Root classes

Overview

Root classes are top level classes that generalize all other classes.

The "Object" class

The "Object" class is the basic class of all the classes present in the J language. All classes, including basic classes (integer, String, etc.), specialize the "Object" class.

The ... operator	corresponds to ...
==	reference equality
!=	reference inequality

The Object[ ] class

The Object[] class represents a set of objects, whatever their nature. It specializes "Object".

A constructor exists with the set type written "SetOf" that can be associated to any J class. The type built in this way specializes Object[]. It is thus possible to handle String[] or Attribute[].

Note: Sets whose elements are sets are not currently implemented. For information on set handling services, please see "Messages to sets (spreading)".

The "Metaclass" class

J classes, either primitive classes like "int" or UML metaclasses such as "Association", are all instances of the "Metaclass" class. This means that without knowing which element is being handled, it is possible to find out its "Metaclass", and then to discover its metaclass name, followed by its parent metaclasses.

All the classes present in the J language are instances of the "Metaclass" class.

Having access to the "meta metamodel" means that more generic processing can be carried out using the J language. This mechanism is similar to the Java introspection mechanism.

Changes to the metamodel in J are not permitted (only changes to the model). Introspection services are, therefore, limited.

ClassOf

MetaClass Object: ClassOf()

"ClassOf" returns an object's metaclass.

Name

String MetaClass: Name()

"Name" returns the metaclass name.

String Object: metaclassName()

Shortcut to obtain the metaclass name of an element.

Symbolic access to metamodel features

boolean Object:getFeature (in String feature, out Object value)

Returns the value of "feature" in "value" for the current object. "feature" is either an attribute, or a dependency with a destination class. The return is true if "feature" is defined on the current object.

Example: We are going to use "getFeature" to recover the name and features of a class:

String name;

Feature[] features;

Object o;

this.getFeature("Name", o);

name ?= o;

getFeature("PartFeature", o);

features ?= o;

ParentOf

MetaClass MetaClass: ParentOf() return MetaClass

"ParentOf" returns the parent metaclass of the current class or returns empty if no parent class exists.

Example:

In this example, we are going to display the name of the metaclass and the name of the parent metaclass of any object.

Note: This example can be run with the "dump" command on any model element ("Object").

Object:dump()

{

MetaClass Mclass = ClassOf();

StdOut.write("Metaclass : ", Mclass.Name, NL);

Mclass.ParentOf()

{

StdOut.write("Parent metaclass: ", Name, NL);

}

}