Structural patterns (Adapter, Decorator, Facade) in Software Engineering - Mini Project: Build & Apply

Understanding Structural Patterns: Adapter, Decorator, Facade

📖 Scenario: Imagine you are building a simple software system that manages different types of devices and their features. You want to organize your code so it is easy to extend and maintain. To do this, you will learn three common structural design patterns: Adapter, Decorator, and Facade.

🎯 Goal: Build a small example that shows how Adapter, Decorator, and Facade patterns work. You will create basic classes and then apply each pattern step-by-step to see how they help organize and improve the code.

📋 What You'll Learn

Create a basic device interface and two device classes

Add an adapter class to make an incompatible device work with the interface

Create a decorator class to add extra features to a device

Build a facade class to simplify interaction with multiple devices

💡 Why This Matters

🌍 Real World

Structural patterns help organize code in software projects where different components need to work together smoothly, especially when integrating legacy systems or adding new features.

💼 Career

Understanding these patterns is valuable for software developers and architects to write flexible, maintainable, and scalable code in real-world applications.

1

Create basic device classes

Create a class called Device with a method get_status() that returns the string 'Device status: OK'. Then create two classes Printer and Scanner that inherit from Device and override get_status() to return 'Printer ready' and 'Scanner ready' respectively.

Software Engineering

# Create class Device with method get_status
# Create classes Printer and Scanner inheriting Device and override get_status
# Your code here

Hint

Define the base class Device with a method that returns a status string. Then create two subclasses that change this status message.

2

Add an Adapter for an incompatible device

Create a class called OldCamera with a method status() that returns 'Old camera active'. Then create an adapter class called CameraAdapter that has a constructor accepting an OldCamera instance and a method get_status() that calls the status() method of OldCamera and returns its result.

Software Engineering

class Device:
    def get_status(self):
        return 'Device status: OK'

class Printer(Device):
    def get_status(self):
        return 'Printer ready'

class Scanner(Device):
    def get_status(self):
        return 'Scanner ready'

# Create class OldCamera with method status
# Create class CameraAdapter with constructor taking OldCamera instance
# Implement get_status in CameraAdapter to call OldCamera's status
# Your code here

Hint

The adapter wraps the old device and translates its method to the expected interface method.

3

Create a Decorator to add features

Create a class called DeviceDecorator with a constructor that takes a Device instance and stores it. Add a method get_status() that calls the wrapped device's get_status() and appends the string ' with extra features' to the result.

Software Engineering

class Device:
    def get_status(self):
        return 'Device status: OK'

class Printer(Device):
    def get_status(self):
        return 'Printer ready'

class Scanner(Device):
    def get_status(self):
        return 'Scanner ready'

class OldCamera:
    def status(self):
        return 'Old camera active'

class CameraAdapter:
    def __init__(self, old_camera):
        self.old_camera = old_camera
    def get_status(self):
        return self.old_camera.status()

# Create class DeviceDecorator with constructor taking Device instance
# Implement get_status to add ' with extra features' to wrapped device's status
# Your code here

Hint

The decorator holds a device and extends its behavior by adding text to the status.

4

Build a Facade to simplify device control

Create a class called DeviceFacade with a constructor that creates instances of Printer, Scanner, and CameraAdapter (using an OldCamera instance). Add a method all_statuses() that returns a list of status strings from all three devices by calling their get_status() methods.

Software Engineering

class Device:
    def get_status(self):
        return 'Device status: OK'

class Printer(Device):
    def get_status(self):
        return 'Printer ready'

class Scanner(Device):
    def get_status(self):
        return 'Scanner ready'

class OldCamera:
    def status(self):
        return 'Old camera active'

class CameraAdapter:
    def __init__(self, old_camera):
        self.old_camera = old_camera
    def get_status(self):
        return self.old_camera.status()

class DeviceDecorator:
    def __init__(self, device):
        self.device = device
    def get_status(self):
        return self.device.get_status() + ' with extra features'

# Create class DeviceFacade with constructor creating Printer, Scanner, and CameraAdapter
# Implement all_statuses method returning list of all device statuses
# Your code here

Hint

The facade creates and holds all devices and provides a simple method to get their statuses together.

Practice

1. Which structural pattern allows incompatible interfaces to work together by converting one interface into another?

easy

A. Decorator

B. Adapter

C. Facade

D. Singleton

Solution

1. Step 1: Understand the role of Adapter pattern

   The Adapter pattern converts the interface of one class into another interface clients expect, enabling incompatible interfaces to work together.

