Many errors when trying to make a tutorial where a database is created: can not resolve, invalid method declaration and Incompatible types

I am using Android Studio 3, I try to follow a YouTube tutorial, this is the code: WarningDBAdapter.java

    package com.example.jhon.cursoandroidstudio;

import android.content.Context;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.provider.ContactsContract;
import android.util.Log;
import android.database.SQLException;


public class AvisoDBAdapter {
    //Estos son los nombres de las columnas
    public static final String COL_ID = "_id";
    public static final String COL_CONTENT = "content";
    public static final String COL_IMPORTANT = "important";

    //Estos son los índices correspondientes
    public static final int INDEX_ID = 0;
    public static final int INDEX_CONTENT = INDEX_ID + 1;
    public static final int INDEX_IMPORTANT = INDEX_ID + 2;

    //Usado for logging
    private static final String TAG = "AvisosDBAdapter";

    private DababaseHelper mDbHelper;
    private SQLiteDatabase mDb;

    private static final String DATABASE_NAME = "dba_remdrs";
    private static final String TABLE_NAME = "tbl_remdrs";
    private static final int DATABASE_VERSION = 1;

    private final Context mCtx;

    //Declaración SQL usada para crear la base de datos
    private static final String DATABASE_CREATE =
            "CREATE TABLE if not exists " + TABLE_NAME + " ( " +
                    COL_ID + "INTEGER PRIMARY KEY autoincrement, " +
                    COL_CONTENT + " TEXT, " +
                    COL_IMPORTANT + " INTEGER );";

    public AvisosDBAdapter(Context ctx){
        this.mCtx = ctx;
    }

    //abrir
    public void open()throws SQLException{
        mDbHelper = new DatabaseHelper(mCtx);
        mDb = mDbHelper.getWritableDatabase();
    }

    //cerrar
    public void close(){
        if(mDbHelper != null){
            mDbHelper.close();
        }
    }

    private static class DatabaseHelper extends SQLiteOpenHelper {
        DatabaseHelper(Context context){
            super(context, DATABASE_NAME, null, DATABASE_VERSION);
        }

        @Override
        public void onCreate(SQLiteDatabase db){
            Log.w(TAG, DATABASE_CREATE);
            db.execSQL(DATABASE_CREATE);
        }

        @Override
        public  void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion){
            Log.w(TAG, "Upgrading database from version " + oldVersion + " to " + newVersion + ", wich will destroy all " +
                    "old data");
            db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);
            onCreate(db);

        }
    }
}

List of errors and where they appear:

 public AvisosDBAdapter(Context ctx){ Invalid method declaration, return type required
 mDbHelper = new DatabaseHelper(mCtx); Incompatible types Required: DatabaseHelper
mDb = mDbHelper.getWritableDatabase(); cannot resolve method getWritableDatabase
mDbHelper.close(); cannot resolve method close

Warning.java

package com.example.jhon.cursoandroidstudio;

public class Aviso {
    private int mId;
    private String mContent;
    private int mImportant;

    public Aviso(int id, String content, int important){
        mId = id;
        mImportant = important;
        mContent = content;
    }

    public  int getId(){
        return mId;
    }

    public void setId(int id){
        mId = id;
    }

    public  int getImportant(){
        return mImportant;
    }

    public void setImportant(int important){
        mImportant = important;
    }

    public  String getContent(){
        return mContent;
    }

    public void setContent(String content){
        mContent = content;
    }
}

I do not know if the one who made the tutorial used an old version of Android Studio or what ... I checked the code so that it was how he put it ... please tell me what changes I should make. Thanks in advance.