Java Invalid Command Line Arguments Exception

Question

Is there an appropriate exception class for invalid command line arguments in the Java API or do I have to create my own? I've tried searching for one but can't find any in the API.

This is for an assignment so I cannot use third party libraries for command line parsing.

Solution 1

The best way to handle unknown command line parameters or combinations that don't make sense to the program is to display an error message and offer a usages output.

Personally, depending on the complexity of the command line, I will create a method called "usage" (usually static) that can have an optional error message passed to it.

While parsing the command line parameters passed into the program, I will call this method and either exit, via a flag or directly, or have usage method call exit for me.

But that's just me

Solution 2

Most of the times, when the received argument(s) are invalid, it's a common idiom to throw an IllegalArgumentException.

public class IllegalArgumentException extends RuntimeException

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

Comments

[Michael]

I did consider using this, but I thought that RuntimeExceptions (unchecked exceptions) are supposed to reflect a problem that the programmer has done wrong whereas checked exceptions would be used for things outside the programmer's control. Or does this only matter when designing API's?

[João Silva]

I tend to use unchecked exceptions for situations where your program cannot recover at runtime. An invalid argument passed to a method is such a case, it's a programmer's error. For example, when you try to instantiate an ArrayList with a negative initial capacity, it will throw an IllegalArgumentException. Why? Because it's not supposed to receive that, and it can't work with a negative capacity; it's a programmer's error. Here's a link that discusses this issue javapractices.com/topic/TopicAction.do?Id=129.

[João Silva]

Also, if you use a checked exception, every method that calls yours needs to catch this exception. Imho, this is somewhat odd, because basically you are acknowledging that you can have errors in your code that makes it pass invalid values/arguments to another method. And when this happens, there's nothing you can do about it, but fix the error that caused this.

[Michael]

When a program is given invalid command line arguments it is not a programmer error though, its a user error.

[João Silva]

@Michael: I agree, but those invalid arguments should never reach a method that doesn't support them, they should be pre-validated. If they do reach, then an IllegalArgumentException can be thrown. Now, if you want to terminate the program and show the user that he has passed invalid arguments, then you can just write to System.err telling why the arguments are invalid. This is what javac does, for example. If you invoke it with, say, javac -abcd, it will terminate with the message javac: invalid flag: -abcd; no reason to throw an Exception in this case, imho.

[Michael]

Yeah, I ended up not throwing an Exception as I was just catching it in the same method anyway.