junit5-nested-test

Example of using nested tests in Junit5.

Reference: https://junit.org/junit5/docs/current/user-guide/#writing-tests-nested

preface

Nested tests give the test writer more capabilities to express the relationship among several group of tests.

• Only non-static nested classes (i.e. inner classes) can serve as @Nested test classes.
• Nesting can be arbitrarily deep
• @BeforeAll and @AfterAll methods cannot be defined in an @Nested class - Java does not allow static members in inner classes.

project description

It is a good practice to group tests of one method in an inner class:

1. suppose we want to test:

   class MathGroup {
       OptionalDouble average(Collection<Integer> ints) {
           if (isNull(ints)) {
               throw new IllegalArgumentException("collection cannot be null");
           }
   
           return ints.stream()
                   .filter(Objects::nonNull)
                   .mapToInt(x -> x)
                   .average();
       }
   }
   

2. method average(Collection<Integer> ints)
   • throws IllegalArgumentException if input is null
   • ignore nulls in non null collection
   • return OptionalDouble
3. we group tests in the inner class

    class NestedTests {
        
       // tests of other methods...
        
       @Nested
       @DisplayName("tests of average(Collection<Integer> ints)")
       class Average_collection {
           
           MathGroup mathGroup = new MathGroup();
           
           @Test
           void nullCollection() {
               assertThrows(IllegalArgumentException.class, () -> mathGroup.average(null));
           }
           
           @Test
           void emptyCollection() {
               assertTrue(mathGroup.average(Collections.emptyList()).isEmpty());
           }
           
           @Test
           @DisplayName("null elements are ignored")
           void collectionWithNullElement() {
               assertThat(mathGroup.average(Arrays.asList(1, null)), is(OptionalDouble.of(1)));
           }
           
           @Test
           void wellFormattedInput() {
               assertThat(mathGroup.average(Arrays.asList(1, 2, 3, 11)), is(OptionalDouble.of(4.25)));
           }
       }
       
       // tests of other methods...
   }
   

4. reports are well formatted:

   Test Results
       NestedTests
           tests of average(Collection<Integer> ints)
               wellFormattedInput()
               collectionWithNullElement()
               emptyCollection()
               nullCollection()