IllegalArgumentException or NullPointerException is in my humble view only a  holy war  for a minority with an incomlete understanding of exception handling in Java  In general  the rules are simple  and as follows    argument constraint violations must be indicated as fast as possible  -  fast fail   in order to avoid illegal states which are much harder to debug in case of an invalid null pointer for whatever reason  throw NullPointerException in case of an illegal array collection index  throw ArrayIndexOutOfBounds in case of a negative array collection size  throw NegativeArraySizeException in case of an illegal argument that is not covered by the above  and for which you don t have another more specific exception type  throw IllegalArgumentException as a wastebasket on the other hand  in case of a constraint violation WITHIN A FIELD that could not be avoided by fast fail for some valid reason  catch and rethrow as IllegalStateException or a more specific checked exception  Never let pass the original NullPointerException  ArrayIndexOutOfBounds  etc in this case    There are at least three very good reasons against the case of mapping all kinds of argument constraint violations to IllegalArgumentException  with the third probably being so severe as to mark the practice bad style    1  A programmer cannot a safely assume that all cases of argument constraint violations result in IllegalArgumentException  because the large majority of standard classes use this exception rather as a wastebasket if there is no more specific kind of exception available  Trying to map all cases of argument constraint violations to IllegalArgumentException in your API only leads to programmer frustration using your classes  as the standard libraries mostly follow different rules that violate yours  and most of your API users will use them as well    2  Mapping the exceptions actually results in a different kind of anomaly  caused by single inheritance  All Java exceptions are classes  and therefore support single inheritance only  Therefore  there is no way to create an exception that is truly say both a NullPointerException and an IllegalArgumentException  as subclasses can only inherit from one or the other  Throwing an IllegalArgumentException in case of a null argument therefore makes it harder for API users to distinguish between problems whenever a program tries to programmatically correct the problem  for example by feeding default values into a call repeat    3  Mapping actually creates the danger of bug masking  In order to map argument constraint violations into IllegalArgumentException  you ll need to code an outer try-catch within every method that has any constrained arguments  However  simply catching RuntimeException in this catch block is out of the question  because that risks mapping documented RuntimeExceptions thrown by libery methods used within yours into IllegalArgumentException  even if they are no caused by argument constraint violations  So you need to be very specific  but even that effort doesn t protect you from the case that you accidentally map an undocumented runtime exception of another API  i e  a bug  into an IllegalArgumentException of your API  Even the most careful mapping therefore risks masking programming errors of other library makers as argument constraint violations of your method s users  which is simply hillareous behavior   With the standard practice on the other hand  the rules stay simple  and exception causes stay unmasked and specific  For the method caller  the rules are easy as well  - if you encounter a documented runtime exception of any kind because you passed an illegal value  either repeat the call with a default  for this specific exceptions are neccessary   or correct your code - if on the other hand you enccounter a runtime exception that is not documented to happen for a given set of arguments  file a bug report to the method s makers to ensure that either their code or their documentation is fixed

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Voted up Jason Cohen s argument because it was well presented  Let me dismember it step by step   -    The NPE JavaDoc explicitly says   other illegal uses of the null object   If it was just limited to situations where the runtime encounters a null when it shouldn t  all such cases could be defined far more succinctly  Can t help it if you assume the wrong thing  but assuming encapsulation is applied properly  you really shouldn t care or notice whether a null was dereferenced inappropriately vs  whether a method detected an inappropriate null and fired an exception off  I d choose NPE over IAE for multiple reasons   It is more specific about the nature of the illegal operation Logic that mistakenly allows nulls tends to be very different from logic that mistakenly allows illegal values  For example  if I m validating data entered by a user  if I get value that is unacceptable  the source of that error is with the end user of the application  If I get a null  that s programmer error  Invalid values can cause things like stack overflows  out of memory errors  parsing exceptions  etc  Indeed  most errors generally present  at some point  as an invalid value in some method call  For this reason I see IAE as actually the MOST GENERAL of all exceptions under RuntimeException   Actually  other invalid arguments can result in all kinds of other exceptions  UnknownHostException  FileNotFoundException  a variety of syntax error exceptions  IndexOutOfBoundsException  authentication failures  etc   etc    In general  I feel NPE is much maligned because traditionally has been associated with code that fails to follow the fail fast principle  That  plus the JDK s failure to populate NPE s with a message string really has created a strong negative sentiment that isn t well founded  Indeed  the difference between NPE and IAE from a runtime perspective is strictly the name  From that perspective  the more precise you are with the name  the more clarity you give to the caller

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Throwing an exception that s exclusive to null arguments  whether NullPointerException or a custom type  makes automated null testing more reliable  This automated testing can be done with reflection and a set of default values  as in Guava s NullPointerTester  For example  NullPointerTester would attempt to call the following method     Foo String string  List lt   gt  list      checkArgument string length    gt  0        missing null check for list    this string   string    this list   list         with two lists of arguments      null and null  ImmutableList of    It would test that each of these calls throws the expected NullPointerException  For this implementation  passing a null list does not produce NullPointerException  It does  however  happen to produce an IllegalArgumentException because NullPointerTester happens to use a default string of     If NullPointerTester expects only NullPointerException for null values  it catches the bug  If it expects IllegalArgumentException  it misses it

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NullPointerException thrown when attempting to access an object with a reference variable whose current value is null   IllegalArgumentException thrown when a method receives an argument formatted differently than the method expects

