25.11. Parameterized Tests

JUnit allows testing a method with multiple sets of parameters.

Parameterized tests in JUnit enable a single test method to be executed multiple times with different inputs. This helps in eliminating redundant test code while still allowing thorough testing of a method's logic under various conditions. Instead of writing separate test methods for each input scenario, you define the method once and specify the varied inputs it should test against. This is particularly useful when the same logic is validated with several sets of data, making your tests easier to manage and understand.

Think of parameterized tests like a recipe that can be made with different ingredients. For example, if you're testing a cake recipe that can have chocolate, vanilla, or lemon flavors, instead of writing separate instructions for each flavor, you can create a single set of instructions (the test) and just change the ingredient (the parameter) for each one. This makes it easier to see how the recipe holds up no matter which flavor you choose.

@ParameterizedTest @ValueSource(ints = {1, 2, 3}) void testEvenNumbers(int number) { assertTrue(number % 2 != 0); }

In this example, we use the @ParameterizedTest annotation to define a test method, testEvenNumbers, that will be run three times—once for each value in the array {1, 2, 3}. The @ValueSource(ints = {1, 2, 3}) annotation is used to provide the different integer parameters to the test. Inside the test, we assert that the number is odd using assertTrue(number % 2 != 0), which means the test will pass if the number is indeed odd. However, since 1 and 3 are odd while 2 is even, this test will fail for the input 2. This highlights how parameterized tests can help confirm logic over a range of input values efficiently.

Consider a health screening process that checks if a person's blood pressure reading is within normal limits. Instead of running separate tests for various readings, a parameterized test could use a range of sample data (like different blood pressure readings). The nurse could apply the same logic to check if each reading is 'normal' or 'high' without having to write separate tests for each reading.

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