When back button is pressed and navigation drawer is open, my app closes instead of closing drawer

To close drawer, when back button is pressed, you have to override onBackPressed(), this way:

@Override
public void onBackPressed() {

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    if (drawer.isDrawerOpen(GravityCompat.START)) {
        drawer.closeDrawer(GravityCompat.START);
    } else {
        super.onBackPressed();
    }
}