Class diagram
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In the Unified Modeling Language (UML), a class diagram is a type of static structure diagram that describes the structure of a system by showing the system's classes, their attributes, and the relationships between the classes.
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[edit] Classes
A class in the software system is represented by a box with the name of the class written inside it. A compartment below the class name can show the class's attributes (i.e. its properties). Each attribute is shown with at least its name, and optionally with its type, initial value, and other properties.
The class's operations (i.e. its methods) can appear in another compartment. Each operation is shown with at least its name, and optionally also with its parameters and return type.
Attributes and operations may have their visibility marked as follows:
- "+" for public
- "#" for protected
- "−" for private
- "~" for package
[edit] Relationships
UML shows the following relationships in a class diagram:
[edit] Types of Logical Connections
These are the types of logical connections found on a class diagram.
[edit] Dependency
No Data Available
[edit] Association
Where we have an A we also have a B.
[edit] Aggregation
Aggregation is a variant of the "has a" or association relationship; composition is more specific than aggregation.
Aggregation occurs when a class is a collection or container of other classes, but where the contained classes do not have a strong life cycle dependency on the container--essentially, if the container is destroyed, its contents are not.
In UML, it is graphically represented as a clear diamond shape on the containing class end of the tree of lines that connect contained class(es) to the containing class.
[edit] Composition
Composition is a variant of the "has a" or association relationship; composition is more specific than aggregation.
Composition indicates a strong life cycle dependency between instances of the container class and instances of the contained class(es): If the container is destroyed, any containees are also destroyed.
The UML graphical representation of a composition relationship is a black diamond shape on the containing class end of the tree of lines that connect containee classes to their containing class.
One way to remember the difference between these types is that aggregation is similar to a clear bottle and the black diamond is frequently associated with destruction.
- Whole – Part relationship
A is a part of B E.g. "an engine is part of a car", "flight segments are part of a flight journey"
[edit] Generalization
The generalization relationship indicates that one of the two related classes (the supertype) is considered to be a more general form of the other (the subtype). In practice, this means that any instance of the subtype is also an instance of the supertype (An exemplary tree of generalizations of this form is found in binomial nomenclature: human beings are a subtype of simian, which are a subtype of mammal, and so on). The relationship is most easily understood by the phrase 'A is a B' (a human is a mammal, a mammal is an animal).
The UML graphical representation of a Generalization is a hollow triangle shape on the supertype end of the line (or tree of lines) that connects it to one or more subtypes.
The generalization relationship is also known as the inheritance or "is a" relationship.
The supertype in the generalization relationship is also known as the "parent", superclass, base class, or base type.
The subtype in the generalization relationship is also known as the "child", subclass, derived class, derived type, inheriting class, or inheriting type.
Note that this relationship bears no resemblance to the biological parent/child relationship: the use of these terms is extremely common, but can be misleading.
- Generalization-Specialization relationship
A is a type of B E.g. "an oak is a type of tree", "a lorry is a type of vehicle"
[edit] Realization
In UML modeling, a realization relationship is a relationship between two model elements, in which one model element (the client) realizes the behavior that the other model element (the supplier) specifies.a realization is displayed in the diagram editor as a dashed line with an unfilled arrowhead
[edit] Multiplicity
The association relationship indicates that (at least) one of the two related classes makes reference to the other. In contrast with the generalization relationship, this is most easily understood through the phrase 'A has a B' (a mother cat has kittens, kittens have a mother cat).
The UML representation of an association is a line with an optional arrowhead indicating the role of the object(s) in the relationship, and an optional notation at each end indicating the multiplicity of instances of that entity (the number of objects that participate in the association). Common multiplicities are:
| 0..1 | No instances, or one instance |
| 1 | Exactly one instance |
| 0..* or * | Zero or more instances |
| 1..* | One or more instances |
The association relationship is also known as the "has a" relationship.
