2. First Steps


Let's get started as simple as possible. We are going to demonstrate how to store, retrieve, update and delete instances of a single class that only contains primitive and String members. In our example this will be a Formula One (F1) pilot whose attributes are his name and the F1 points he has already gained this season.

First we create a class to hold our data. It looks like this:

package com.db4odoc.f1.chapter1;
public class Pilot {    
    private String name;
    private int points;  
    
    public Pilot(String name,int points) {
        this.name=name;
        this.points=points;
    }
        
    public int getPoints() {
        return points;
    }
    
    public void addPoints(int points) {
        this.points+=points;
    }
    
    public String getName() {
        return name;
    }
    
    public String toString() {
        return name+"/"+points;
    }
}


Notice that this class does not contain any db4o-related code.


    2.1. Opening the database


    To access a db4o database file or create a new one, call Db4oEmbedded.openFile() and provide Db4oEmbedded.newConfiguration() as a configuration template and the path to your database file as the second parameter, to obtain an ObjectContainer instance. ObjectContainer represents "The Database", and will be your primary interface to db4o. Closing the ObjectContainer with the #close() method will close the database file and release all resources associated with it.

    // accessDb4o
    ObjectContainer db = Db4oEmbedded.openFile(Db4oEmbedded
            .newConfiguration(), DB4OFILENAME);
    try {
        // do something with db4o
    } finally {
        db.close();
    }


    DB4OFILENAME  is just a string value representing any filename. If the file with this name already exists, it will be opened as db4o database, otherwise a new db4o database will be created.
    For the following examples we will assume that our environment takes care of opening and closing the ObjectContainer automagically, and stores the reference in a variable named 'db'.


    2.2. Storing objects


    To store an object, we simply call #store() on our database, passing any object as a parameter.

    // storeFirstPilot
    Pilot pilot1 = new Pilot("Michael Schumacher", 100);
    db.store(pilot1);
    System.out.println("Stored " + pilot1);


    We'll need a second pilot, too.

    // storeSecondPilot
    Pilot pilot2 = new Pilot("Rubens Barrichello", 99);
    db.store(pilot2);
    System.out.println("Stored " + pilot2);



    2.3. Retrieving objects


    The easiest way to see the content of our database is to use Object Manager Enterprise, which will be introduced in the next chapter . For now let's continue with the API overview and learn how to build db4o queries.
    db4o supplies several different querying systems, Query by Example (QBE), , Native Queries (NQ) and the SODA Query API (SODA). In this first example we will introduce QBE. Once you are familiar with storing objects, we encourage you to use Native Queries .

    When using Query-By-Example, you create a prototypical object for db4o to use as an example of what you wish to retrieve. db4o will retrieve all objects of the given type that contain the same (non-default) field values as the example. The results will be returned as an  ObjectSet instance. We will use a convenience method #listResult() to display the contents of our result ObjectSet :

    public static void listResult(List<?> result){
        System.out.println(result.size());
        for (Object o : result) {
            System.out.println(o);
        }
    }


    To retrieve all pilots from our database, we provide an 'empty' prototype:

    // retrieveAllPilotQBE
    Pilot proto = new Pilot(null, 0);
    ObjectSet result = db.queryByExample(proto);
    listResult(result);


    Note that we specify 0 points, but our results were not constrained to only those Pilots with 0 points; 0 is the default value for int fields.

    db4o also supplies a shortcut to retrieve all instances of a class:

    // retrieveAllPilots
    ObjectSet result = db.queryByExample(Pilot.class);
    listResult(result);


    For JDK 5  there also is a generics shortcut, using the query method:

    List <Pilot> pilots = db.query(Pilot.class);



    To query for a pilot by name:

    // retrievePilotByName
    Pilot proto = new Pilot("Michael Schumacher", 0);
    ObjectSet result = db.queryByExample(proto);
    listResult(result);


    And to query for Pilots with a specific number of points:

    // retrievePilotByExactPoints
    Pilot proto = new Pilot(null, 100);
    ObjectSet result = db.queryByExample(proto);
    listResult(result);



    Of course there's much more to db4o queries. They will be covered in more depth in later chapters.


    2.4. Updating objects


    Updating objects is just as easy as storing them. In fact, you use the same #store() method to update your objects: just call #store() again after modifying any object.

    // updatePilot
    ObjectSet result = db
            .queryByExample(new Pilot("Michael Schumacher", 0));
    Pilot found = (Pilot) result.next();
    found.addPoints(11);
    db.store(found);
    System.out.println("Added 11 points for " + found);
    retrieveAllPilots(db);


    Notice that we query for the object first. This is an importaint point. When you call #store() to modify a stored object, if the object is not 'known' (having been previously stored or retrieved during the current session), db4o will insert a new object. db4o does this because it does not automatically match up objects to be stored, with objects previously stored. It assumes you are inserting a second object which happens to have the same field values.

