What exception to throw?

Question

I have a function which calculates the mean of a list passed as an argument. I would like to know which of Java exception should I throw when I try to compute the mean of a list of size 0.

public double mean (MyLinkedList<? extends Number> list)
{
    if (list.isEmpty())
        throw new ????????; //If I am not mistaken Java has some defined exception for this case

    //code goes here
}

Thanks.

Answer 1

You can throw a new IllegalArgumentException().

Thrown to indicate that a method has been passed an illegal or inappropriate argument.

Just don't forget to pass a clear message as a first argument. This will really help you to understand what happend.

For example "Can't use mean on an empty List".

Answer 2

The question to ask yourself first is whether you should be throwing at all and then, if so, whether it should be a checked or unchecked exception.

Unfortunately, there's no industry best practice on deciding these things, as shown by this StackOverflow answer:

In Java, when should I create a checked exception, and when should it be a runtime exception?

Nevertheless, there are some key considerations:

• Your design/vision for how this method is supposed to work (Is it reasonable/normal for the method to be called with 0-size list)?

• Consistency with other methods in the class/package

• Compliance with your applicable coding standard (if any)

My opinion:

• Return Double.NAN or 0 If calling the method with a 0-size list is reasonable/expected/normal, I'd consider returning Double.NAN or 0 if 0 is appropriate for your problem domain.

• Throw anIllegalArgumentException If my design says that checking for an empty List is strongly the responsibility of the caller and the documentation for the method is going to clearly state that it is the responsibility of the caller, then I'd use the standard unchecked IllegalArgumentException.

• Throw a custom checked exception If the method is part of a statistics package or library where several statistics functions need to deal with an possible empty data set, I'd think this is an exception condition that is part of the problem domain. I'd create a custom (probably checked) exception (e.g. EmptyDataSetException) to be part of the class/package/library and use it across all applicable methods. Making it a checked exceptions helps remind the client to consider how to handle the condition.