Software Testing

Packages and Object Diagrams

To simplify complex class diagrams, you can group classes into packages. A package is a collection of logically related UML elements. The diagram below is a business model in which the classes are grouped into packages.

Packages appear as rectangles with small tabs at the top. The package name is on the tab or inside the rectangle. The dotted arrows are dependencies. One package depends on another if changes in the other could possibly force changes in the first.

Object diagrams show instances instead of classes. They are useful for explaining small pieces with complicated relationships, especially recursive relationships.

This small class diagram shows that a university Department can contain lots of other Departments.

object diagram below instantiates the class diagram, replacing it by a concrete example.

Each rectangle in the object diagram corresponds to a single instance. Instance names are underlined in UML diagrams. Class or instance names may be omitted from object diagrams as long as the diagram meaning is still clear.

Class Diagram:

A Class diagram gives an overview of a system by showing its classes and the relationships among them. Class diagrams are static — they display what interacts but not what happens when they do interact.

The class diagrams below models a customer order from a retail catalog. The central class is the Order. Associated with it are the Customer making the purchase and the Payment. A Payment is one of three kinds: Cash, Check, or Credit. The order contains OrderDetails (line items), each with its associated Item.

UML class notation is a rectangle divided into three parts: class name, attributes, and operations. Names of abstract classes, such as Payment, are in italics. Relationships between classes are the connecting links.

Our class diagram has three kinds of relationships.

• Association — a relationship between instances of the two classes. There is an association between two classes if an instance of one class must know about the other in order to perform its work. In a diagram, an association is a link connecting two classes.
• Aggregation — an association in which one class belongs to a collection. An aggregation has a diamond end pointing to the part containing the whole. In our diagram, Order has a collection of OrderDetails.
• Generalization — an inheritance link indicating one class is a superclass of the other. A generalization has a triangle pointing to the superclass. Payment is a superclass of Cash, Check, and Credit.

An association has two ends. An end may have a role name to clarify the nature of the association. For example, an OrderDetail is a line item of each Order.

A navigability arrow on an association shows which direction the association can be traversed or queried. An OrderDetail can be queried about its Item, but not the other way around. The arrow also lets you know who “owns” the association’s implementation; in this case, OrderDetail has an Item. Associations with no navigability arrows are bi-directional.

The multiplicity of an association end is the number of possible instances of the class associated with a single instance of the other end. Multiplicities are single numbers or ranges of numbers. In our example, there can be only one Customer for each Order, but a Customer can have any number of Orders.

This table gives the most common multiplicities.

Every class diagram has classes, associations, and multiplicities. Navigability and roles are optional items placed in a diagram to provide clarity.

Use Case diagram:

Use case diagrams describe what a system does from the standpoint of an external observer. The emphasis is on what a system does rather than how

Use case diagrams are closely connected to scenarios. A scenario is an example of what happens when someone interacts with the system

Let’s take an example of a medical clinic:

“”A patient calls the clinic to make an appointment for a yearly checkup. The receptionist finds the nearest empty time slot in the appointment book and schedules the appointment for that time slot. “”

A use case is a summary of scenarios for a single task or goal. An actor is who or what initiates the events involved in that task. Actors are simply roles that people or objects play. The picture below is a Make Appointment use case for the medical clinic. The actor is a Patient. The connection between actor and use case is a communication association (or communication for short).

Actors are stick figures. Use cases are ovals. Communications are lines that link actors to use cases.

A use case diagram is a collection of actors, use cases, and their communications. We’ve put Make Appointment as part of a diagram with four actors and four use cases. Notice that a single use case can have multiple actors.

Use case diagrams are helpful in three areas.

• Determining features (requirements). New use cases often generate new requirements as the system is analyzed and the design takes shape.
• Communicating with clients. Their notational simplicity makes use case diagrams a good way for developers to communicate with clients.
• Generating test cases. The collection of scenarios for a use case may suggest a suite of test cases for those scenarios.

What Is UML?

The OMG specification states:

“The Unified Modeling Language (UML) is a graphical language for visualizing, specifying, constructing, and documenting the artifacts of a software-intensive system. The UML offers a standard way to write a system’s blueprints, including conceptual things such as business processes and system functions as well as concrete things such as programming language statements, database schemas, and reusable software components.”

In simple words:

UML (Unified Modeling Language):

• Is a graphical modeling language used to express designs.
• Is a standardized language used to specify artifacts and components of a software system.
• It describes notation not a process.
• It does not put forth a single method or process of design, but rather is a standardized tool that can be used in a design process.

There are nine kinds of UML diagrams. Let’s discuss them one by one with an example.