If you put everything important into the custID object, what is left for the customer object? Just a reference to its custID?
And how does a custID object help you to avoid breaking old contracts? Adding stuff directly to the customer object wouldn't break your old contracts either.
What could break your contracts however, is omitting getters and setters and just making variables public. What if you have to add some "processing" of an input later? If you have had a setter from the beginning, it can be done transparently without changing any classes that use the setter.
I think you are calling "customer" what I'm calling "customer ID."
You then add getters and setters for the customer (custID) object. Any pre-processing is handled there. Also, some vars in the custID can certainly be handled with get/set--vars set via user input, for instance.
But I also use this for building, for example, custom buttons. Things like color, stroke, etc. all set in a "buttonData" object. My custom button comes from a class that extends JButton with a constructor that accepts the buttonData object.
The beauty of this is that I can add a new feature to my buttons, say transparency, but (i) modifying the new Button class to paint transparently, and (ii) adding the transparency value to the buttonData objects. Nothing else in the code has to change, and I have custom transparency.
If I want a particular set of buttons to have a unique transparency, say the "help" buttons, I simply vary the value of the buttonData object before calling that constructor.
This is the power of OOP: encapsulating what changes. Is it slower to build? Absolutely. But you can probably already see how simple it is to modify and maintain.
And how does a custID object help you to avoid breaking old contracts? Adding stuff directly to the customer object wouldn't break your old contracts either.
What could break your contracts however, is omitting getters and setters and just making variables public. What if you have to add some "processing" of an input later? If you have had a setter from the beginning, it can be done transparently without changing any classes that use the setter.