<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="arquivo.estudos"
      android:versionCode="1"
      android:versionName="1.0">
    <application android:label="@string/app_name" >
        <activity android:name="EstudosActivity"
......

Spinner
  android:id="@+id/cidade"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:layout_marginTop="10dp"
......

<string-array name="cidades_nomes">
  <item>Goiânia</item>
  <item>Cuiabá</item>
  <item>São Paulo</item>
......

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="
  http://schemas.android.com/apk/res/android"
  android:orientation="vertical"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent">
  tools:context=".MainActivity">

  <Button xmlns:android="
......

package com.example.estudosandroid;

import androidx.appcompat.app.AppCompatActivity;
import android.content.DialogInterface;
import android.os.Bundle;
import android.view.View;
import android.app.AlertDialog;

public class MainActivity extends AppCompatActivity {
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
  }

  // método público que será chamado ao clicarmos no botão
  public void enviarMensagem(View view) {
......

java.lang.Object 
......

package arquivo.estudos;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

public class EstudosActivity extends Activity
{
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
......

<Button 
    android:id="@+id/btn"
    android:layout_height="wrap_content"     
......