Помогите сделать проверку на правильность введенного названия города

package com.example.weather;

import androidx.appcompat.app.AppCompatActivity;

import android.annotation.SuppressLint;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import org.json.JSONException;
import org.json.JSONObject;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;

public class MainActivity extends AppCompatActivity {
    private EditText cityname;
    private Button button1;
    private TextView textView2;
    private TextView textView3;
    private TextView textView4;
    private TextView textView5;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        cityname = findViewById(R.id.cityname);
        button1 = findViewById(R.id.button1);
        textView2 = findViewById(R.id.textView2);
        textView3 = findViewById(R.id.textView3);
        textView4 = findViewById(R.id.textView4);
        textView5 = findViewById(R.id.textView5);

        button1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if (cityname.getText().toString().trim().equals(""))
                    Toast.makeText(MainActivity.this,R.string.warning, Toast.LENGTH_LONG).show();
                else {
                    String city = cityname.getText().toString();
                    String key = "cba05fac32e986f325878b497331cfc8";
                    String url = "https://api.openweathermap.org/data/2.5/weather?q=" + city +"&appid="+ key +"&units=metric&lang=ru";
                    new GerUrlData().execute(url);

                }
            }
        });

    }
    private class GerUrlData extends AsyncTask<String, String, String>{
        protected void onPreExecute(){
            super.onPreExecute();
            textView2.setText("Ожидайте...");
            textView3.setText("Ожидайте...");
            textView4.setText("Ожидайте...");
            textView5.setText("Ожидайте...");

        }

        @Override
        protected String doInBackground(String... strings) {
            HttpURLConnection connection = null;
            BufferedReader reader = null;
            try {
                URL url = new URL(strings[0]);
                connection = (HttpURLConnection) url.openConnection();
                connection.connect();
                InputStream stream = connection.getInputStream();
                reader = new BufferedReader(new InputStreamReader(stream));
                StringBuffer buffer = new StringBuffer();
                String line = "";
                while ((line = reader.readLine()) != null)
                    buffer.append(line).append("\n");
                return buffer.toString();


            } catch (MalformedURLException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }finally {
                if(connection != null)
                    connection.disconnect();
                try {
                    if(reader != null)
                        reader.close();
                }catch (IOException e){
                    e.printStackTrace();
                }
            }
            return null;
        }
        @SuppressLint("SetTextI18n")
        @Override
        protected void onPostExecute(String result){
            super.onPostExecute(result);
            try {
                JSONObject jsonObject = new JSONObject(result);
                textView2.setText("Температура: " + jsonObject.getJSONObject("main").getDouble("temp"));
                textView3.setText("Ощущаеться: " + jsonObject.getJSONObject("main").getDouble("feels_like"));
                textView4.setText("Влажность: " + jsonObject.getJSONObject("main").getDouble("humidity")+"%");
                textView5.setText("Порыв ветра: " + jsonObject.getJSONObject("wind").getDouble("gust")+"м/c");
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }
    }
}