Interface Segregation Principle in LLD - Architecture Diagram

System Overview - Interface Segregation Principle

The Interface Segregation Principle (ISP) is a design guideline that suggests splitting large, general interfaces into smaller, more specific ones. This helps systems avoid forcing components to depend on methods they do not use, improving modularity and maintainability.

Key requirements include designing interfaces that are focused and tailored to client needs, reducing unnecessary dependencies, and enabling easier system evolution.

Architecture Diagram

  +----------------+       +----------------+       +----------------+
  |  Client A      |       |  Client B      |       |  Client C      |
  +-------+--------+       +-------+--------+       +-------+--------+
          |                        |                        |
          |                        |                        |
  +-------v--------+       +-------v--------+       +-------v--------+
  | Interface A    |       | Interface B    |       | Interface C    |
  | (Specific API) |       | (Specific API) |       | (Specific API) |
  +-------+--------+       +-------+--------+       +-------+--------+
          |                        |                        |
          +-----------+------------+------------+-----------+
                      |                         |
               +------v------+           +------v------+  
               | Service X   |           | Service Y   |  
               +-------------+           +-------------+  

Components

Client A

client

Uses Interface A to access specific service features it needs

Client B

client

Uses Interface B to access different specific features

Client C

client

Uses Interface C for yet another set of features

Interface A

interface

Defines a small, focused set of methods for Client A

Interface B

interface

Defines a small, focused set of methods for Client B

Interface C

interface

Defines a small, focused set of methods for Client C

Service X

service

Implements Interface A and Interface B to provide required features

Service Y

service

Implements Interface C to provide its specific features

Request Flow - 9 Hops

Client A → Interface A

Interface A → Service X

Service X → Client A

Client B → Interface B

Interface B → Service X

Service X → Client B

Client C → Interface C

Interface C → Service Y

Service Y → Client C

Failure Scenario

Component Fails:Interface B

Impact:Client B cannot access Service X features through Interface B, causing failure in Client B's specific operations. Other clients and interfaces remain unaffected.

Mitigation:Implement fallback interfaces or degrade Client B features gracefully. Use interface versioning and monitoring to detect and fix issues quickly.

Architecture Quiz - 3 Questions

Why does the system use multiple small interfaces instead of one large interface?

A To make the system slower

B To ensure clients only depend on methods they use

C To force clients to implement all methods

D To reduce the number of services

Design Principle

The Interface Segregation Principle improves system modularity by ensuring clients depend only on the interfaces relevant to them. This reduces unnecessary coupling and makes the system easier to maintain and extend.

Practice

1. What is the main idea behind the Interface Segregation Principle in system design?

easy

A. Interfaces should be as large as possible to cover all features.

B. All classes must implement every method of an interface.

C. Clients should only depend on methods they actually use.

D. Interfaces should be avoided to reduce complexity.

Solution

1. Step 1: Understand the principle's goal

   The Interface Segregation Principle aims to keep interfaces small and focused so clients are not forced to depend on unused methods.

2. Step 2: Match the correct description

   Clients should only depend on methods they actually use. correctly states that clients should only depend on methods they use, which aligns with the principle.

3. Final Answer:

   Clients should only depend on methods they actually use. -> Option C

4. Quick Check:

   Interface Segregation Principle = Clients depend only on used methods [OK]

Hint: Focus on clients using only needed methods [OK]

Common Mistakes:

• Thinking interfaces must be large and cover all features
• Believing all classes must implement every interface method
• Assuming interfaces should be avoided

2. Which of the following interface definitions follows the Interface Segregation Principle?

easy

A. interface Printer { void print(); void scan(); void fax(); }

B. interface Machine { void doEverything(); }

C. interface Device { void operate(); void print(); void scan(); void fax(); }

D. interface Printer { void print(); } interface Scanner { void scan(); }

Solution

1. Step 1: Analyze interface sizes

   interface Printer { void print(); } interface Scanner { void scan(); } splits responsibilities into separate interfaces for printing and scanning, keeping them focused and small.

