How to create Test Case from Use Case document

Use cases are based on the Unified Modeling Language (UML) and can be visually represented in use-case diagrams. Figure 1 shows a use-case diagram depicting requirements for a university course registration system

Figure 1: Use Case Diagram for a University Course Registration System

The ovals represent use cases, and the stick figures represent "actors," which can be either humans or other systems. The lines represent communication between an actor and a use case. As you can see, this use case diagram provides the big picture: Each use case represents a big chunk of functionality that will be implemented, and each actor represents someone or something outside our system that interacts with it. It is a significant step to identify use cases and actors, but now there is more to be done. Each use case also requires a significant amount of text to describe it. This text is usually formatted in sections, as shown in



















  

Table 1: Format for a Use-Case Textual Description

Use Case Section	Description
Name	An appropriate name for the use case (see Leslee Probasco’s article in the March issue of The Rational Edge).
Brief Description	A brief description of the use case’s role and purpose.
Flow Event	A textual description of what the system does with regard to the use case (not how specific problems are solved by the system). The description should be understandable to the customer.
Spacial Requirment	A textual description that collects all requirements, such as non-functional requirements, on the use case, that are not considered in the use-case model, but that need to be taken care of during design or implementation.
Precondition	A textual description that defines any constraints on the system at the time the use case may start.
Post Condition	A textual description that defines any constraints on the system at the time the use case will terminate.

The most important part of a use case for generating test cases is the flow of events. The two main parts of the flow of events are the basic flow of events and the alternate flows of events. The basic flow of events should cover what "normally" happens when the use case is performed. The alternate flows of events cover behavior of an optional or exceptional character relative to normal behavior, and also variations of the normal behavior. You can think of the alternate flows of events as "detours" from the basic flow of events.

Figure 2: Basic Flow of Events and Alternate Flows of Events for a Use Case

Figure 3: Textual Description for the University Course Registration Use-Case Basic Flow of Events.

Register For Courses

Basic Flow

1. Logon this use case starts when a Student accesses the Wylie University Web site. The system asks for, and the Student enters, the student ID and password.

2. Select 'Create a Schedule' the system displays the functions available to the student. The student selects "Create a Schedule."

3. Obtain Course Information The system retrieves a list of available course offerings from the Course Catalog System and displays the list to the Student.

4. Select Courses the Student selects four primary course offerings and two alternate course offerings from the list of available course offerings.

5. Submit Schedule The student indicates that the schedule is complete. For each selected course offering on the schedule, the system verifies that the Student has the necessary prerequisites.

6. Display Completed Schedule The system displays the schedule containing the selected course offerings for the Student and the confirmation number for the schedule.

Figure 4: Textual Description for University Course Registration Use-Case Alternate Flows

Register For Courses

Alternate Flows

1. Unidentified Student in Step 1 of the Basic Flow, Logon, if the system determines that the student ID and/or password is not valid, an error message is displayed.

2. Quit the Course Registration System allows the student to quit at any time during the use case. The Student may choose to save a partial schedule before quitting. All courses that are not marked as "enrolled in" are marked as "selected" in the schedule. The schedule is saved in the system. The use case ends.

3. Unfulfilled Prerequisites, Course Full, or Schedule Conflicts In Step 5 of the Basic Flow, Submit Schedule, if the system determines that prerequisites for a selected course are not satisfied, that the course is full, or that there are schedule conflicts, the system will not enroll the student in the course. A message is displayed that the student can select a different course. The use case continues at Step 4, Select Courses, in the basic flow.

4. Course Catalog System Unavailable In Step 3 of the Basic Flow, Obtain Course Information, if the system is down, a message is displayed and the use case ends.

5. Course Registration Closed If, when the use case starts, it is determined that registration has been closed, a message is displayed, and the use case ends.

As you can see, a significant amount of detail goes into fully specifying a use case. Ideally, the flows should be written as "dialogs" between the system and the actors. Each step should explain what the actor does and what the system does in response; it should also be numbered and have a title. Alternate flows always specify where they start in the basic flow and where they go when they end.

Use-Case Scenarios: There is one more thing to describe before we concentrate on how use cases can be used to generate test cases: a use-case scenario. A use-case scenario is an instance of a use case, or a complete "path" through the use case. End users of the completed system can go down many paths as they execute the functionality specified in the use case. Following the basic flow would be one scenario. Following the basic flow plus alternate flow 1A would be another. The basic flow plus alternate flow 2A would be a third, and so on. Table 2 lists all possible scenarios for the diagram shown in Figure 2, beginning with the basic flow and then combining the basic flow with alternate flows.

