This  is code of android hardware back button. Here you can perform your action

@Override
public boolean onKeyDown(int keyCode, KeyEvent event)  {
    if (android.os.Build.VERSION.SDK_INT < android.os.Build.VERSION_CODES.ECLAIR
            && keyCode == KeyEvent.KEYCODE_BACK // your code here
      }
 
    return super.onKeyDown(keyCode, event);
}