MainActivity.java

package io.bennofs.androidplayground;

import android.arch.persistence.room.ColumnInfo;
import android.arch.persistence.room.Dao;
import android.arch.persistence.room.Database;
import android.arch.persistence.room.Delete;
import android.arch.persistence.room.Entity;
import android.arch.persistence.room.Insert;
import android.arch.persistence.room.PrimaryKey;
import android.arch.persistence.room.Query;
import android.arch.persistence.room.Room;
import android.arch.persistence.room.RoomDatabase;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

import java.util.List;

import static android.arch.persistence.room.OnConflictStrategy.REPLACE;

@Entity
class User {
    @PrimaryKey
    private int uid;

    @ColumnInfo(name = "first_name")
    private String firstName;

    @ColumnInfo(name = "last_name")
    private String lastName;

    public User(int uid, String firstName, String lastName) {
        this.uid = uid;
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public int getUid() {
        return uid;
    }

    public void setUid(int uid) {
        this.uid = uid;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    @Override
    public String toString() {
        return "User{" +
                "uid=" + uid +
                ", firstName='" + firstName + '\'' +
                ", lastName='" + lastName + '\'' +
                '}';
    }
}

@Dao
interface UserDao {
    @Query("SELECT * FROM user")
    List<User> getAll();

    @Query("SELECT * FROM user WHERE uid IN (:userIds)")
    List<User> loadAllByIds(int[] userIds);

    @Query("SELECT * FROM user WHERE first_name LIKE :first AND "
            + "last_name LIKE :last LIMIT 1")
    User findByName(String first, String last);

    @Insert(onConflict = REPLACE)
    void insertAll(User... users);

    @Delete
    void delete(User user);
}

@Database(entities = {User.class}, version = 1)
abstract class AppDatabase extends RoomDatabase {
    public abstract UserDao userDao();
}


public class MainActivity extends AppCompatActivity {

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

        final AppDatabase db = Room.databaseBuilder(getApplicationContext(),
                AppDatabase.class, "database-name").build();

        new Thread(new Runnable() {
            @Override
            public void run() {
                db.userDao().insertAll(new User(1, null, "lastname"));
                System.out.println("first name: " + db.userDao().getAll().get(0).getFirstName());

            }
        }).start();
    }
}