Cannot be resolved or is not a field

Mobile Android
#1

Estou sem computador, mas descobri um aplicativo que lhe permite programar aplicativos (um aplicativo para fazer aplicativos), mas, estou tendo estes erros:

3kMtLSs
3kMtLSs.png720×1280 86.9 KB 

MainActivity.java
package com.pixelfight;

import android.app.*;
import android.widget.*;
import android.view.*;
import android.os.*;

import com.pixelfight.R;

public class MainActivity extends Activity {

    protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);

         setContentView(R.layout.linear_layout);

         final Button button = findViewById(R.id.button_id);
         button.setOnClickListener(new View.OnClickListener() {
             public void onClick(View v) {
                 // Code here executes on main thread after user presses button
             }
         });
     }

}

activity_main.xml
        <?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/linear_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <TextView
        android:id="@+id/txt_hello"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:text="Olá, Mundo!" />
    <Button
    android:id="@+id/btnAddARoom"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Teste" />
</LinearLayout>
#2

Tem certeza que esse aplicativo está atualizando os membros da classe R quando você altera o activity_main.xml?