A very important concept in object-oriented design, inheritance, refers to the ability of one class (child class) to inherit the identical functionality of another class (super class), and then add new functionality of its own. (In a very non-technical sense, imagine that I inherited my mother's general musical abilities, but in my family I'm the only one who plays electric guitar.) To model inheritance on a class diagram, a solid line is drawn from the child class (the class inheriting the behavior) with a closed, unfilled arrowhead (or triangle) pointing to the super class. Consider types of bank accounts
Reflexive Associations
A class can also be associated with itself, using a reflexive association. This may not make sense at first, but remember that classes are abstractions. Figure 14 shows how an Employee class could be related to itself through the manager/manages role. When a class is associated to itself, this does not mean that a class's instance is related to itself, but that an instance of the class is related to another instance of the class.
‘Extend’ and ‘Include’ in use cases
‘Extend’ and ‘Include’ define relationships between use cases. Below figure ‘Extend and Include’ shows how these two fundamentals are implemented in a project. The below use case represents a system which is used to maintain customer. When a customer is added successfully it should send an email to the admin saying that a new customer is added. Only admin have rights to modify the customer. First lets define extend and include and then see how the same fits in this use case scenario.
‘Extend’ and ‘Include’ define relationships between use cases. Below figure ‘Extend and Include’ shows how these two fundamentals are implemented in a project. The below use case represents a system which is used to maintain customer. When a customer is added successfully it should send an email to the admin saying that a new customer is added. Only admin have rights to modify the customer. First lets define extend and include and then see how the same fits in this use case scenario.
Include: - Include relationship represents an invocation of one use case by the other. If you think from the coding perspective its like one function been called by the other function.
Extend: - This relationship signifies that the extending use case will work exactly like the base use case only that some new steps will inserted in the extended use case.
Below figure ‘Extend and Include’ shows that ‘add customer’ is same as the ‘add discounted customer’. The ‘Add discounted customer’ has an extra process, to define discount for the discounted customer which is not available for the simple customer. One of the requirements of the project was that when we add a customer, the system should send an email. So after the customer is added either through ‘Add simple customer’ use case or ‘Add discounted customer’ use case it should invoke ‘send a email’ use case. So we have defined the same with a simple dotted line with <> as the relationship.
Associations in Class diagrams
A single Class cannot represent the whole module in a project so we need one or more classes to represent a module. For instance, a module named ‘customer detail’ cannot be completed by the customer class alone , to complete the whole module we need customer class, address class, phone class in short there is relationship between the classes. So by grouping and relating between the classes we create module and these are termed as Association. In order to associate them we need to draw the arrowed lines between the classes as shown in the below figure.
In the figure ‘Order is paid by payments class’, we can see Order class and the Payment class and arrowed line showing relationship that the order class is paid using payment class in other words order class is going to be used by payment class to pay the order. The left to right marked arrow basically shows the flow that order class uses the payment class.In case payment class using the order class then the marked arrow should be right to left showing the direction of the flow.
Aggregation and composition in class diagrams
In this Association there are two types mainly Aggregation Association and Composition Association.
Aggregation Association signifies that the whole object can exist without the Aggregated Object. For example in the below figure we have three classes university class, department class and the Professor Class. The university cannot exist without department which means that university will be closed as the department is closed. In other words lifetime of the university depend on the lifetime of department.
In the same figure we have defined second Association between the department and the Professor. In this case, if the professor leaves the department still the department continues in other words department is not dependent on the professor this is called as Composition Association.
Note: - The filled diamond represents the aggregation and the empty diamond represents the composition. You can see the figure below for more details.
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