This also definitely does not apply to .NET where there are many classes that override ToString for meaningful, well-documented behavior (StringBuilder is one, funnily enough). It's best to check the docs or look at IntelliSense before applying this rule in a blanket fashion.

That said, it does seem like Microsoft has since moved away from overriding ToString for such things. In all honesty, Object.ToString was probably a mistake to begin with.

There's nothing wrong with returning something well-specified from toString, but I agree this should never be relied upon. And even if toString actually return something well-specified, the same functionality should be available as a distinct method.

Any form of (semi-)automatic SomeObject -> String should be treated as a convenience for the developer to avoid getting a non-descriptive memory location reported.

Having the automatic toString isn't the problem, but maybe the name of the method is. It could be called "asDebugHelpRepresentation" or something more descriptive.

This also goes for logging and even string-ification in a lot of popular Java libraries. It's certainly good to write methods like getName() if you foresee an issue with changes down the line, but precisely because toString is universal, it may well be invoked somewhere outside what you're writing.

The typical usage for StringBuilder tends to make it pretty obvious you're using a StringBuilder and everybody expects you will convert to a string at the end. Usually StringBuilders are short lived and all the calls and the declaration are close to each other.

Ok, but then the rule is not to use toString() for behavior unless it "makes sense" to do so, which is not a rule at all.

I think that misses my point. It's an established pattern recognizable to anyone who has read or written a lot of code. And locality of use and declaration is high. The danger of it being mixed up with another type in a refactor in faraway code is low.

Whereas in the general case, where code can be sparse and types can be more freely altered, you could very easily change the type of something and not realize that a ToString() call on that type lurks somewhere else in the code base.