Apex Test Class Skill

Core Principles

1. Bulkify tests - Always test with 200+ records to catch governor limit issues
2. Isolate test data - Use @TestSetup and TestDataFactory; never rely on org data
3. Assert meaningfully - Test behavior, not just coverage; include failure messages
4. Mock external dependencies - Use HttpCalloutMock, Test.setMock() for integrations
5. Test negative paths - Validate error handling, not just happy paths

Test Class Structure

@IsTest
private class MyServiceTest {