boolean doubleBackToExitPressedOnce = false;

@Override
public void onBackPressed() {
if (doubleBackToExitPressedOnce) {
super.onBackPressed();
return;
}

this.doubleBackToExitPressedOnce = true;
Toast.makeText(this, "Please click BACK again to exit", Toast.LENGTH_SHORT).show();

new Handler().postDelayed(new Runnable() {

@Override
public void run() {
doubleBackToExitPressedOnce=false;
}
}, 2000);
}