Adapter pattern in LLD - System Design Exercise

Design: Adapter Pattern Implementation

Design and implement the adapter pattern to connect incompatible interfaces in a low-level design context. Out of scope are other design patterns or large system integrations.

Functional Requirements

FR1: Allow two incompatible interfaces to work together without changing their existing code

FR2: Enable reuse of existing classes with different interfaces

FR3: Support multiple types of adapters for different incompatible interfaces

FR4: Ensure the adapter is transparent to the client using the target interface

Non-Functional Requirements

NFR1: The adapter should not modify the existing adaptee or target interfaces

NFR2: The solution should be simple and maintainable

NFR3: Adapters should be easy to add or remove without affecting other parts of the system

NFR4: Performance overhead introduced by the adapter should be minimal

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Target interface expected by the client

Adaptee interface that is incompatible

Adapter class that implements the target interface and holds a reference to the adaptee

Client code that uses the target interface

Design Patterns

Adapter pattern (object adapter and class adapter variants)

Facade pattern for simplifying complex interfaces

Decorator pattern if additional behavior is needed

Proxy pattern if controlling access is required

Reference Architecture

Client --> Target Interface --> Adapter --> Adaptee

Where:
- Client calls methods on Target Interface
- Adapter implements Target Interface and translates calls to Adaptee
- Adaptee has incompatible interface but provides needed functionality

Components

Target Interface

Interface or Abstract Class

Defines the interface expected by the client

Adaptee

Existing Class with incompatible interface

Provides the actual functionality but with a different interface

Adapter

Class implementing Target Interface

Converts calls from Target Interface to Adaptee's interface

Client

Any

Uses the Target Interface without knowing about the Adaptee

Request Flow

1. Client calls a method on the Target Interface

2. Adapter receives the call and translates it to the Adaptee's method call

3. Adapter calls the corresponding method on the Adaptee

4. Adaptee executes the method and returns the result to the Adapter

5. Adapter returns the result back to the Client

Database Schema

Not applicable for this design pattern as it focuses on interface compatibility and object structure.

Scaling Discussion

Bottlenecks

Multiple adapters increase code complexity and maintenance overhead

Performance overhead if adapters add significant translation logic

Difficulty managing many adapters if interfaces change frequently

Solutions

Use automated tools or code generation to create adapters when possible

Keep adapter logic simple and focused on translation only

Use interface segregation to minimize the number of methods adapters must implement

Document adapters clearly and group related adapters for easier maintenance

Interview Tips

Time: Spend 10 minutes explaining the problem and requirements, 15 minutes designing the adapter pattern with diagrams and code examples, and 5 minutes discussing scaling and trade-offs.

Explain the problem of incompatible interfaces and why adapter pattern is useful

Describe the difference between object adapter and class adapter

Show how the adapter translates calls between client and adaptee

Discuss how this pattern promotes code reuse and flexibility

Mention potential performance impacts and maintenance considerations

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