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8.1 Development testing 215

3 11
4 7 10

Less than 4 Between 4 and 10 More than 10
Number of Input Values

9999 100000
10000 50000 99999

Less than 10000 Between 10000 and 99999 More than 99999
Figure 8.6 Equivalence
partitions Input Values

You identify partitions by using the program specification or user documentation
and from experience where you predict the classes of input value that are likely to
detect errors. For example, say a program specification states that the program
accepts 4 to 8 inputs which are five-digit integers greater than 10,000. You use this
information to identify the input partitions and possible test input values. These are
shown in Figure 8.6.
When you use the specification of a system to identify equivalence partitions, this
is called ‘black-box testing’. Here, you don’t need any knowledge of how the system
works. However, it may be helpful to supplement the black-box tests with ‘white-
box testing’, where you look at the code of the program to find other possible tests.
For example, your code may include exceptions to handle incorrect inputs. You can
use this knowledge to identify ‘exception partitions’—different ranges where the
same exception handling should be applied.
Equivalence partitioning is an effective approach to testing because it helps
account for errors that programmers often make when processing inputs at the edges
of partitions. You can also use testing guidelines to help choose test cases.
Guidelines encapsulate knowledge of what kinds of test cases are effective for dis-
covering errors. For example, when you are testing programs with sequences, arrays,
or lists, guidelines that could help reveal defects include:

1. Test software with sequences that have only a single value. Programmers natu-
rally think of sequences as made up of several values and sometimes they embed
this assumption in their programs. Consequently, if presented with a single-
value sequence, a program may not work properly.
2. Use different sequences of different sizes in different tests. This decreases the
chances that a program with defects will accidentally produce a correct output
because of some accidental characteristics of the input.
3. Derive tests so that the first, middle, and last elements of the sequence are
accessed. This approach is reveals problems at partition boundaries.