Intercepting the back button

In an activity you can just override

onBackPressed()

edit: that is api lvl 5+ :/ for 4 and below you gotta override onKeyDown()

By the docs, do not use onBackPressed if you can. Now is recommended using onBackPressedDispatcher.addCallback(this) {}. There's a Fragment example:

class MyFragment : Fragment() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        // This callback will only be called when MyFragment is at least Started.
        val callback = requireActivity().onBackPressedDispatcher.addCallback(this) {
            // Handle the back button event
        }

        // The callback can be enabled or disabled here or in the lambda
    }
    ...
}

Simply override the onKeyDown(int, KeyEvent) method in your activity and look for the back button. Return true so that the event is consumed.

public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK) {
        //Do something here
        return true;
    }
    return super.onKeyDown(keyCode, event);
}

As schwiz pointed out, you'll want to override the onBackPressed() method in your activity class (http://developer.android.com/reference/android/app/Activity.html#onBackPressed()).

I just wanted to add that if you did want to at some stage continue with or access the standard the back operation (after say, displaying a dialog), then you simply call super.onBackPressed() or ActivityName.super.onBackPressed() from anywhere in the Activity.