Integration testing validates that individual modules or components work together correctly when combined. It focuses on testing interfaces, data flow, and communication between integrated components rather than individual unit functionality.

Key Objectives:

• Interface Validation: Ensures proper data exchange between components
• Communication Testing: Verifies protocols and message formats
• Data Flow Verification: Confirms correct data transformation across boundaries
• Dependency Management: Tests how components handle shared resources
• End-to-End Functionality: Validates complete business processes

Why It's Critical:

• Detects integration-specific bugs not found in unit testing
• Validates system architecture and design decisions
• Ensures components meet integration contracts
• Provides confidence before system-level testing

Big Bang Integration Testing:

• All modules integrated simultaneously and tested as complete system
• Quick to implement but difficult to isolate defects
• Best for small systems with few integration points

Incremental Integration Testing:

• Modules integrated and tested one by one
• Better fault localization and easier debugging
• Requires more planning but provides better control

Top-Down Integration:

• Starts from top-level modules, progressively integrates lower levels
• Uses stubs to simulate lower-level modules not yet integrated
• Good for systems with well-defined hierarchical structure

Bottom-Up Integration:

• Starts from lowest-level modules, progressively builds upward
• Uses drivers to simulate higher-level modules
• Effective when lower-level modules are stable and reliable

Sandwich/Hybrid Integration:

• Combines top-down and bottom-up approaches
• Tests critical paths from both directions simultaneously
• Balances benefits of both approaches

Stubs:

• Dummy modules that simulate lower-level components
• Called by the module being tested (downward calls)
• Used in top-down integration testing
• Provide controlled responses to test upper-level logic

Drivers:

• Dummy modules that simulate higher-level components
• Call the module being tested (upward calls)
• Used in bottom-up integration testing
• Initiate test scenarios and validate responses

Example Implementation:

// Stub example for top-down testing
public class PaymentServiceStub {
    public PaymentResult processPayment(PaymentRequest request) {
        // Return controlled test response
        return new PaymentResult("SUCCESS", "TXN-123");
    }
}

// Driver example for bottom-up testing  
public class OrderServiceDriver {
    public void testInventoryService() {
        InventoryService service = new InventoryService();
        boolean result = service.checkAvailability("PRODUCT-123", 5);
        assert result == true;
    }
}