The Idea behind unit testing is to verify that each “separate” unit of your software is working as expected. Tests that verify combined components at the same time are generally called integration tests.

Unit tests and integration tests both enhance the overall quality of production code but the reason of existing of each type of tests is different.

When writing a test case, you should validate only one “thing” at a time. This way you will be focused only on that “thing” and when the test fails, you can address the problem without confusion or too much investigation.

Example​

In the following example, the SortUtils class offers a few methods to sort arrays. The test case testSortUtils() is bad because it verifies three methods at once.

@Test
void testSortUtils(){

    final int [] NUMBERS_ARRAY = {10, 1, 5 , -5, 0};
    final int [] SORTED_ARRAY = {-5, 0, 1, 5, 10};

    int [] quickSortResult = SortUtils.quicksort(NUMBERS_ARRAY);
    int [] bubbleSortResult = SortUtils.bubbleSort(NUMBERS_ARRAY);
    int [] insertionSortResult = SortUtils.insertionSort(NUMBERS_ARRAY);

    assertArrayEquals(SORTED_ARRAY, quickSortResult);
    assertArrayEquals(SORTED_ARRAY, bubbleSortResult);
    assertArrayEquals(SORTED_ARRAY, insertionSortResult);
    
}

Improved example​

The following example is better than the last one, because each test cases verifies one sorting method at a time. Of course, these are minimal examples and in a real test class more cases should be covered.

final static int [] NUMBERS_ARRAY = {10, 1, 5 , -5, 0};
final static int [] SORTED_ARRAY  = {-5, 0, 1, 5, 10};

void quickSort_Should_Return_Sorted_Array(){
    int [] quickSortResult = SortUtils.quicksort(NUMBERS_ARRAY);
    assertArrayEquals(SORTED_ARRAY, quickSortResult);    
}

void bubbleSort_Should_Return_Sorted_Array(){
    int [] bubbleSortResult = SortUtils.quicksort(NUMBERS_ARRAY);
    assertArrayEquals(SORTED_ARRAY, bubbleSortResult);
}

void insertionSort_Should_Return_Sorted_Array(){
    int [] insertionSortResult = SortUtils.quicksort(NUMBERS_ARRAY);
    assertArrayEquals(SORTED_ARRAY, insertionSortResult);
}