Test Design

How to test and which test cases to use

When designing tests you determine how you are going to test. At the design stage, test conditions are used to create test cases (or sequences of test cases). Here, you will usually use one of the test techniques.

Test cases can be specified on two “levels”: abstract and concrete (see Case Study: Abstract and concrete test cases below).

Abstract and concrete test cases

An abstract (or “high-level”) test case doesn’t include specific input values or expected results, and is described using logical operators. The advantage of such cases is that they can be used during multiple test cycles and with varying data but can still adequately document the scope of each case. A concrete (or “low-level”) test case uses specific input data and expected result values.

When you begin to design a test, you can use abstract as well as concrete test cases. Because only concrete test cases can be executed on a computer, abstract test cases have to be fleshed out with real input and output values. In order to utilize the advantages of abstract test cases (see above), you can derive abstract test cases from concrete cases too.

Test cases involve more than just test data

Preconditions have to be defined for every test case. A test also requires clear constraints that must be adhered to. Furthermore, you need to establish in advance what results or behavior you expect the test to produce. In addition to output data, results include any changes to global (persistent) data and states, as well as any other reactions to the execution of the test case.

The test oracle

You need to use adequate sources to predict test results. In this context, people talk about the “test oracle” that has to be “consulted” regarding the expected results. For example, specifications can serve as an oracle. There are two basic approaches:

• The tester derives the expected output value from the input value based on the test object’s requirements and specifications as defined in the test basis.
• If the inverse of a function exists, you can execute the inverse and compare its output with the input value for your original test case. This technique can, for example, be used when testing encryption and decryption algorithms

The following example illustrates the difference between abstract and concrete test cases.