Null Pointer Exception
Hi,
I'am new to Java and OOPS and I would like to know the best pratices programmers follow in the following situation:
I have a method defined as
public String concatenate(String argument1, String argument2){
//Method defined here
}
Ideally my method does not expect any argument as null.
My question is :
1. Should my method handle null objects gracefully? (I beleive it should otherwise it may throw a NullPointerException...)
2. If any of the arguments is null, what should it do. Should it return back a null object or should it throw a application-specific user-defined Exception?
3. What if my method does not return anything; return type is void? How will it tell the invoking entity that this method has not been executed due to error condition.
4. Should such null-checks be made in private methods also.
I fully understand this is not a Java specific query. Its like there are so many people logged in this site that I do not really understand where else I will get a prompt and precise reply.
Thanks in advance.
Jay
[1121 byte] By [
Jay@Javaa] at [2007-10-2 22:01:41]
> Hi,
>
> I'am new to Java and OOPS and I would like to know
> the best pratices programmers follow in the following
> situation:
>
> I have a method defined as
>
> public String concatenate(String argument1, String
> argument2){
>//Method defined here
> Ideally my method does not expect any argument as
> null.
>
>
> My question is :
> 1. Should my method handle null objects gracefully?
> (I beleive it should otherwise it may throw a
> NullPointerException...)
> 2. If any of the arguments is null, what should it
> do. Should it return back a null object or should it
> throw a application-specific user-defined Exception?
>
> 3. What if my method does not return anything; return
> type is void? How will it tell the invoking entity
> that this method has not been executed due to error
> condition.
surround the call to the method with a try/catch, and inside the method throw an exception if an error happens.
> 4. Should such null-checks be made in private methods
> also.
>
> I fully understand this is not a Java specific query.
> Its like there are so many people logged in this site
> that I do not really understand where else I will get
> a prompt and precise reply.
>
> Thanks in advance.
> Jay
> 1. Should my method handle null objects gracefully?
Always - (Note, I'm not specifiying what gracefully is though)
> 2. If any of the arguments is null, what should it
> do. Should it return back a null object or should it
> throw a application-specific user-defined Exception?
This depends on how your method is used.
> 3. What if my method does not return anything; return
> type is void? How will it tell the invoking entity
> that this method has not been executed due to error
> condition.
See the answer given by the other poster
> 4. Should such null-checks be made in private methods
> also.
Depends on how they are used.
I think your question is both relevent to this forum and an excelent example of someone who is asking the right question to become and good OO programmer.
> My question is :
> 1. Should my method handle null objects gracefully?
> (I beleive it should otherwise it may throw a
> NullPointerException...)
As already mentioned you should always handle exceptions gracfully, but that depends what gracful is in the perticular situation. You have to decide whether a null for one of the values is appropriate for your method.
It may be the case that you have a set method that allows you to set a value to null. This may be a desired behaviour. If however your method cannot perform it's intended purpouse with the value provided, you should provide feedback to the calling method, using exceptions.
> 2. If any of the arguments is null, what should it
> do. Should it return back a null object or should it
> throw a application-specific user-defined Exception?
>
Again, it depends on your method. It may be that returning null instead of an object is appropriate. if you want to get a value that hasn't been set yet, maybe null is appropriate, or maye an exception is more appropriate, it is up to you to decide. A general rule you can apply to help you decide is: "has the method performed the task it is suppost to": yes? return the null value - no? throw an exception
> 3. What if my method does not return anything; return
> type is void? How will it tell the invoking entity
> that this method has not been executed due to error
> condition.
Exceptions are not returns. An exception can be thrown from a void method in exactly that same way as a non-void method. Exceptions should always be used to privide feedback of errounouse behaviour or failures. It is not the job of any return value to carry faliure feedback information.
> 4. Should such null-checks be made in private methods
> also.
>
This is a good question. I would say, if you don't have 100% control over weither the value is null and a null value could cause the method to fail, then putting full exception handelling into your private methods is a good idea. Also if the class is expected to grow very large and/or will have multiple people working on it, then it is always better safe than sorry.
As a final note I would like to add that quite oftern nulls are valid states for refrences to objects, and in many cases a null can be as useful as the object it's self.
