Equivalence classes and boundary values
Choosing test data without testing everything: splitting the inputs into valid and invalid equivalence classes, then aiming at the values at each class's boundaries, just before, on and just after.
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Equivalence class: what is it?
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A set of inputs that the program must handle in the same way, so that a single one of them is enough to stand for it in the tests.
Testing 40 and 41 when the class is "between 26 and 60" teaches nothing more: if one passes, so does the other, unless the code has a splitting flaw.
Delivery is free for a basket strictly above €50, in whole euros. Which pair of values straddles the boundary?
- €50 (charged) and €51 (free)
- €49 (charged) and €50 (free)
- €50 (free) and €51 (free)
- €0 (charged) and €100 (free)
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€50 (charged) and €51 (free)
"Strictly above" excludes 50: the last charged value is 50 and the first free one is 51. Thinking 50 is free means reading
>=instead of>; 0 and 100 are far from the boundary and let that mistake through.For each boundary, one generally tests the value …, the boundary itself and the value ….
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For each boundary, one generally tests the value just before, the boundary itself and the value just after.
These three points cover the
<for<=mistakes,>for>=and off-by-one shifts.Invalid class: what is it?
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A set of inputs that the program must reject or flag, because they fall outside what it accepts.
For a month from 1 to 12, "too small" (0 and below), "too large" (13 and above) and "not a number" are three distinct invalid classes, each to be tested.
This function must return true from age 18. Which test case fails with this code?
```java
static boolean estMajeur(int age) {
return age > 18;
}`- age = 17, expected result: false
- age = 18, expected result: true
- age = 30, expected result: true
- age = 19, expected result: true
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age = 18, expected result: true
With
> 18, 18 is rejected: only the test on the exact boundary sees it. 17, 19 and 30 give the right result with>as with>=.