@IsTest
private class MyServiceTest {

    @TestSetup
    static void setupTestData() {
        // Create shared test data using TestDataFactory
        List<Account> accounts = TestDataFactory.createAccounts(200, true);
    }

    @IsTest
    static void shouldPerformExpectedBehavior_WhenValidInput() {
        // Given: Setup specific test state
        List<Account> accounts = [SELECT Id, Name FROM Account];
        
        // When: Execute the code under test
        Test.startTest();
        MyService.processAccounts(accounts);
        Test.stopTest();
        
        // Then: Assert expected outcomes
        List<Account> updated = [SELECT Id, Status__c FROM Account];
        System.Assert.areEqual(200, updated.size(), 'All accounts should be processed');
        for (Account acc : updated) {
            System.Assert.areEqual('Processed', acc.Status__c, 'Status should be updated');
        }
    }

    @IsTest
    static void shouldThrowException_WhenInvalidInput() {
        // Given
        List<Account> emptyList = new List<Account>();
        
        // When/Then
        Test.startTest();
        try {
            MyService.processAccounts(emptyList);
            System.Assert.fail('Expected MyCustomException to be thrown');
        } catch (MyCustomException e) {
            System.Assert.isTrue(e.getMessage().contains('cannot be empty'), 
                'Exception message should indicate empty input');
        }
        Test.stopTest();
    }
}