Get back press work only at second press and notify user to press again to exit.

DoubleBackPressExit.java

private static long back_pressed;

@Override
public void onBackPressed(){
  if (back_pressed + 2000 > System.currentTimeMillis()) super.onBackPressed();
  else Toast.makeText(getBaseContext(), "Press once again to exit!", Toast.LENGTH_SHORT).show();
  back_pressed = System.currentTimeMillis();
}