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 {

    @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();
    }
}

Naming Convention

Use descriptive method names: should[ExpectedBehavior]_When[Condition]

Examples:

• shouldCreateContact_WhenAccountIsActive
• shouldThrowException_WhenEmailIsInvalid
• shouldSendNotification_WhenOpportunityClosedWon
• shouldBypassTrigger_WhenRunningAsBatch

Test.startTest() / Test.stopTest()

Always wrap the code under test:

• Resets governor limits for accurate limit testing
• Executes async operations synchronously (queueables, batch, future)
• Fires scheduled jobs immediately

Reference Files

Detailed patterns for specific scenarios:

• references/test-data-factory.md - TestDataFactory class patterns and field defaults
• references/assertion-patterns.md - Assertion best practices and common pitfalls
• references/mocking-patterns.md - HttpCalloutMock, Test.setMock(), stubbing
• references/async-testing.md - Batch, Queueable, Future, Scheduled job testing

Quick Reference: What to Test

Component	Key Test Scenarios
Trigger	Bulk insert/update/delete, recursion, field changes
Service	Valid/invalid inputs, bulk operations, exceptions
Controller	Page load, action methods, view state
Batch	Start/execute/finish, chunking, error records
Queueable	Chaining, bulkification, error handling
Callout	Success response, error response, timeout
Scheduled	Execution, CRON validation