    To make sure you've updated the pilot, please return to any of the retrieval examples above and run them again.


    2.5. Deleting objects


    Objects are removed from the database using the #delete() method.

    // deleteFirstPilotByName
    ObjectSet result = db
            .queryByExample(new Pilot("Michael Schumacher", 0));
    Pilot found = (Pilot) result.next();
    db.delete(found);
    System.out.println("Deleted " + found);
    retrieveAllPilots(db);


    Let's delete the other one, too.

    // deleteSecondPilotByName
    ObjectSet result = db
            .queryByExample(new Pilot("Rubens Barrichello", 0));
    Pilot found = (Pilot) result.next();
    db.delete(found);
    System.out.println("Deleted " + found);
    retrieveAllPilots(db);


    Please check the deletion with the retrieval examples above.

    As with updating objects, the object to be deleted has to be 'known' to db4o. It is not sufficient to provide a prototype object with the same field values.


    2.6. Conclusion


    That was easy, wasn't it? We have stored, retrieved, updated and deleted objects with a few lines of code. Now you are probably interested to see how the database looks like. Let's have a look using db4o graphical tool - Object Manager in the next chapter .


    2.7. Full source


    package com.db4odoc.f1.chapter1;
    import java.io.*;
    import com.db4o.*;
    import com.db4odoc.f1.*;
    public class FirstStepsExample extends Util {
        final static String DB4OFILENAME = System.getProperty("user.home")
                + "/formula1.db4o";
        public static void main(String[] args) {
            new File(DB4OFILENAME).delete();
            accessDb4o();
            new File(DB4OFILENAME).delete();
            ObjectContainer db = Db4oEmbedded.openFile(Db4oEmbedded
                    .newConfiguration(), DB4OFILENAME);
            try {
                storeFirstPilot(db);
                storeSecondPilot(db);
                retrieveAllPilots(db);
                retrievePilotByName(db);
                retrievePilotByExactPoints(db);
                updatePilot(db);
                deleteFirstPilotByName(db);
                deleteSecondPilotByName(db);
            } finally {
                db.close();
            }
        }
        public static void accessDb4o() {
            ObjectContainer db = Db4oEmbedded.openFile(Db4oEmbedded
                    .newConfiguration(), DB4OFILENAME);
            try {
                // do something with db4o
            } finally {
                db.close();
            }
        }
        public static void storeFirstPilot(ObjectContainer db) {
            Pilot pilot1 = new Pilot("Michael Schumacher", 100);
            db.store(pilot1);
            System.out.println("Stored " + pilot1);
        }
        public static void storeSecondPilot(ObjectContainer db) {
            Pilot pilot2 = new Pilot("Rubens Barrichello", 99);
            db.store(pilot2);
            System.out.println("Stored " + pilot2);
        }
        public static void retrieveAllPilotQBE(ObjectContainer db) {
            Pilot proto = new Pilot(null, 0);
            ObjectSet result = db.queryByExample(proto);
            listResult(result);
        }
        public static void retrieveAllPilots(ObjectContainer db) {
            ObjectSet result = db.queryByExample(Pilot.class);
            listResult(result);
        }
        public static void retrievePilotByName(ObjectContainer db) {
            Pilot proto = new Pilot("Michael Schumacher", 0);
            ObjectSet result = db.queryByExample(proto);
            listResult(result);
        }
        public static void retrievePilotByExactPoints(ObjectContainer db) {
            Pilot proto = new Pilot(null, 100);
            ObjectSet result = db.queryByExample(proto);
            listResult(result);
        }
        public static void updatePilot(ObjectContainer db) {
            ObjectSet result = db
                    .queryByExample(new Pilot("Michael Schumacher", 0));
            Pilot found = (Pilot) result.next();
            found.addPoints(11);
            db.store(found);
            System.out.println("Added 11 points for " + found);
            retrieveAllPilots(db);
        }
        public static void deleteFirstPilotByName(ObjectContainer db) {
            ObjectSet result = db
                    .queryByExample(new Pilot("Michael Schumacher", 0));
            Pilot found = (Pilot) result.next();
            db.delete(found);
            System.out.println("Deleted " + found);
            retrieveAllPilots(db);
        }
        public static void deleteSecondPilotByName(ObjectContainer db) {
            ObjectSet result = db
                    .queryByExample(new Pilot("Rubens Barrichello", 0));
            Pilot found = (Pilot) result.next();
            db.delete(found);
            System.out.println("Deleted " + found);
            retrieveAllPilots(db);
        }
    }