I have developed an app that works on Android devices and iPhones. I went to great lengths to ensure that the interface is exactly the same on both systems.
However, one significant difference that I can not change is that most, maybe all, Android devices have, on the front of the device below the screen, two extra physical buttons along with a main button, whereas the iPhone only has one main button. One is a menu button, the other is a back button.
When using my app, whether on the iPhone or Android, pressing the main central button will exit to the desktop screen. This seems to be the default behaviour as I made no attempt to configure it, and that behaviour is fine with me.
Since I have taken care of all other interface needs in order to accomodate the iPhone's lack of any other buttons, the two extra buttons on the Android are not needed for me. However, I've discovered with testing that users expect at least something to happen. If nothing else, to at least in some way let them know that it's the case that the button was pressed but has no function, as opposed to not responding because of some kind of system error.
I have a settings and configuration screen, so the menu button could conceivably take a user to that.
However in my application, there is nowhere to conceptually go "back" to, rendering the back button completely superfluous. In some ways it might make sense to have it exit the user out of the application, but the main button already does that, so it seems a little odd to have that function duplicated.
In any case, what would be the best action to apply to these buttons as a way of letting a user know that those buttons don't serve a purpose? Or, what would be a logical default action for those buttons that would least not confuse a user?
I'm open to other suggestions as well if there are other possibilities I haven't touched on.