It is being checked by the compiler though. For your example to work both would need to extend a common object (unless the only thing you ever do to that object is call fire(), which is nonsensical) or you would get a compile time error right off the bat.
It's a ridiculous thing to worry about in either of your cases. That incredibly specific situation simply isn't ever going to happen in practice because so many other conditions need to be met for the compiler to let you proceed.
> For your example to work both would need to extend a common object
Not necessarily at all.
> unless the only thing you ever do to that object is call fire(), which is nonsensical
In this example, when the object is returned by a particular method, only fire() is being called, but other ways of obtaining the object can have more methods being called.
> It's a ridiculous thing to worry about in either of your cases.
Not at all... if your program is running inside a NASA rover, or if your program is running inside a robotic surgery machine you have to worry about safety and you want to maximize compile-time checks.
It's a ridiculous thing to worry about in either of your cases. That incredibly specific situation simply isn't ever going to happen in practice because so many other conditions need to be met for the compiler to let you proceed.