CoreDomainTest: This is the name of this test suite. By convention, test suite classes end with the word Test. It’s not uncommon for them to end with UnitTest for unit tests, IntegrationTest for integration tests, and any other qualifiers.

@Test: This JUnit annotation signals that this method is a test case. Be sure to use the org.junit.jupiter.api version of @Test, and not the org.junit version. The former package is JUnit 5, while the latter package is JUnit 4 (both are on the classpath to support backward compatibility).

newVideoEntityShouldHaveNullId: The name of the test method is important as it should convey the gist of what it verifies. This isn’t a technical requirement but instead an opportunity to capture information. This method verifies that when we create a new instance of VideoEntity, its id field should be null.

The first line of the method creates an instance of VideoEntity.

assertThat(): An AssertJ static helper method that takes a value and verifies it with a collection of clauses.

isNull(): This verifies that the id field is null.

isEqualTo(): This verifies that the various fields are equal to their expected values.