Naked Objects - Creating persistent objects

Naked Objects
By Richard Pawson and Robert Matthews

Enriching object behaviours

Creating persistent objects

Because of the way that objects are persisted within the framework Java's new operator should not be used to create new naked objects. Instead, create a new persistent object using the utility method createInstance provided by the org.nakedobjects.object.AbstractNakedObject superclass. Like most factory methods this returns an instance using a superclass reference - in this case org.nakedobjects.object.NakedObject - and so must be cast into its real type before it can be used. This method takes a java.lang.Class object, which specifies what class of naked object to create. The example below creates a Location object:

Location home;
home = (Location) createInstance(Location.class);

Sometimes it is necessary to create non-persistent (or 'transient') naked objects, either for use outside the shared object space or as part of a longer creation process, where the user will place the object into the shared space later. A transient object can be created using the createTransientInstance utility method. This will return a properly initialized transient object. As with the createInstance method, this will need its reference to be cast to its real type. Note also that it is illegal (because it is illogical) for any persistent object to reference a transient object. Therefore, do not attempt to set up a field within a persistent object using this method.

Calling makePersistent on a transient object will make that object, and all the objects it contains, persistent. The following example shows a non-persistent object being created, and then being made persistent:

Location away;
away = (Location) createTransientInstance(Location.class);
    :
    :
away.makePersistent();

This following method is an actual object creation example taken from the City class:

public Location actionNewLocation() {
    Location loc = (Location) createInstance(Location.class);
    loc.setCity(this);
    return loc;
}

As these two creation methods are available you should avoid using the new operator. If, however, you have not subclassed org.nakedobjects.object.AbstractNakedObject then you will have to create an object this way. It then becomes your responsibility to call the created method, and if the object is to be persisted, to call the makePersistent method as well.