This question is not really about Stacks though You should be able to answer it even if you know nothing about the Stack class.
I did not know what behavior push() has on a Stack object or if the object and method were just made up. I was not able to know if it is going to affect the reference passed in as an argument or not and how. I did not know what Stack.toString() method would return.
I just learned the hard way that Stack and push() exist and what they do.