@Database(entities = {Word.class}, version = 2)
public abstract class AppDatabase extends RoomDatabase {
private static AppDatabase sInstance;

public abstract WordDao wordDao();

public static AppDatabase getInstance(Context context) {
    if (sInstance == null) {
        synchronized (AppDatabase.class) {
            if (sInstance == null) {
                sInstance = Room.databaseBuilder(context.getApplicationContext(),
                        AppDatabase.class, "word.db").addMigrations(new Migration(1, 2) {
                    @Override
                    public void migrate(@NonNull SupportSQLiteDatabase database) {
                    }
                }).build();
            }
        }
    }

    return sInstance;
}

@Entity
public class Word {
@PrimaryKey(autoGenerate = true)
public int uid;

public String word;

public Word(String word) {
    this.word = word;
}

@Dao
public interface WordDao {
@Query("Select * from word")
LiveData<List<Word>> getAll();

@Update
void update(Word word);

@Insert
void insert(Word word);

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

    TextView tv = findViewById(R.id.textview);

    WordDao dao = AppDatabase.getInstance(this).wordDao();

    dao.getAll().observeForever(wordlist -> {
        StringBuilder builder = new StringBuilder();

        for (Word w : wordlist ) {
            builder.append(w.word).append(", ");
        }

        tv.setText(builder.toString());
    });

    findViewById(R.id.add).setOnClickListener(view -> new Thread(() -> {
        dao.insert(new Word("haha"));
    }).start());
}

dependencies {
    def lifecycle_version = "1.1.1"
    implementation "android.arch.lifecycle:extensions:$lifecycle_version"
    implementation "android.arch.lifecycle:common-java8:$lifecycle_version"
    testImplementation "android.arch.core:core-testing:$lifecycle_version"

    def room_version = "1.1.1"
    implementation "android.arch.persistence.room:runtime:$room_version"
    annotationProcessor "android.arch.persistence.room:compiler:$room_version"
    testImplementation "android.arch.persistence.room:testing:$room_version"
}