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<body><div id="pagecontainer"><table><tr><td width="5"> </td><td><a name="Inheritance"></a><br>
<a name="outline183"></a><br><h1>7. Inheritance</h1><br>
So far we have always been working with the concrete (i.e. most specific type of an object. What about subclassing and interfaces?<br>
<br>
To explore this, we will differentiate between different kinds of sensors.<br>
<br>
<table width="100%" cellpadding="3" cellspacing="0" border="0"><tr><td class="lg">
<code>using System;<br>
namespace Db4odoc.Tutorial.F1.Chapter5<br>
{ <br>
public class SensorReadout<br>
{<br>
DateTime _time;<br>
Car _car;<br>
string _description;<br>
<br>
public SensorReadout(DateTime time, Car car, string description)<br>
{<br>
_time = time;<br>
_car = car;<br>
_description = description;<br>
}<br>
<br>
public Car Car<br>
{<br>
get<br>
{<br>
return _car;<br>
}<br>
}<br>
<br>
public DateTime Time<br>
{<br>
get<br>
{<br>
return _time;<br>
}<br>
}<br>
<br>
public string Description<br>
{<br>
get<br>
{<br>
return _description;<br>
}<br>
}<br>
<br>
override public string ToString()<br>
{<br>
return string.Format("{0}:{1}:{2}", _car, _time, _description);<br>
}<br>
}<br>
}<br>
</code></td></tr></table>
<br>
<br>
<table width="100%" cellpadding="3" cellspacing="0" border="0"><tr><td class="lg">
<code>using System;<br>
namespace Db4odoc.Tutorial.F1.Chapter5<br>
{ <br>
public class TemperatureSensorReadout : SensorReadout<br>
{<br>
double _temperature;<br>
public TemperatureSensorReadout(DateTime time, Car car, string description, double temperature)<br>
: base(time, car, description)<br>
{<br>
_temperature = temperature;<br>
}<br>
<br>
public double Temperature<br>
{<br>
get<br>
{<br>
return _temperature;<br>
}<br>
}<br>
<br>
override public string ToString()<br>
{<br>
return string.Format("{0} temp: {1}", base.ToString(), _temperature);<br>
}<br>
}<br>
}<br>
</code></td></tr></table>
<br>
<br>
<table width="100%" cellpadding="3" cellspacing="0" border="0"><tr><td class="lg">
<code>using System;<br>
namespace Db4odoc.Tutorial.F1.Chapter5<br>
{<br>
public class PressureSensorReadout : SensorReadout<br>
{<br>
double _pressure;<br>
<br>
public PressureSensorReadout(DateTime time, Car car, string description, double pressure)<br>
: base(time, car, description)<br>
{<br>
_pressure = pressure;<br>
}<br>
<br>
public double Pressure<br>
{<br>
get<br>
{<br>
return _pressure;<br>
}<br>
}<br>
<br>
override public string ToString()<br>
{<br>
return string.Format("{0} pressure: {1}", base.ToString(), _pressure);<br>
}<br>
}<br>
}<br>
</code></td></tr></table>
<br>
<br>
Our car's snapshot mechanism is changed accordingly.<br>
<br>
<table width="100%" cellpadding="3" cellspacing="0" border="0"><tr><td class="lg">
<code>using System;<br>
using System.Collections;<br>
namespace Db4odoc.Tutorial.F1.Chapter5<br>
{ <br>
public class Car<br>
{<br>
string _model;<br>
Pilot _pilot;<br>
IList _history;<br>
<br>
public Car(string model)<br>
{<br>
_model = model;<br>
_pilot = null;<br>
_history = new ArrayList();<br>
}<br>
<br>
public Pilot Pilot<br>
{<br>
get<br>
{<br>
return _pilot;<br>
}<br>
<br>
set<br>
{<br>
_pilot = value;<br>
}<br>
}<br>
<br>
public string Model<br>
{<br>
get<br>
{<br>
return _model;<br>
}<br>
}<br>
<br>
public SensorReadout[] GetHistory()<br>
{<br>
SensorReadout[] history = new SensorReadout[_history.Count];<br>
_history.CopyTo(history, 0);<br>
return history;<br>
}<br>
<br>
public void Snapshot()<br>
{<br>
_history.Add(new TemperatureSensorReadout(DateTime.Now, this, "oil", PollOilTemperature()));<br>
_history.Add(new TemperatureSensorReadout(DateTime.Now, this, "water", PollWaterTemperature()));<br>
_history.Add(new PressureSensorReadout(DateTime.Now, this, "oil", PollOilPressure()));<br>
}<br>
<br>
protected double PollOilTemperature()<br>
{<br>
return 0.1*_history.Count;<br>
}<br>
<br>
