When should an IllegalArgumentException be thrown?

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The API doc for IllegalArgumentException:

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

From looking at how it is used in the JDK libraries, I would say:

  • It seems like a defensive measure to complain about obviously bad input before the input can get into the works and cause something to fail halfway through with a nonsensical error message.

  • It’s used for cases where it would be too annoying to throw a checked exception (although it makes an appearance in the java.lang.reflect code, where concern about ridiculous levels of checked-exception-throwing is not otherwise apparent).

I would use IllegalArgumentException to do last ditch defensive argument checking for common utilities (trying to stay consistent with the JDK usage). Or where the expectation is that a bad argument is a programmer error, similar to an NullPointerException. I wouldn’t use it to implement validation in business code. I certainly wouldn’t use it for the email example.

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