When to throw IllegalStateException vs IllegalArgumentException?

Question

Let's start with the Javadocs:

IllegalStateException

Signals that a method has been invoked at an illegal or inappropriate time. In other words, the Java environment or Java application is not in an appropriate state for the requested operation.

IllegalArgumentException

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

The problem with the above is that they are very black and white. Consider a use case where a method is parsing a file provided by the caller. The file exists, is readable, and is in the correct format. However, some content in the file is non-compliant with the business rules. What would be an appropriate exception to throw in this case - IllegalStateException or IllegalArgumentException?

Looking at various libraries that provide assertions, like Guava Preconditions or Spring Assert, it appears that there is no consensus. There are some good discussions here and here, but none provide a conclusive answer to the common use case I stated above.

Answer 1

IllegalStateException is for coding errors, not input errors. It's for when the invariants of a class have been violated, or a method is called when an object is in the wrong state. Examples are using a closed resource, or closing a resource twice.

IllegalArgumentException is when an argument has an invalid value per the method API. Passing -1 when only positive numbers are allowed.

In this case neither exception is appropriate. I would create a sub-class of IOException since there's an error in an input file.