protected double PollWaterTemperature()<br>
{<br>
return 0.2*_history.Count;<br>
}<br>
<br>
protected double PollOilPressure()<br>
{<br>
return 0.3*_history.Count;<br>
}<br>
<br>
override public string ToString()<br>
{<br>
return string.Format("{0}[{1}]/{2}", _model, _pilot, _history.Count);<br>
}<br>
}<br>
}<br>
</code></td></tr></table>
<br>
<br>
<ul>
<a name="outline184"></a><br><h2>7.1. Storing</h2><br>
Our setup code has not changed at all, just the internal workings of a snapshot.<br>
<br>
<table width="100%" cellpadding="3" cellspacing="0" border="0"><tr><td class="lg">
<code>// storeFirstCar<br>
Car car1 = new Car("Ferrari");<br>
Pilot pilot1 = new Pilot("Michael Schumacher", 100);<br>
car1.Pilot = pilot1;<br>
db.Store(car1);</code></td><td class="lg" align="left" valign="bottom" width=43><input type='button' class='button' value='Run' onclick='window.external.RunExample("com.db4odoc.f1.chapter5.InheritanceExample", "storeFirstCar")' /></td></tr></table>
<br>
<br>
<table width="100%" cellpadding="3" cellspacing="0" border="0"><tr><td class="lg">
<code>// storeSecondCar<br>
Pilot pilot2 = new Pilot("Rubens Barrichello", 99);<br>
Car car2 = new Car("BMW");<br>
car2.Pilot = pilot2;<br>
car2.Snapshot();<br>
car2.Snapshot();<br>
db.Store(car2);</code></td><td class="lg" align="left" valign="bottom" width=43><input type='button' class='button' value='Run' onclick='window.external.RunExample("com.db4odoc.f1.chapter5.InheritanceExample", "storeSecondCar")' /></td></tr></table>
<br>
<br>
<a name="outline185"></a><br><h2>7.2. Retrieving</h2><br>
db4o will provide us with all objects of the given type. To collect all instances of a given class, no matter whether they are subclass members or direct instances, we just provide a corresponding prototype.<br>
<br>
<table width="100%" cellpadding="3" cellspacing="0" border="0"><tr><td class="lg">
<code>// retrieveTemperatureReadoutsQBE<br>
SensorReadout proto = new TemperatureSensorReadout(DateTime.MinValue, null, null, 0.0);<br>
IObjectSet result = db.QueryByExample(proto);<br>
ListResult(result);</code></td><td class="lg" align="left" valign="bottom" width=43><input type='button' class='button' value='Run' onclick='window.external.RunExample("com.db4odoc.f1.chapter5.InheritanceExample", "retrieveTemperatureReadoutsQBE")' /></td></tr></table>
<br>
<br>
<table width="100%" cellpadding="3" cellspacing="0" border="0"><tr><td class="lg">
<code>// retrieveAllSensorReadoutsQBE<br>
SensorReadout proto = new SensorReadout(DateTime.MinValue, null, null);<br>
IObjectSet result = db.QueryByExample(proto);<br>
ListResult(result);</code></td><td class="lg" align="left" valign="bottom" width=43><input type='button' class='button' value='Run' onclick='window.external.RunExample("com.db4odoc.f1.chapter5.InheritanceExample", "retrieveAllSensorReadoutsQBE")' /></td></tr></table>
<br>
<br>
This is one more situation where QBE might not be applicable: What if the given type is an interface or an abstract class? Well, there's a little trick to keep in mind: Type objects receive special handling with QBE.<br>
<br>
<table width="100%" cellpadding="3" cellspacing="0" border="0"><tr><td class="lg">
<code>// retrieveAllSensorReadoutsQBEAlternative<br>
IObjectSet result = db.QueryByExample(typeof(SensorReadout));<br>
ListResult(result);</code></td><td class="lg" align="left" valign="bottom" width=43><input type='button' class='button' value='Run' onclick='window.external.RunExample("com.db4odoc.f1.chapter5.InheritanceExample", "retrieveAllSensorReadoutsQBEAlternative")' /></td></tr></table>
<br>
<br>
And of course there's our SODA API:<br>
<br>
<table width="100%" cellpadding="3" cellspacing="0" border="0"><tr><td class="lg">
<code>// retrieveAllSensorReadoutsQuery<br>
IQuery query = db.Query();<br>
query.Constrain(typeof(SensorReadout));<br>
IObjectSet result = query.Execute();<br>
