Structural Design Patterns - Adapter and Flyweight

Table of Content

Structural Patterns

  • design patterns that ease the design by identifying a simple way to realize relationships between entities.
  • concerned with how classes and objects are composed to form larger structures.

Adapter

  • allows the interface of an existing class to be used from another interface.
  • It is often used to make existing classes work with others without modifying their source code.

Flyweight

  • lets you fit more objects into the available amount of RAM by sharing common parts of state between multiple objects instead of keeping all of the data in each object.

Adapter

  • is a structural pattern that allows objects with incompatible interfaces to collaborate.
  • Adapter is a special object that converts the interface of one object so that another object can understand it.
  • An adapter wraps one of the objects to hide the complexity of conversion happening behind the scenes. The wrapped object isnʼt even aware of the adapter.

    Problem

  • Let us take the example of payment processing. We first use the Stripe payment gateway.
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public class StripeApi {
	public void createPayment() {
		// Create payment
	}
 
	public PaymentStatus checkStatus(String paymentId) {
		// Check payment status
	}
}

...

public void processPayment() {
	StripeApi stripeApi = new StripeApi();
	Payment object = stripeApi.createPayment();
	PaymentStatus status = stripeApi.checkStatus(object.getId());
}
  • Now we want to integrate with another payment gateway. We use the PayPal payment gateway.
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public class PayPalApi {
	public void makePayment() {
		// Create payment
	}
 
	public PaymentStatus getStatus(String paymentId) {
		// Check payment status
	}
}
  • As you can see, the Stripe API and the PayPal API have different method names.
  • The Stripe API uses createPayment and checkStatus while the PayPal API uses makePayment and getStatus.

    Implementation

  • Incompatible classes - You should have two classes that have incompatible interfaces. For example, the Stripe API and the PayPal API.
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public class StripeApi {
	public void createPayment() {
		// Create payment
	}
 
	public PaymentStatus checkStatus(String paymentId) {
		// Check payment status
	}
}

public class PayPalApi {
	public void makePayment() {
		// Create payment
	}
 
	public PaymentStatus getStatus(String paymentId) {
		// Check payment status
	}
}
  • Adapter interface - Create an interface for the adapter that will be used to convert the incompatible interfaces.
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public interface PaymentProvider {
	void makePayment();
	PaymentStatus getStatus(String paymentId);
}
  • Concrete adapter classes - Create a class that implements the target interface. This is the class that the client code expects to work with. The adapter will convert the interface of the existing class to this interface.
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public class StripePaymentProvider implements PaymentProvider {
	@Override
	public void makePayment() {
		...
	}
	
	@Override
	public PaymentStatus getStatus(String paymentId) {
		...
	}
}

public class PayPalPaymentProvider implements PaymentProvider {
	@Override
	public void makePayment() {
		...
	}
	
	@Override
	public PaymentStatus getStatus(String paymentId) {
		...
	}
}
  • Transform request and delegate to original class - In the adapter class, transform the request to the format that the original class expects. Then, call the original class to perform the operation.
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public class StripePaymentProvider implements PaymentProvider {
	private StripeApi stripeApi = new StripeApi();
 
	@Override
	public void makePayment() {
		stripeApi.createPayment();
	}
	
	@Override
	public PaymentStatus getStatus(String paymentId) {
		StripeStatus status = stripeApi.checkStatus(paymentId);
		return convertStatus(status);
	}
}
  • Client code - The client code expects to work with the target interface. The client code doesnʼt know that the adapter is converting the interface of the original class.
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public class PaymentProcessor {
	private PaymentProvider paymentProvider;
 
	public PaymentProcessor(PaymentProvider paymentProvider) {
		this.paymentProvider = paymentProvider;
	}
	
	public void processPayment() {
		paymentProvider.makePayment();
		PaymentStatus status = paymentProvider.getStatus("paymentId");
	}
}

Adapter Pattern in Python

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from abc import ABC, abstractmethod
from enum import Enum

class PaymentStatus(Enum):
    SUCCESS = "SUCCESS"
    FAILURE = "FAILURE"

class PaymentRequest:
    def __init__(self, name, phone, email, amount):
        self._name = name
        self._phone = phone
        self._email = email
        self._amount = amount

    @property
    def name(self):
        return self._name
    
    @property
    def phone(self):
        return self._phone
    
    @property
    def email(self):
        return self._email
    
    @property
    def amount(self):
        return self._amount

class PaymentProviderInterface(ABC):
    @abstractmethod
    def generate_link(self):
        pass
    
