4.2.2 Advanced principles

From Geostandards

Contents 1 Visibility of attributes 2 DataType 3 Enumerations 4 Class attributes 5 Direction of association 6 Aggregation or composition 7 Exclusivity {XOR} 8 Association class 9 Organization in packages

Visibility of attributes

Visibility of attributes can be defined to what extent they are known by external objects. This is what is meant by the scope or visibility. UML has four categories with separate types of notation:

• + public: The attribute is visible for each class that references this class
• -private: The attribute is only visible for the class itself
• #protected: The attribute is only visible for this class and its subclasses
• /derived: The attribute can be derived from the rest of the model. This makes the attribute redundant.
• ~ package: Attribute is visible for all other classes within the same package For the domain modeling in the field of geo information, as for instance seen in the Inspire data specifications, the most commonly used scope is public. The means the less restricted option.

UML Notation:

DataType

Datatypes are used to associate attributes with defined value formats or types. Typical datatypes are:

• Character,
• Integer,
• Boolean,
• Date.

The ISO19107 Spatial schema defines amongst others the datatypes for geometry. Examples are:

• GM_Surface,
• GM_Point,
• GM_Curve,
• GM_MultiPoint.

When defining geometry types these datatypes should be used. Datatypes are also often created by the modeler. A common use is to define complex datatypes that are a combination of several related attributes.

Enumerations

Enumerations are types of classes that contain domain values. For instance the enumeration ‘VegetationType’ can contain the defined values: forest, grass, heather. Enumerations show lists of which a value has to be chosen. The list is not extensible. CodeList are used when lists are supposed to be extensible. In the example, for the vegetationType the options are limited to ‘forest’, ‘grass’ or ‘heather’. For the types of animal ‘bear’, ‘monkey’, ‘bird’ or any other suggestion can be added.

Class attributes

Class attributes are used to specify properties of a class in stead of instances. In that respect can the class be considered as an object. The class attribute can have only one value. Class attributes are recognizable as underlined attributes.

Direction of association

Associations generally are not directed. Objects at both sides of an association are referenced to each other. However it can be necessary to specify that one class can find the other but not visa versa. For instance apartments might be associated to a apartment building and the building not to its separate apartments.

Aggregation or composition

Aggregation and composition are both special types of associations.
The aggregation specifies that one or more classes are part of another class. What the part of relation exactly implies is not specified. The composition is more strict. It is used when an object (or instance) can only be part of exactly one other object. Furthermore is there a lifetime dependency, the ‘part of object’ can never outlive the composed object. The aggregation therefore gives more freedom in use, but also provides less information and can easily be substituted by a normal association. Aggregations are therefore much avoided.

When using compositions one has to be sure that the lifetime dependency really exists. This often can be debated. An engine in car in to some has a lifetime dependency. When the car is gone the engine is gone. To others the engine can easily lead a life of its own. Two examples are given by the figure below.

Exclusivity {XOR}

When modeling associations or generalization-specialization relations it sometimes is needed to explicitly state that the relation is exclusive. This exclusivity constraint can be modeled using the {XOR} constraint. It indicates that only one of the relations pointed at is true at any one time. Two examples:

• An address is either a Visiting address or a Postbox it cannot be both at the same time.
• An inhabitant of a building is either a renter or an owner.

Association class

Organization in packages

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