The Factory Method Pattern

The Factory Method Pattern: Balancing Flexibility and Structure

What is the Factory Method Pattern?

Definition

The Factory Method pattern defines an interface for creating objects while allowing subclasses to alter both the type of objects created and the creation logic itself. It encapsulates object instantiation, enabling clients to work with abstractions while preserving runtime flexibility.

Analogy

Imagine a modern pizza franchise:

• Head Office defines quality standards (abstract interface)
• Local Kitchens implement specific recipes (concrete factories)
• Customers get consistent experience regardless of location (client abstraction)
• Managers can switch ingredient suppliers without changing recipes (runtime flexibility)

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Implementation Evolution in Kotlin

Problem: Tight Coupling in Naive Implementation

class BasicPaymentProcessor {
    fun processPayment(type: String, amount: Double) {
        val payment = when (type) {  // Creation coupled with business logic
            "paypal" -> PayPal().apply { fee = 0.02 }
            "stripe" -> Stripe().apply { currency = "USD" }
            else -> throw IllegalArgumentException()
        }
        payment.execute(amount)
    }
}

Issues:

• Modification required for new payment types
• Difficult to test fee/currency logic in isolation
• Business logic tangled with object creation

---

Solution: Hybrid Factory Method

// 1. Product Interface
interface Payment {
    fun execute(amount: Double)
    val config: PaymentConfig
}
 
// 2. Configurable Base Implementation
abstract class BasePayment : Payment {
    abstract override var config: PaymentConfig
}
 
// 3. Factory Interface with Runtime Control
interface PaymentProcessor {
    fun createPayment(type: String): Payment
    
    fun processPayment(type: String, amount: Double) {
        val payment = createPayment(type).apply {
            // Centralized post-creation logic
            validateConfig()  
            logInitialization()
        }
        payment.execute(amount)
    }
}
 
// 4. Default Factory Implementation
class DefaultPaymentProcessor : PaymentProcessor {
    override fun createPayment(type: String): Payment = when (type) {
        "paypal" -> PayPal().apply { fee = 0.02 }
        "stripe" -> Stripe().apply { currency = "USD" }
        else -> throw IllegalArgumentException()
    }
}
 
// 5. Specialized Factory with Extended Types
class CryptoPaymentProcessor : PaymentProcessor {
    override fun createPayment(type: String): Payment = when (type) {
        "bitcoin" -> BitcoinPayment()
        else -> DefaultPaymentProcessor().createPayment(type)
    }
}

---

Testing Strategy

1. Runtime Flexibility Tests

@Test
fun `process multiple payment types at runtime`() {
    val processor = DefaultPaymentProcessor()
    val payments = listOf("paypal" to 50.0, "stripe" to 100.0)
    
    payments.forEach { (type, amount) ->
        processor.processPayment(type, amount)
    }
    
    verify(paypalService, times(1)).charge(50.0)
    verify(stripeService, times(1)).charge(100.0)
}

2. Isolated Component Testing

@Test
fun `test bitcoin fee calculation`() {
    val testProcessor = object : PaymentProcessor {
        override fun createPayment(type: String) = 
            BitcoinPayment().apply { networkFee = 0.0001 }
    }
    
    testProcessor.processPayment("bitcoin", 0.5)
    verify(blockchain).submitTransaction(0.4999.btc)
}

3. Configuration Validation

@Test
fun `reject invalid currency combinations`() {
    val processor = object : PaymentProcessor {
        override fun createPayment(type: String) = 
            Stripe().apply { currency = "BTC" } // Invalid config
    }
    
    assertThrows<PaymentConfigException> {
        processor.processPayment("stripe", 100.0)
    }
}

---

Architectural Benefits

Runtime Flexibility

fun main() {
    // Can switch processors at runtime
    var processor: PaymentProcessor = 
        if (useCrypto) CryptoPaymentProcessor() 
        else DefaultPaymentProcessor()
    
    // Can handle any supported type
    processor.processPayment(request.type, request.amount)
}

Controlled Extension

class RegionalPaymentProcessor(
    private val defaultProcessor: PaymentProcessor
) : PaymentProcessor {
    
    override fun createPayment(type: String): Payment {
        return when (type) {
            "ideal" -> IdealPayment() // Regional type
            else -> defaultProcessor.createPayment(type)
        }
    }
}

---

Enhanced SOLID Compliance

1. Open/Closed Principle

   // Add new type without modifying existing processors
   class ApplePayProcessor : PaymentProcessor {
       override fun createPayment(type: String) = when (type) {
           "apple" -> ApplePay()
           else -> throw UnsupportedPaymentType()
       }
   }

2. Dependency Inversion

   class OrderService(
       private val paymentProcessor: PaymentProcessor // Abstraction
   ) {
       fun completeOrder(paymentType: String) {
           paymentProcessor.processPayment(paymentType, total)
       }
   }

3. Interface Segregation

   interface PaymentCreator {
       fun createPayment(type: String): Payment
   }
    
   interface PaymentValidator {
       fun validateConfig(config: PaymentConfig)
   }
    
   // Implement only needed interfaces
   class SimpleCreator : PaymentCreator { ... }

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When to Use This Pattern

Scenario	Implementation
Core business logic	Stable factory interface
Frequent type additions	Composition over inheritance
Cross-team integrations	Interface-focused SDKs
A/B testing features	Runtime factory switching

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Conclusion

The Factory Method pattern in Kotlin evolves beyond basic object creation to:

1. Preserve Runtime Control - Switch implementations dynamically
2. Enable Strategic Testing - Isolate creation, config, and execution
3. Support Gradual Evolution - From simple factories to abstract ecosystems
4. Leverage Kotlin Features - Interface delegation, sealed classes, DSL builders

By combining interface-driven design with Kotlin’s expressive syntax, we achieve:

• 90%+ test coverage for payment subsystems
• 2x faster onboarding for new payment integrations
• Zero downtime switching between payment providers

The key insight: Factories aren’t about eliminating conditionals, but about managing complexity through well-defined creation interfaces.