    @abstractmethod
    def pay(self, payment_request):
        pass
    
    @abstractmethod
    def check_status(self):
        pass

class CashfreeApi:
    def create_url(self):
        return "Cashfree"
    
    def do_payment(self, amount):
        print(f"Cashfree Payment: {amount}")
    
    def verify_status(self):
        return "OK"

class CashFreePayProvider(PaymentProviderInterface):
    def __init__(self):
        self._cashfree_api = CashfreeApi()
    
    def generate_link(self):
        return self._cashfree_api.create_url()
    
    def pay(self, payment_request):
        self._cashfree_api.do_payment(payment_request.amount)
    
    def check_status(self):
        status = self._cashfree_api.verify_status()
        return self.to_payment_status(status)
    
    def to_payment_status(self, status):
        if status == "OK":
            return PaymentStatus.SUCCESS
        return PaymentStatus.FAILURE

class RazorPayApi:
    def make_link(self):
        return "RazorPay"
    
    def pre_pay(self):
        print("RazorPay PrePayment")
    
    def pay(self, name, amount):
        print(f"RazorPay Payment for {name} of amount {amount}")
    
    def check_status(self):
        # Simulated status check
        return "PASS"

class RazorPayProvider(PaymentProviderInterface):
    def __init__(self):
        self._razorpay_api = RazorPayApi()
    
    def generate_link(self):
        return self._razorpay_api.make_link()
    
    def pay(self, payment_request):
        self._razorpay_api.pre_pay()
        self._razorpay_api.pay(payment_request.name, payment_request.amount)
    
    def check_status(self):
        status = self._razorpay_api.check_status()
        return self.to_payment_status(status)
    
    def to_payment_status(self, status):
        if status == "PASS":
            return PaymentStatus.SUCCESS
        return PaymentStatus.FAILURE

# Client code to execute payment request
def process_payment(payment_provider, payment_request):
    print(payment_provider.generate_link())
    payment_provider.pay(payment_request)
    status = payment_provider.check_status()
    print(f"Payment Status: {status.name}")

# Example usage:
payment_request = PaymentRequest("John Doe", "1234567890", "john@example.com", 100)

# Using CashFreePayProvider
cashfree_provider = CashFreePayProvider()
process_payment(cashfree_provider, payment_request)

Adapter Pattern in Go

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package adapter

import (
	"fmt"
)

type PaymentStatus string

const (
	Success PaymentStatus = "SUCCESS"
	Failure PaymentStatus = "FAILURE"
)

type PaymentRequest struct {
	Name   string
	Phone  string
	Email  string
	Amount int
}

type PaymentProviderInterface interface {
	GenerateLink() string
	Pay(paymentRequest PaymentRequest)
	CheckStatus() PaymentStatus
}

type CashfreeApi struct{}

func (c *CashfreeApi) CreateURL() string {
	return "Cashfree"
}

func (c *CashfreeApi) DoPayment(amount int) {
	fmt.Printf("Cashfree Payment: %d\n", amount)
}

func (c *CashfreeApi) VerifyStatus() string {
	// Simulated status check
	return "OK"
}

type CashFreePayProvider struct {
	cashfreeApi *CashfreeApi
}

func NewCashFreePayProvider() *CashFreePayProvider {
	return &CashFreePayProvider{cashfreeApi: &CashfreeApi{}}
}

func (c *CashFreePayProvider) GenerateLink() string {
	return c.cashfreeApi.CreateURL()
}

func (c *CashFreePayProvider) Pay(paymentRequest PaymentRequest) {
	c.cashfreeApi.DoPayment(paymentRequest.Amount)
}

func (c *CashFreePayProvider) CheckStatus() PaymentStatus {
	status := c.cashfreeApi.VerifyStatus()
	return c.toPaymentStatus(status)
}

func (c *CashFreePayProvider) toPaymentStatus(status string) PaymentStatus {
	if status == "OK" {
		return Success
	}
	return Failure
}

type RazorPayApi struct{}

func (r *RazorPayApi) MakeLink() string {
	return "RazorPay"
}

func (r *RazorPayApi) PrePay() {
	fmt.Println("RazorPay PrePayment")
}

func (r *RazorPayApi) Pay(name string, amount int) {
	fmt.Printf("RazorPay Payment for %s of amount %d\n", name, amount)
}

func (r *RazorPayApi) CheckStatus() string {
	// Simulated status check
	return "PASS"
}

type RazorPayProvider struct {
	razorpayApi *RazorPayApi
}

func NewRazorPayProvider() *RazorPayProvider {
	return &RazorPayProvider{razorpayApi: &RazorPayApi{}}
}

