'what does it mean when he says "we may have to change the object that is passed over here"' - you often wouldn't pass a bunch of parameters, but a single object with all required data, and if a new field was required, you'd have to add it to that object (new field in "that class"). You've done something slightly different, you have 'T value, U value2' - but you can also think of that as of a two-field object "in disguise". If you needed a new parameter, you'd have to add it in, and change the interface - and that would mean you'd also have to change every concrete subject and every observer.
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