What exception should I throw when code which isn't supposed to run is run?

Question

if (stuff) doThings();
else if (something) doOtherThings();
else if (otherStuff) doStuff();
else // .. this isn't supposed to be reached

In situations such as the one above, I like to put the else in the end so I can be notified if the program runs code it shouldn't run, meaning something went wrong (i.e. one of the above conditions should be true, but none are, which means something is wrong).

What exception should I throw in that final else, to notify me something is wrong?

Answer 1

The IllegalStateException is a good candidate.

As the Java API states:

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.

In other words, your program is in a state for which there is no transition defined for that "this isn't supposed to be reached" input.

If your stuff, something and otherStuff are all related to your method arguments, you could also use IllegalArgumentException, like:

if (arg == null) doThings();
else if (arg.endsWith("foo")) doOtherThings();
else if (arg.endsWith("bar")) doStuff();
else throw new IllegalArgumentException(
        "arg should be null or end with \"foo\" or \"bar\"");

Answer 2

You can create your own custom exception class by extending the Exception class.

For example:

class CustomException extends Exception {
    ...
}

And then:

if (stuff)
    doThings();
else if (something) 
    doOtherThings();
else if (otherStuff) 
    doStuff();
else
    throw new CustomException();