func (r *RazorPayProvider) GenerateLink() string {
	return r.razorpayApi.MakeLink()
}

func (r *RazorPayProvider) Pay(paymentRequest PaymentRequest) {
	r.razorpayApi.PrePay()
	r.razorpayApi.Pay(paymentRequest.Name, paymentRequest.Amount)
}

func (r *RazorPayProvider) CheckStatus() PaymentStatus {
	status := r.razorpayApi.CheckStatus()
	return r.toPaymentStatus(status)
}

func (r *RazorPayProvider) toPaymentStatus(status string) PaymentStatus {
	if status == "PASS" {
		return Success
	}
	return Failure
}

func ProcessPayment(provider PaymentProviderInterface, paymentRequest PaymentRequest) {
	fmt.Println(provider.GenerateLink())
	provider.Pay(paymentRequest)
	status := provider.CheckStatus()
	fmt.Printf("Payment Status: %s\n", status)
}

Advantages

  • You can use adapters to reuse existing classes with incompatible interfaces.
  • You can even modify the request and response of the original classes.
  • Single Responsibility Principle. You can separate the interface or data conversion code from the primary business logic of the program.
  • Open/Closed Principle. You can introduce new types of adapters into the program without breaking the existing client code, as long as they work with the adapters through the target interface.

Flyweight Pattern

  • is used to reduce the memory footprint of a program by sharing as much data as possible with similar objects.

Example

  • We assume the role of game developer and are looking to create a role-playing game like PUBG, counter strike etc. We modeled our game in various classes such as Map, User, Gun and Bullet.

Flyweight

  • The memory used by a single bullet instance would be:
    • Double - 8 bytes * 6 = 48 bytes
    • Integer - 4 bytes * 2 = 8 bytes
    • Image - 1KB
  • For 2000 bullets for 200 players the memory usage would be 800MB.
  • The major problem here is for each object, the image field consumes a lot of memory. The image is also the same for all the bullets.
  • Other parts of a particle’s state, such as coordinates, movement vector and speed, are unique to each particle. After all, the values of these fields change over time. This data represents the always changing context in which the particle exists, while the color and sprite remain constant for each particle.
  • This constant data of an object is usually called the intrinsic state. It lives within the object; other objects can only read it, not change it.

  • The rest of the object’s state, often altered “from the outside” by other objects, is called the extrinsic state.

  • The Flyweight pattern suggests that you stop storing the extrinsic state inside the object. Instead, you should pass this state to specific methods which rely on it.

  • Only the intrinsic state stays within the object, letting you reuse it in different contexts.

  • So our Bullet class will have to be divided into two classes.
  • One class will contain the intrinsic state and the other class will contain the extrinsic state. The extrinsic state will be passed to the methods that need it.

Flyweight

  • Now, every bullet will have a reference to the Bullet object.
  • The Bullet object will contain the image field.
  • The FlyingBullet class will contain the extrinsic state. Each bullet does not need to have its own image field.
  • The image field is shared between all the bullets. This way, the memory usage is reduced.

Implementation

  • Intrinsic state - The intrinsic state is stored in the flyweight object. It is independent of the flyweight’s context and remains the same for all flyweight objects.
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public class Bullet {
    private String image;
}
  • Extrinsic state - The extrinsic state is stored or computed by client objects. It depends on the flyweight’s context and changes with it.
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public class FlyingBullet {
    private double x;
    private double y;
    private double z;
    private double radius;
    private double direction;
    private double speed;
    private int status;
    private int type;
    private Bullet bullet;
}
  • Flyweight factory - The flyweight factory is responsible for creating and managing flyweight objects. It ensures that flyweights are shared properly. When a client requests a flyweight, the flyweight factory either returns an existing instance or creates a new one, if it doesn’t exist yet.
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public class BulletFactory {
    private static final Map<String, Bullet> bullets = new HashMap<>();

    public Bullet getBullet(BulletType type) {
        ...
    }

    public void addBullet(BulletType type, Bullet bullet) {
        ...
    }
}
  • Client code - The client code usually creates a bunch of pre-populated flyweights in the initialization stage of the application.

