使用什么异常来防止方法被多次调用？

Question

I have a method that should only be called once during an object's lifetime. In order to ensure that this is the case, the method sets a boolean flag in the Object to true so it can later check if this method has already run. I am currently throwing an IllegalArgumentException (with a descriptive message) if this method is called a second time during a single object's lifetime, but that doesn't feel quite right to me, since the problem is not actually with the arguments themselves. Is there a better exception to use than an IllegalArgumentException ?

I chose not to use an assert statement in this case, because the class and method are both visible outside the package, so the problem may be caused by code outside of my package. Is that correct thinking?

Answer 1

Throw an IllegalStateException .

But since exceptions shouldn't be part of the ordinary control flow, you should add a companion method, which returns a boolean that indicates whether the next call to the method will be successful.

An example for such a companion method is Iterator#hasNext() .

A well-designed API must not force its clients to use exceptions for ordinary control flow. A class with a “state-dependent” method that can be invoked only under certain unpredictable conditions should generally have a separate “state-testing” method indicating whether it is appropriate to invoke the state-dependent method. For example, the Iterator interface has the state-dependent method next and the corresponding state-testing method hasNext. ¹

_{1: from Effective Java, Chapter 9: Exceptions}

Answer 2

What should worry you more than the specific exception type is the fact that you created a bad design here.

Good interfaces make it easy to do the right thing and hard to do the wrong thing.

Meaning: your current implementation makes it easy to call that method twice; respectively you now force your clients to always check if that method was already called.

So, instead of spending your time on the exception type: step back and figure how to dissect your one class into two classes for example. And find a nice so that calling that specific method gives you a different object to work on. Or check if you should rather use a state machine to solve this problem.