ListResult(result);</code></td><td class="lg" align="left" valign="bottom" width=43><input type='button' class='button' value='Run' onclick='window.external.RunExample("com.db4odoc.f1.chapter5.InheritanceExample", "retrieveAllSensorReadoutsQuery")' /></td></tr></table>
<br>
<br>
<a name="outline186"></a><br><h2>7.3. Updating and deleting</h2><br>
is just the same for all objects, no matter where they are situated in the inheritance tree.<br>
<br>
Just like we retrieved all objects from the database above, we can delete all stored objects to prepare for the next chapter.<br>
<br>
<table width="100%" cellpadding="3" cellspacing="0" border="0"><tr><td class="lg">
<code>// deleteAll<br>
IObjectSet result = db.QueryByExample(typeof(Object));<br>
foreach (object item in result)<br>
{<br>
db.Delete(item);<br>
}</code></td><td class="lg" align="left" valign="bottom" width=43><input type='button' class='button' value='Run' onclick='window.external.RunExample("com.db4odoc.f1.Util", "deleteAll")' /></td></tr></table>
<br>
<br>
<a name="outline187"></a><br><h2>7.4. Conclusion</h2><br>
Now we have covered all basic OO features and the way they are handled by db4o. We will complete the first part of our db4o walkthrough in the <a href="Deep.html#Deep">next chapter </a> by looking at deep object graphs, including recursive structures.<br>
<br>
<a name="outline188"></a><br><h2>7.5. Full source</h2><br>
<table width="100%" cellpadding="3" cellspacing="0" border="0"><tr><td class="lg">
<code>using System;<br>
using System.IO;<br>
using Db4objects.Db4o;<br>
using Db4objects.Db4o.Query;<br>
namespace Db4odoc.Tutorial.F1.Chapter5<br>
{ <br>
public class InheritanceExample : Util<br>
{<br>
readonly static string YapFileName = Path.Combine(<br>
Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),<br>
"formula1.yap"); <br>
<br>
public static void Main(string[] args)<br>
{<br>
File.Delete(YapFileName);<br>
using(IObjectContainer db = Db4oEmbedded.OpenFile(YapFileName))<br>
{<br>
StoreFirstCar(db);<br>
StoreSecondCar(db);<br>
RetrieveTemperatureReadoutsQBE(db);<br>
RetrieveAllSensorReadoutsQBE(db);<br>
RetrieveAllSensorReadoutsQBEAlternative(db);<br>
RetrieveAllSensorReadoutsQuery(db);<br>
RetrieveAllObjects(db);<br>
}<br>
}<br>
<br>
public static void StoreFirstCar(IObjectContainer db)<br>
{<br>
Car car1 = new Car("Ferrari");<br>
Pilot pilot1 = new Pilot("Michael Schumacher", 100);<br>
car1.Pilot = pilot1;<br>
db.Store(car1);<br>
}<br>
<br>
public static void StoreSecondCar(IObjectContainer db)<br>
{<br>
Pilot pilot2 = new Pilot("Rubens Barrichello", 99);<br>
Car car2 = new Car("BMW");<br>
car2.Pilot = pilot2;<br>
car2.Snapshot();<br>
car2.Snapshot();<br>
db.Store(car2);<br>
}<br>
<br>
public static void RetrieveAllSensorReadoutsQBE(IObjectContainer db)<br>
{<br>
SensorReadout proto = new SensorReadout(DateTime.MinValue, null, null);<br>
IObjectSet result = db.QueryByExample(proto);<br>
ListResult(result);<br>
}<br>
<br>
public static void RetrieveTemperatureReadoutsQBE(IObjectContainer db)<br>
{<br>
SensorReadout proto = new TemperatureSensorReadout(DateTime.MinValue, null, null, 0.0);<br>
IObjectSet result = db.QueryByExample(proto);<br>
ListResult(result);<br>
}<br>
<br>
public static void RetrieveAllSensorReadoutsQBEAlternative(IObjectContainer db)<br>
{<br>
IObjectSet result = db.QueryByExample(typeof(SensorReadout));<br>
ListResult(result);<br>
}<br>
<br>
public static void RetrieveAllSensorReadoutsQuery(IObjectContainer db)<br>
{<br>
IQuery query = db.Query();<br>
query.Constrain(typeof(SensorReadout));<br>
IObjectSet result = query.Execute();<br>
ListResult(result);<br>
}<br>
<br>
public static void RetrieveAllObjects(IObjectContainer db)<br>
{<br>
IObjectSet result = db.QueryByExample(new object());<br>
ListResult(result);<br>
}<br>
}<br>
}<br>
</code></td></tr></table>
<br>
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