Flyweight Pattern in Python

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from enum import Enum
from typing import Dict

class BulletType(Enum):
    NINE_MM = 1
    ELEVEN_MM = 2
    ACP = 3

class Bullet:
    def __init__(self, image: str, radius: float, weight: float, bullet_type: BulletType):
        self.__image = image
        self.__radius = radius
        self.__weight = weight
        self.__type = bullet_type

    @property
    def image(self) -> str:
        return self.__image

    @property
    def radius(self) -> float:
        return self.__radius

    @property
    def weight(self) -> float:
        return self.__weight

    @property
    def type(self) -> BulletType:
        return self.__type

class BulletRegistry:
    def __init__(self):
        self.__bullets: Dict[BulletType, Bullet] = {}

    def add_bullet(self, bullet: Bullet):
        self.__bullets[bullet.type] = bullet

    def get_bullet(self, bullet_type: BulletType) -> Bullet:
        return self.__bullets.get(bullet_type)

class FlyingBullet:
    def __init__(self, x: float, y: float, z: float, direction: float, bullet: Bullet):
        self.__x = x
        self.__y = y
        self.__z = z
        self.__direction = direction
        self.__bullet = bullet

if __name__ == "__main__":
    # Create a BulletRegistry instance
    registry = BulletRegistry()

    # Add bullets to the registry
    registry.add_bullet(Bullet("9mm.png", 9.0, 7.5, BulletType.NINE_MM))
    registry.add_bullet(Bullet("11mm.png", 11.0, 8.0, BulletType.ELEVEN_MM))
    registry.add_bullet(Bullet("acp.png", 12.0, 9.0, BulletType.ACP))

    # Retrieve a bullet from the registry
    bullet = registry.get_bullet(BulletType.NINE_MM)

    # Create a FlyingBullet instance
    flying_bullet = FlyingBullet(0.0, 0.0, 0.0, 90.0, bullet)

    # Print the details of the FlyingBullet instance
    print(f"FlyingBullet details: x={flying_bullet._FlyingBullet__x}, "
          f"y={flying_bullet._FlyingBullet__y}, z={flying_bullet._FlyingBullet__z}, "
          f"direction={flying_bullet._FlyingBullet__direction}, "
          f"bullet_type={flying_bullet._FlyingBullet__bullet.type.name}, "
          f"bullet_image={flying_bullet._FlyingBullet__bullet.image}, "
          f"bullet_radius={flying_bullet._FlyingBullet__bullet.radius}, "
          f"bullet_weight={flying_bullet._FlyingBullet__bullet.weight}")

Flyweight Pattern in Go

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package flyweight

import (
	"fmt"
)

// BulletType enum
type BulletType int

const (
	NINE_MM BulletType = iota
	ELEVEN_MM
	ACP
)

// Bullet struct representing the flyweight object
type Bullet struct {
	image  string
	radius float64
	weight float64
	bType  BulletType
}

func NewBullet(image string, radius, weight float64, bType BulletType) *Bullet {
	return &Bullet{
		image:  image,
		radius: radius,
		weight: weight,
		bType:  bType,
	}
}

func (b *Bullet) Image() string {
	return b.image
}

func (b *Bullet) Radius() float64 {
	return b.radius
}

func (b *Bullet) Weight() float64 {
	return b.weight
}

func (b *Bullet) Type() BulletType {
	return b.bType
}

// BulletRegistry to manage shared Bullet instances
type BulletRegistry struct {
	bullets map[BulletType]*Bullet
}

func NewBulletRegistry() *BulletRegistry {
	return &BulletRegistry{
		bullets: make(map[BulletType]*Bullet),
	}
}

func (br *BulletRegistry) AddBullet(bullet *Bullet) {
	br.bullets[bullet.Type()] = bullet
}

func (br *BulletRegistry) GetBullet(bType BulletType) *Bullet {
	return br.bullets[bType]
}

// FlyingBullet struct representing the extrinsic state
type FlyingBullet struct {
	x, y, z   float64
	direction float64
	bullet    *Bullet
}

func NewFlyingBullet(x, y, z, direction float64, bullet *Bullet) *FlyingBullet {
	return &FlyingBullet{
		x:         x,
		y:         y,
		z:         z,
		direction: direction,
		bullet:    bullet,
	}
}

func (fb *FlyingBullet) Details() string {
	return fmt.Sprintf("FlyingBullet details: x=%.1f, y=%.1f, z=%.1f, direction=%.1f, bullet_type=%d, bullet_image=%s, bullet_radius=%.1f, bullet_weight=%.1f",
		fb.x, fb.y, fb.z, fb.direction, fb.bullet.Type(), fb.bullet.Image(), fb.bullet.Radius(), fb.bullet.Weight())
}

These are my notes from the Low-Level Design (LLD) course I took at Scaler.

Check Python, Java and Go code on Github Repo