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Black Box Testing

Black box testing uncovers following types of errors.

1. Incorrect or missing functions

2. Interface errors

3. External database access

4. Performance errors

5. Initialization and termination errors.

The following techniques are employed during black box testing

Equivalence Partitioning

In equivalence partitioning, a test case is designed so as to uncover a group or class of error. This limits the number of test cases that might need to be developed otherwise.

Here input domain is divided into classes or group of data. These classes are known as equivalence classes and the process of making equivalence classes is called equivalence partitioning. Equivalence classes represent a set of valid or invalid states for input condition.

An input condition can be a range, a specific value, a set of values, or a boolean value. Then depending upon type of input equivalence classes is defined. For defining equivalence classes the following guidelines should be used.

1. If an input condition specifies a range, one valid and two invalid equivalence classes are defined.

2. If an input condition requires a specific value, then one valid and two invalid equivalence classes are defined.

3. If an input condition specifies a member of a set, then one valid and one invalid equivalence class are defined.

4. If an input condition is Boolean, then one valid and one invalid equivalence class are defined.

For example, the range is say, 0 < count < Max1000. Then form a valid equivalence class with that range of values and two invalid equivalence classes, one with values less than the lower bound of range (i.e., count < 0) and other with values higher than the higher bound( count > 1000).

Boundary Value Analysis

It has been observed that programs that work correctly for a set of values in an equivalence class fail on some special values. These values often lie on the boundary of the equivalence class. Test cases that have values on the boundaries of equivalence classes are therefore likely to be error producing so selecting such test cases for those boundaries is the aim of boundary value analysis.

In boundary value analysis, we choose input for a test case from an equivalence class, such that the input lies at the edge of the equivalence classes. Boundary values for each equivalence class, including the equivalence classes of the output, should be covered. Boundary value test cases are also called “extreme cases”.

Hence, a boundary value test case is a set of input data that lies on the edge or boundary of a class of input data or that generates output that lies at the boundary of a class of output data.

In case of ranges, for boundary value analysis it is useful to select boundary elements of the range and an invalid value just beyond the two ends (for the two invalid equivalence classes. For example, if the range is 0.0 <= x <= 1.0, then the test cases are 0.0,1.0for valid inputs and –0.1 and 1.1 for invalid inputs.

For boundary value analysis, the following guidelines should be used:

For input ranges bounded by a and b, test cases should include values a and b and just above and just below a and b respectively.

If an input condition specifies a number of values, test cases should be developed to exercise the minimum and maximum numbers and values just above and below these limits.

If internal data structures have prescribed boundaries, a test case should be designed to exercise the data structure at its boundary.

Now we know how the testing for software product is done. But testing software is not an easy task since the size of software developed for the various systems is often too big. Testing needs a specific systematic procedure, which should guide the tester in performing different tests at correct time. This systematic procedure is testing strategies, which should be followed in order to test the system developed thoroughly. Performing testing without some testing strategy would be very cumbersome and difficult. Testing strategies are discussed the following pages of this chapter.

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