What's your definition of 'know'? It's very common to have surface level knowledge of the class only to dive in and see that it does something else. So do you 'know' the code as well as you thought so? Probably not. Fact of the matter is you can't magically 'know' everything about a class based off a class name. And this includes your 'seasoned Java developers'.

You're throwing out some very large assumptions as well.