How to stop notification in android studio using switch buttons?

Question

I'm using this code to stop a notification. I'm using Android Studio and switch buttons (via the mainActivity):

public class MainActivity extends AppCompatActivity {

    public static final String NOTIFICATION_CHANNEL_ID = "channel_id";
    public static final String CHANNEL_NAME = "Notification Channel";
    SharedPreferences.Editor prefEditor;
    SharedPreferences prefs;
    public Switch switchbtn;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        prefEditor= PreferenceManager.getDefaultSharedPreferences(getBaseContext()).edit();
        prefs=PreferenceManager.getDefaultSharedPreferences(getBaseContext());

        switchbtn= findViewById(R.id.switch2);
        switchbtn.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
            @Override
            public void onCheckedChanged(CompoundButton compoundButton, boolean b) {

                if(switchbtn.isChecked()){
                    shownotifi();
                    prefEditor.putString("checked","yes");
                    prefEditor.apply();
                }
                else{
                    //To Stop notification?????
                    prefEditor.putString("checked","no");
                    prefEditor.apply();
                }
            }
        });

        if(prefs.getString("checked","no").equals("yes")) {
            switchbtn.setChecked(true);
        }
        else{
            switchbtn.setChecked(false);
        }

    }

   public void shownotifi() {
       NotificationCompat.Builder builder = new NotificationCompat.Builder(this, NOTIFICATION_CHANNEL_ID);
       builder.setContentTitle("This is heading");
       builder.setContentText("This is description");
       builder.setSmallIcon(R.drawable.ic_android_black_24dp);
       Notification notification = builder.build();
       NotificationManager notificationManager=(NotificationManager)getSystemService(Context.NOTIFICATION_SERVICE);
       notificationManager.notify(0,builder.build());
   }
}

What changes do I need to make to the code above to stop the notifications? I've commented where I need to stop them.