import java.nio.charset.StandardCharsets;
import java.util.List;
import org.bson.Document;
import com.mongodb.MongoClientSettings;
import com.mongodb.MongoCommandException;
import com.mongodb.MongoCredential;
import com.mongodb.ServerAddress;
import com.mongodb.client.MongoClient;
import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoCollection;
public class CreateInvalidUTF8 {
    
    public static void main(String[] args) {
        String fname = "Firstname";
        String lname = "🐎🐎🐎🐎";
        
        MongoClient mongoCli = MongoClients.create(MongoClientSettings.builder()
                .applyToClusterSettings(builder ->
                   builder
                       .hosts(List.of(new ServerAddress(args[0]))))
                    .credential(MongoCredential.createCredential(args[1], "admin", args[2].toCharArray()))
            .build());        MongoCollection<Document> coll = mongoCli.getDatabase("test_utf8").getCollection("foo");
        MongoCollection<Document> coll2 = mongoCli.getDatabase("test_utf8").getCollection("bar");
        
        Document d = new Document();
        // This is a common mistake in Java code that takes user input.
        // But it could also be a String created from any byte array 
        // containing random sequences that cannot be encoded
        d.put("name", fname + " " + lname.charAt(0));
        coll.insertOne(d);
        
        // fails with MongoCommandException "text contains invalid UTF-8"
        try {
            coll.createIndex(new Document("name", "text"));
        } catch (MongoCommandException e) {
            e.printStackTrace();
        }
        
        Document d2 = new Document();
        // String.getBytes(UTF_8) validates UTF-8 
        // and substitutes valid replacement character so the UTF-8 is valid 
        d2.put("name", new String((fname + " " + lname.charAt(0)).getBytes(StandardCharsets.UTF_8), StandardCharsets.UTF_8));
        coll2.insertOne(d2);
        
        // succeeds
        coll2.createIndex(new Document("name", "text"));
        
    }
    
}

db.baz.insertOne({name:('Firstname ' + '🐎🐎🐎🐎'.substring(0,1))})
db.baz.createIndex({name:'text'})