Software Engineering for Students A Programming Approach

second. So we devise two tests:
Test number Data Outcome
1 12 cannot vote
2 21 can vote
19.4 Black box (functional) testing 271
We have reasoned that we need two sets of test data to test this program. These two
sets, together with a statement of the expected outcomes from testing, constitute a test
specification. We run the program with the two sets of data and note any discrepancies
between predicted and actual outcome.
Unfortunately we can see that these tests have not investigated the important distinction
between someone aged 17 and someone aged 18. Anyone who has ever written a program
knows that using if statements is error prone, so it is advisable to investigate this particular
region of the data. This is the same as recognizing that data values at the edges of the
partitions are worthy of inclusion in the testing. Therefore we create two additional tests:
Test number Data Outcome
3 17 cannot vote
4 18 can vote
In summary, the rules for selecting test data for black box testing using equivalence
partitioning are:
1. partition the input data values
2. select representative data from each partition (equivalent data)
3. select data at the boundaries of partitions.
In the last program, there is a single input; there are four data values and therefore
four tests. However, most programs process a number of inputs. Suppose we
wish to test a program that displays the larger of two numbers, each in the range
0–10,000, entered into a pair of text boxes. If the values are equal, the program displays
either value.
Each input is within a partition that runs from 0 to 10,000. We choose values at
each end of the partitions and sample values somewhere in the middle:
first number: 0 54 10,000
second number: 0 142 10,000
Now that we have selected representative values, we need to consider what combinations
of values we should use. Exhaustive testing would mean using every possible
combination of every possible data value, but this is, of course, infeasible. Instead, we
use every combination of the representative values. So the tests are: