Click again to exit from the app using Navigation drawer

I have a problem with this code. I wanna when the drawer is close and the user clicks back the drawer open, and when it opens and the user wanna to exit from the app show message said: "press again to exit". how can I do that with this code?

I wanna it be like this

press back -- open drawer;

press back again -- show message;

press back again -- exit the app;

Here's my code

private boolean doubleBackToExitPressedOnce = false;
@Override
public void onBackPressed() {
    if (drawer.isDrawerOpen(GravityCompat.START) && doubleBackToExitPressedOnce) {
        super.onBackPressed();
    }

    else {
        drawer.openDrawer(GravityCompat.START);
    }
}

And here is the message code and I don't know where to but it

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

    new Handler().postDelayed(new Runnable() {
        @Override
        public void run() {
            doubleBackToExitPressedOnce = false;
        }
    }, 2000);

This should solve your problem.

@Override
public void onBackPressed() {
    // If navigation drawer is not open yet, open it.
    if (!drawer.isDrawerOpen(GravityCompat.START)) {
        drawer.openDrawer(GravityCompat.START);
    } else if (doubleBackToExitPressedOnce) {
        super.onBackPressed();
    } else {
        doubleBackToExitPressedOnce = true;
        Toast.makeText(this, "Press again to exit", Toast.LENGTH_SHORT).show();

        new Handler().postDelayed(new Runnable() {
            @Override
            public void run() {
                doubleBackToExitPressedOnce = false;
            }
        }, 2000);
    }
}