2. Step 2: Compare with other patterns

   Decorator adds features, Facade simplifies interfaces, Singleton controls instance creation. Only Adapter changes interfaces for compatibility.

3. Final Answer:

   Adapter -> Option B

4. Quick Check:

   Interface conversion = Adapter [OK]

Hint: Adapter = interface converter between incompatible parts [OK]

Common Mistakes:

• Confusing Decorator with Adapter
• Thinking Facade changes interfaces
• Mixing Singleton with structural patterns

2. Which of the following is the correct way to describe the Decorator pattern?

easy

A. It converts one interface to another.

B. It provides a simple interface to a complex system.

C. It adds new responsibilities to objects dynamically.

D. It ensures a class has only one instance.

Solution

1. Step 1: Identify the purpose of Decorator pattern

   Decorator adds new features or responsibilities to objects dynamically without changing their structure.

2. Step 2: Eliminate other options

   It provides a simple interface to a complex system. describes Facade, It converts one interface to another. describes Adapter, It ensures a class has only one instance. describes Singleton pattern.

3. Final Answer:

   It adds new responsibilities to objects dynamically. -> Option C

4. Quick Check:

   Decorator = add features dynamically [OK]

Hint: Decorator = add features without changing original object [OK]

Common Mistakes:

• Mixing Decorator with Facade
• Confusing Adapter with Decorator
• Thinking Singleton is a structural pattern

3. Consider a system where a Facade pattern is used to simplify multiple complex subsystems. What is the main benefit when a client uses the Facade?

medium

A. The client interacts with a simple interface hiding complexity.

B. The client must adapt its interface to each subsystem.

C. The client can access all subsystem details directly.

D. The client can add new features to subsystems dynamically.

Solution

1. Step 1: Understand Facade pattern role

   Facade provides a simple unified interface to a set of interfaces in a subsystem, hiding complexity from the client.

2. Step 2: Analyze options

   The client can access all subsystem details directly. is wrong because Facade hides details. The client must adapt its interface to each subsystem. describes Adapter. The client can add new features to subsystems dynamically. describes Decorator.

3. Final Answer:

   The client interacts with a simple interface hiding complexity. -> Option A

4. Quick Check:

   Facade = simple interface to complex system [OK]

Hint: Facade hides complexity behind a simple interface [OK]

Common Mistakes:

• Thinking Facade exposes all subsystem details
• Confusing Facade with Adapter or Decorator
• Assuming Facade adds features dynamically

4. A developer tries to use an Adapter pattern but the client code still fails due to interface mismatch. What is the most likely error?

medium

A. The client code is using the correct interface.

B. The Decorator pattern was used instead of Adapter.

C. The Facade pattern was applied to simplify the interface.

D. The Adapter does not implement the target interface correctly.

Solution

1. Step 1: Identify Adapter pattern requirement

   Adapter must implement the target interface expected by the client to convert calls properly.

2. Step 2: Analyze failure cause

   If the client still fails, likely the Adapter does not correctly implement or map the target interface methods.

3. Final Answer:

   The Adapter does not implement the target interface correctly. -> Option D

4. Quick Check:

   Adapter interface mismatch = implementation error [OK]

Hint: Adapter must match target interface exactly [OK]

Common Mistakes:

• Confusing Decorator or Facade with Adapter errors
• Assuming client code is always correct
• Ignoring interface method signatures

5. You have a legacy payment system with a complex API. You want to provide a simple interface for new clients and also add logging features without changing the legacy code. Which combination of structural patterns should you use?

hard

A. Facade to simplify interface and Decorator to add logging

B. Decorator to simplify interface and Adapter to add logging

C. Adapter to add logging and Facade to simplify interface

D. Adapter to simplify interface and Decorator to add logging

Solution

1. Step 1: Identify pattern for simplifying complex API

   Facade provides a simple interface to a complex system, ideal for the legacy payment system.

2. Step 2: Identify pattern for adding features without changing code

   Decorator adds new responsibilities like logging dynamically without modifying original code.

3. Step 3: Combine patterns logically

   Use Facade to simplify the interface for clients, then wrap with Decorator to add logging features.

4. Final Answer:

   Facade to simplify interface and Decorator to add logging -> Option A

5. Quick Check:

   Simplify = Facade, Add features = Decorator [OK]

Hint: Facade simplifies, Decorator adds features without code change [OK]

Common Mistakes:

• Using Adapter to simplify interface instead of Facade
• Confusing roles of Adapter and Decorator
• Trying to add logging via Adapter