Step 3: Add a Main Class
In this step we will add a main class to the project with code that will store Point objects in the database and then retrieve them from the database:
- Open the [New Java Class] dialog box by right clicking the tutorial package node
(in the [Package Explorer] window) and selecting New > Class.
- The package name should be tutorial.
- Enter Main as the class name - use exactly that case sensitive class name.
- Click Finish to create the new class.
Use copy and paste to fill the new source file with the following content:
package tutorial;
import javax.persistence.*;
import java.util.*;
public class Main {
public static void main(String[] args) {
// Open a database connection
// (create a new database if it doesn't exist yet):
EntityManagerFactory emf =
Persistence.createEntityManagerFactory("$objectdb/db/points.odb");
EntityManager em = emf.createEntityManager();
// Store 1000 Point objects in the database:
em.getTransaction().begin();
for (int i = 0; i < 1000; i++) {
Point p = new Point(i, i);
em.persist(p);
}
em.getTransaction().commit();
// Find the number of Point objects in the database:
Query q1 = em.createQuery("SELECT COUNT(p) FROM Point p");
System.out.println("Total Points: " + q1.getSingleResult());
// Find the average X value:
Query q2 = em.createQuery("SELECT AVG(p.x) FROM Point p");
System.out.println("Average X: " + q2.getSingleResult());
// Retrieve all the Point objects from the database:
TypedQuery<Point> query =
em.createQuery("SELECT p FROM Point p", Point.class);
List<Point> results = query.getResultList();
for (Point p : results) {
System.out.println(p);
}
// Close the database connection:
em.close();
emf.close();
}
}
The main method creates an ObjectDB database file, stores 1000 Point objects in the database, and then run a few queries, including a query that retrieves all the 1000 Point objects from the database. Finally it closes the database. A detailed explanation of the code is provided in the Quick Tour chapter of the ObjectDB Manual.
The next step (and the last in this tutorial) is running the project.