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The accepted practice if to use the IllegalArgumentException  String message   to declare a parameter to be invalid and give as much detail as possible    So to say that a parameters was found to be null while exception non-null  you would do something like this   if  variable    null       throw new IllegalArgumentException  The object  variable  cannot be null      You have virtually no reason to implicitly use the  NullPointerException   The NullPointerException is an exception thrown by the Java Virtual Machine when you try to execute code on null reference  Like toString

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the dichotomy    Are they non-overlapping  Only non-overlapping parts of a whole can make a dichotomy  As i see it   throw new IllegalArgumentException new NullPointerException NULL ARGUMENT IN METHOD BAD BOY BAD

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Apache Commons Lang has a NullArgumentException that does a number of the things discussed here  it extends IllegalArgumentException and its sole constructor takes the name of the argument which should have been non-null   While I feel that throwing something like a NullArgumentException or IllegalArgumentException more accurately describes the exceptional circumstances  my colleagues and I have chosen to defer to Bloch s advice on the subject

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You should throw an IllegalArgumentException  as it will make it obvious to the programmer that he has done something invalid   Developers are so used to seeing NPE thrown by the VM  that any programmer would not immediately realize his error  and would start looking around randomly  or worse  blame your code for being  buggy

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I was all in favour of throwing IllegalArgumentException for null parameters  until today  when I noticed the java util Objects requireNonNull method in Java 7   With that method  instead of doing   if  param    null        throw new IllegalArgumentException  param cannot be null         you can do   Objects requireNonNull param     and it will throw a NullPointerException if the parameter you pass it is null   Given that that method is right bang in the middle of java util I take its existence to be a pretty strong indication that throwing NullPointerException is  the Java way of doing things    I think I m decided at any rate   Note that the arguments about hard debugging are bogus because you can of course provide a message to NullPointerException saying what was null and why it shouldn t be null   Just like with IllegalArgumentException   One added advantage of NullPointerException is that  in highly performance critical code  you could dispense with an explicit check for null  and a NullPointerException with a friendly error message   and just rely on the NullPointerException you ll get automatically when you call a method on the null parameter   Provided you call a method quickly  i e  fail fast   then you have essentially the same effect  just not quite as user friendly for the developer   Most times it s probably better to check explicitly and throw with a useful message to indicate which parameter was null  but it s nice to have the option of changing that if performance dictates without breaking the published contract of the method constructor

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I wanted to single out Null arguments from other illegal arguments  so I derived an exception from IAE named NullArgumentException  Without even needing to read the exception message  I know that a null argument was passed into a method and by reading the message  I find out which argument was null  I still catch the NullArgumentException with an IAE handler  but in my logs is where I can see the difference quickly

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According to your scenario  IllegalArgumentException is the best pick  because null is not a valid value for your property

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In general  a developer should never throw a NullPointerException  This exception is thrown by the runtime when code attempts to dereference a variable who s value is null  Therefore  if your method wants to explicitly disallow null  as opposed to just happening to have a null value raise a NullPointerException  you should throw an IllegalArgumentException

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You should be using IllegalArgumentException  IAE   not NullPointerException  NPE  for the following reasons   First  the NPE JavaDoc explicitly lists the cases where NPE is appropriate   Notice that all of them are thrown by the runtime when null is used inappropriately   In contrast  the IAE JavaDoc couldn t be more clear   Thrown to indicate that a method has been passed an illegal or inappropriate argument    Yup  that s you   Second  when you see an NPE in a stack trace  what do you assume   Probably that someone dereferenced a null   When you see IAE  you assume the caller of the method at the top of the stack passed in an illegal value   Again  the latter assumption is true  the former is misleading   Third  since IAE is clearly designed for validating parameters  you have to assume it as the default choice of exception  so why would you choose NPE instead   Certainly not for different behavior -- do you really expect calling code to catch NPE s separately from IAE and do something different as a result   Are you trying to communicate a more specific error message   But you can do that in the exception message text anyway  as you should for all other incorrect parameters   Fourth  all other incorrect parameter data will be IAE  so why not be consistent   Why is it that an illegal null is so special that it deserves a separate exception from all other types of illegal arguments   Finally  I accept the argument given by other answers that parts of the Java API use NPE in this manner   However  the Java API is inconsistent with everything from exception types to naming conventions  so I think just blindly copying  your favorite part of  the Java API isn t a good enough argument to trump these other considerations

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In this case  IllegalArgumentException conveys clear information to the user using your API that the   should not be null   As other forum users pointed out you could use NPE if you want to as long as you convey the right information to the user using your API    GaryF and tweakt dropped  Effective Java   which I swear by  references which recommends using NPE  And looking at how other good APIs are constructed is the best way to see how to construct your API   Another good example is to look at the Spring APIs  For example  org springframework beans BeanUtils instantiateClass Constructor ctor  Object   args  has a Assert notNull ctor   Constructor must not be null   line  org springframework util Assert notNull Object object  String message  method checks to see if the argument  object  passed in is null and if it is it throws a new IllegalArgumentException message  which is then caught in the org springframework beans BeanUtils instantiateClass      method

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Couldn t agree more with what s being said   Fail early  fail fast   Pretty good Exception mantra    The question about which Exception to throw is mostly a matter of personal taste   In my mind IllegalArgumentException seems more specific than using a NPE since it s telling me that the problem was with an argument I passed to the method and not with a value that may have been generated while performing the method   My 2 Cents

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It s a  Holy War  style question  In others words  both alternatives are good  but people will have their preferences which they will defend to the death

[java] IllegalArgumentException or NullPointerException for a null parameter?

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