Creating Test Data

@isTest
static void testSomething() {
    List<Account> accounts = [SELECT Id FROM Account]; // returns EMPTY, even in a full production org
}

1. @isTest — marks the test method.
2. static void testSomething() { — the test.
3. List<Account> accounts = [SELECT Id FROM Account]; — by design, Salesforce automatically isolates every test's data — this query never sees real, existing records, even if the org has millions of Account records.
4. } — closes the test method.

@isTest
private class EmployeeDirectoryTest {
    @isTest
    static void addEmployeeStoresTheRecord() {
        EmployeeDirectory directory = new EmployeeDirectory();
        Employee emp = new Employee('E001', 'Amara Nkosi', 'amara@example.com');

        Boolean added = directory.addEmployee(emp);

        System.assertEquals(true, added);
        System.assertEquals(1, directory.getEmployeeCount());
    }
}

1. @isTest — marks the class.
2. private class EmployeeDirectoryTest { — the class.
3. @isTest — marks the test method.
4. static void addEmployeeStoresTheRecord() { — the test.
5. EmployeeDirectory directory = new EmployeeDirectory(); — since EmployeeDirectory (Module 17) is a plain Apex class, not an sObject, this test just creates it directly with new.
6. Employee emp = new Employee('E001', 'Amara Nkosi', 'amara@example.com'); — same for Employee — no DML or SOQL involved at all.
7. (blank line)
8. Boolean added = directory.addEmployee(emp); — runs the code being tested.
9. (blank line)
10. System.assertEquals(true, added); — asserts the add succeeded.
11. System.assertEquals(1, directory.getEmployeeCount()); — asserts the count reflects it.
12. } — closes the test method.
13. } — closes the class.

@isTest
private class LibraryServiceTest {
    @isTest
    static void addBookInsertsARecord() {
        LibraryService library = new LibraryService();

        Book__c book = library.addBook('The Long Walk', 'Richard Bachman', '978-0-451-14098-6');

        System.assertNotEquals(null, book.Id);

        List<Book__c> allBooks = [SELECT Id, Title__c FROM Book__c];
        System.assertEquals(1, allBooks.size());
    }
}

1. @isTest — marks the class.
2. private class LibraryServiceTest { — the class.
3. @isTest — marks the test method.
4. static void addBookInsertsARecord() { — the test.
5. LibraryService library = new LibraryService(); — creates the service.
6. (blank line)
7. Book__c book = library.addBook('The Long Walk', 'Richard Bachman', '978-0-451-14098-6'); — performs a real insert (Module 22).
8. (blank line)
9. System.assertNotEquals(null, book.Id); — confirms it was actually saved.
10. (blank line)
11. List<Book__c> allBooks = [SELECT Id, Title__c FROM Book__c]; — because tests run in their own isolated data, this confidently finds exactly the one book this test itself created.
12. System.assertEquals(1, allBooks.size()); — never any leftover data from another test or from production.
13. } — closes the test method.
14. } — closes the class.