Table 2: Scenarios for the Use Case Shown in Figure 2

Scenario   1	Basic Flow
Scenario 2	Basic Flow	Alternet Flow 1
Scenario 3	Basic Flow	Alternet Flow 1	Alternet Flow 2
Scenario 4	Basic Flow	Alternet Flow 3
Scenario 5	Basic Flow	Alternet Flow 3	Alternet Flow 1
Scenario 6	Basic Flow	Alternet Flow 3	Alternet Flow 1	Alternet Flow 2
Scenario 7	Basic Flow	Alternet Flow 4
Scenario 8	Basic Flow	Alternet Flow 3	Alternet Flow 4

These scenarios will be used as the basis for creating test cases.

Generating Test Cases

A test case is a set of test inputs, execution conditions, and expected results developed for a particular objective: to exercise a particular program path or verify compliance with a specific requirement, for example.

The purpose of a test case is to identify and communicate conditions that will be implemented in test. Test cases are necessary to verify successful and acceptable implementation of the product requirements (use cases). We will describe a three-step process for generating test cases from a fully detailed

Use case:

1. For each use case, generate a full set of use-case scenarios.

2. For each scenario, identify at least one test case and the conditions that will make it "execute."

3. For each test case, identify the data values with which to test.

Step One: Generate Scenarios

Read the use-case textual description and identify each combination of main and alternate flows -- the scenarios -- and create a scenario matrix. Table 3 shows a partial scenario matrix for the Register for Courses use case. This is a simple example with no nested alternate flows.

Table 3: Partial Scenario Matrix for the Register for Courses Use Case

Scenatio Name	Starting Flow	Alternate
Scenario 1 - Successful registration	Basic Flow
Scenario 2 - Unidentified student	Basic Flow	A1
Scenario 3 - User quits	Basic Flow	A2
Scenario 4 - Course catalog system unavailable	Basic Flow	A4
Scenario 5 - Registration closed	Basic Flow	A5
Scenario 6 – Cannot enroll	Basic Flow	A3

Step Two: Identify Test Cases

Once the full set of scenarios has been identified, the next step is to identify the test cases. We can do this by analyzing the scenarios and reviewing the use case textual description as well. There should be at least one test case for each scenario, but there will probably be more. For example, if the textual description for an alternate flow is written in a very cursory way, like the description below, 3A. Unfulfilled Prerequisites, Course Full, or Schedule Conflicts then additional test cases may be required to test all the possibilities. In addition, we may wish to add test cases to test boundary conditions. The next step in fleshing out the test cases is to reread the use-case textual description and find the conditions or data elements required to execute the various scenarios. For the Register for Course use case, conditions would be student ID, password, courses selected, etc. To clearly document the test cases, once again, a matrix format is useful, like the one in Table 4. Notice the top row. The first column contains the test case ID, the second column has a brief description of the test case, including the scenario being tested, and all other columns except the last one contain data elements that will be used in implementing the tests. The last column contains a description of the test case's expected output.

Table 4: Test Case Matrix for the Register for Courses Use Case

Test Case ID	Scenario/ Condition	Student ID	Password	Courses selected	Prerequisites fulfilled	Course Open	Schedule Open	Expected Result
RC 1	Scenario 1- successful registration	V	V	V	V	V	V	Schedule and confirmation number displayed
RC 2	Scenario 2- unidentified student	I	N/A	N/A	N/A	N/A	N/A	Error message; back to login screen
RC 3	Scenario 3- valid user quits	V	V	N/A	N/A	N/A	N/A	Login screen appears
RC 4	Scenario 3- valid user quits	V	V	N/A	N/A	N/A	N/A	Error message; back to step 2
RC 5	Scenario 4- course registration system unavailable	V	V	N/A	N/A	N/A	N/A	Error message; back to step 2
RC 6	Scenario 5- registration closed	V	V	V	V	I	V	Error message; back to step 3
RC 7	Scenario 6- cannot enroll -- course full	V	V	V	I	V	V	Error message; back to step 4
RC 8	Scenario 6- cannot enroll -- prerequisite not fulfilled	V	V	V	V	V	I	Error message; back to step 4

Notice that in this matrix no data values have actually been entered. The cells of the table contain a V, I, or n/a. V indicates valid, I is for invalid, and n/a means that it is not necessary to supply a data value in this case. This specific matrix is a good intermediate step; it clearly shows what conditions are being tested for each test case. It is also very easy to determine by looking at the Vs and Is whether you have identified a sufficient number of test cases. In addition to the "happy day" scenarios in which everything works fine, each row in the matrix should have at least one I indicating an invalid condition being tested. In the test case matrix in Table 4, some conditions are obviously missing -- e.g., Registration Closed -- because RC3, RC4, and RC5 each has the same combination of Is and Vs.