Adapter pattern in LLD - Architecture Diagram

System Overview - Adapter pattern

The Adapter pattern helps two incompatible interfaces work together. It acts like a translator between a client and a service that have different interfaces. This pattern is useful when you want to reuse existing code but the interfaces don’t match.

Architecture Diagram

User
  |
  v
Client Interface
  |
  v
Adapter
  |
  v
Adaptee (Existing Service)

Components

User

actor

Initiates requests using the client interface

Client Interface

interface

Defines the expected methods the user calls

Adapter

adapter

Converts client interface calls to adaptee interface calls

Adaptee (Existing Service)

service

Provides existing functionality with an incompatible interface

Request Flow - 6 Hops

User → Client Interface

Client Interface → Adapter

Adapter → Adaptee (Existing Service)

Adaptee (Existing Service) → Adapter

Adapter → Client Interface

Client Interface → User

Failure Scenario

Component Fails:Adapter

Impact:User requests cannot be translated to the adaptee interface, causing failures or errors in service calls.

Mitigation:Implement error handling in the adapter to catch translation errors and fallback to default behavior or notify the user gracefully.

Architecture Quiz - 3 Questions

What is the main role of the Adapter in this pattern?

A To replace the existing service

B To translate calls between incompatible interfaces

C To store data for the client

D To act as the user interface

Design Principle

The Adapter pattern allows reuse of existing services with incompatible interfaces by introducing a translator component. This keeps client code unchanged and promotes flexibility.

Practice

1. What is the main purpose of the Adapter pattern in system design?

easy

A. To improve the performance of a system by caching results

B. To allow incompatible interfaces to work together by converting one interface to another

C. To store data persistently in a database

D. To create multiple instances of a class

Solution

1. Step 1: Understand the Adapter pattern role

   The Adapter pattern acts as a bridge between two incompatible interfaces, allowing them to communicate.

2. Step 2: Compare with other options

   The other options describe different design goals unrelated to interface compatibility.

3. Final Answer:

   To allow incompatible interfaces to work together by converting one interface to another -> Option B

4. Quick Check:

   Adapter pattern = interface compatibility [OK]

Hint: Adapter = bridge between incompatible interfaces [OK]

Common Mistakes:

• Confusing Adapter with Singleton or Factory patterns
• Thinking Adapter improves performance directly
• Assuming Adapter stores data

2. Which of the following is the correct way to declare an Adapter class in a typical object-oriented language?

easy

A. class Adapter implements Adaptee { private TargetInterface target; }

B. class Adapter extends Adaptee implements TargetInterface { }

C. class Adapter extends TargetInterface { private Adaptee adaptee; }

D. class Adapter implements TargetInterface { private Adaptee adaptee; }

Solution

1. Step 1: Identify Adapter pattern structure

   The Adapter class implements the TargetInterface and holds a reference to the Adaptee to delegate calls.

2. Step 2: Analyze options

   class Adapter implements TargetInterface { private Adaptee adaptee; } correctly shows Adapter implementing TargetInterface and containing Adaptee. The other options misuse inheritance or interfaces.

3. Final Answer:

   class Adapter implements TargetInterface { private Adaptee adaptee; } -> Option D

4. Quick Check:

   Adapter implements target and wraps adaptee [OK]

Hint: Adapter implements target interface, holds adaptee instance [OK]

Common Mistakes:

• Using inheritance incorrectly instead of composition
• Confusing which interface to implement
• Swapping adaptee and target roles

3. Given this Adapter pattern code snippet, what will be the output?

class Adaptee {
  String specificRequest() { return "Adaptee response"; }
}

interface Target {
  String request();
}

class Adapter implements Target {
  private Adaptee adaptee = new Adaptee();
  public String request() {
    return adaptee.specificRequest();
  }
}

public class Main {
  public static void main(String[] args) {
    Target target = new Adapter();
    System.out.println(target.request());
  }
}

medium

A. Adaptee response

B. Target response

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Trace method calls in Adapter

   The Adapter's request() calls adaptee.specificRequest(), which returns "Adaptee response".

2. Step 2: Check main method output

   Main creates Adapter as Target and prints target.request(), so output is "Adaptee response".

3. Final Answer:

   Adaptee response -> Option A

4. Quick Check:

   Adapter delegates request() to adaptee.specificRequest() [OK]

Hint: Adapter.request() returns adaptee.specificRequest() result [OK]

Common Mistakes:

• Expecting 'Target response' instead of adaptee's
• Assuming compilation or runtime errors without cause
• Confusing method names

4. Identify the error in this Adapter pattern implementation:

interface Target {
  void request();
}

class Adaptee {
  void specificRequest() { System.out.println("Adaptee called"); }
}

class Adapter implements Target {
  Adaptee adaptee = new Adaptee();
  public void request() {
    specificRequest();
  }
}

medium

A. Adapter calls specificRequest() without referencing adaptee instance

B. Adaptee should implement Target interface

C. Adapter should extend Adaptee instead of implementing Target

D. Target interface method signature is incorrect

Solution

1. Step 1: Review Adapter's request() method

   Adapter calls specificRequest() directly, but specificRequest() belongs to Adaptee instance, so it should call adaptee.specificRequest().

2. Step 2: Validate other options

   Adaptee does not need to implement Target. Adapter implements Target correctly. Target interface method is correct.

3. Final Answer:

   Adapter calls specificRequest() without referencing adaptee instance -> Option A

4. Quick Check:

   Adapter must call adaptee.specificRequest() [OK]

Hint: Call adaptee methods via adaptee instance inside Adapter [OK]

Common Mistakes:

• Calling adaptee methods without instance prefix
• Misusing inheritance instead of composition
• Changing Target interface unnecessarily

5. You need to integrate a legacy payment system with a new e-commerce platform. The legacy system has a method processLegacyPayment(), but the platform expects processPayment(). How should you apply the Adapter pattern to solve this?

hard

A. Create a new payment system and discard the legacy system

B. Modify the legacy system to rename processLegacyPayment() to processPayment()

C. Create an Adapter class implementing the platform's payment interface and inside it call processLegacyPayment() of the legacy system

D. Use inheritance to make the legacy system extend the platform's payment interface

Solution

1. Step 1: Understand integration requirement

   The platform expects processPayment(), but legacy system only has processLegacyPayment().

2. Step 2: Apply Adapter pattern

   Create an Adapter that implements the platform interface and inside calls legacy's processLegacyPayment() to adapt calls without changing legacy code.

3. Step 3: Evaluate other options

   Modifying the legacy code is risky. Discarding the legacy system is costly. Using inheritance is not possible if the legacy system is not designed for it.

4. Final Answer:

   Create an Adapter class implementing the platform's payment interface and inside it call processLegacyPayment() of the legacy system -> Option C

5. Quick Check:

   Adapter wraps legacy method to match platform interface [OK]

Hint: Adapter wraps legacy calls to match new interface [OK]

Common Mistakes:

• Changing legacy code directly
• Ignoring Adapter pattern benefits
• Trying inheritance when not supported