2. Step 2: Compare with other options

   Options A, B, and C combine multiple unrelated methods into one interface, violating the principle.

3. Final Answer:

   Separate interfaces for each responsibility -> Option D

4. Quick Check:

   Small focused interfaces = interface Printer { void print(); } interface Scanner { void scan(); } [OK]

Hint: Look for interfaces with single focused responsibility [OK]

Common Mistakes:

• Choosing interfaces with many unrelated methods
• Ignoring separation of concerns
• Confusing interface segregation with inheritance

3. Given these interfaces and classes, what will be the output?

interface Printer { void print(); }
interface Scanner { void scan(); }
class MultiFunctionDevice implements Printer, Scanner {
  public void print() { System.out.println("Printing document"); }
  public void scan() { System.out.println("Scanning document"); }
}

MultiFunctionDevice device = new MultiFunctionDevice();
device.print();
device.scan();

medium

A. Printing document\nScanning document

B. Printing document

C. Scanning document

D. Compilation error

Solution

1. Step 1: Understand class implementation

   The class implements both Printer and Scanner interfaces and provides both methods.

2. Step 2: Trace method calls

   Calling device.print() prints "Printing document" and device.scan() prints "Scanning document".

3. Final Answer:

   Printing document\nScanning document -> Option A

4. Quick Check:

   Both methods print messages = Printing document\nScanning document [OK]

Hint: Check which interfaces are implemented and methods called [OK]

Common Mistakes:

• Assuming only one method runs
• Expecting compilation error due to multiple interfaces
• Confusing method outputs

4. Identify the problem in this interface design violating the Interface Segregation Principle:

interface Worker {
  void work();
  void eat();
}

class Robot implements Worker {
  public void work() { System.out.println("Robot working"); }
  public void eat() { throw new UnsupportedOperationException(); }
}

medium

A. Worker interface is too broad, forcing Robot to implement unused methods.

B. Robot should implement eat() properly.

C. Robot class should not implement Worker interface.

D. No problem, this design is correct.

Solution

1. Step 1: Analyze interface methods

   The Worker interface combines work and eat methods, but Robot cannot eat.

2. Step 2: Identify violation

   Robot is forced to implement eat() which it does not support, violating Interface Segregation Principle.

3. Final Answer:

   Worker interface is too broad, forcing Robot to implement unused methods. -> Option A

4. Quick Check:

   Broad interface forcing unused methods = Worker interface is too broad, forcing Robot to implement unused methods. [OK]

Hint: Check if all interface methods apply to all implementers [OK]

Common Mistakes:

• Thinking Robot must implement eat() properly
• Believing no problem exists
• Suggesting Robot should avoid interface

5. You are designing a payment system with interfaces for different payment methods. Which design best follows the Interface Segregation Principle?

hard

A. interface Payment { void payWithCard(); void payWithCash(); void payWithCrypto(); }

B. interface CardPayment { void payWithCard(); } interface CashPayment { void payWithCash(); } interface CryptoPayment { void payWithCrypto(); }

C. interface Payment { void pay(); } class AllPayments implements Payment { void pay() { /* all methods */ } }

D. interface Payment { void payWithCard(); void payWithCash(); } class CryptoPayment implements Payment { void payWithCard() {} void payWithCash() {} }

Solution

1. Step 1: Evaluate interface segregation

   interface CardPayment { void payWithCard(); } interface CashPayment { void payWithCash(); } interface CryptoPayment { void payWithCrypto(); } splits payment methods into separate interfaces, allowing clients to implement only what they need.

2. Step 2: Compare other options

   Options A, B, and D force clients to implement methods they may not use, violating the principle.

3. Final Answer:

   Separate interfaces for each payment method -> Option B

4. Quick Check:

   Separate interfaces = interface CardPayment { void payWithCard(); } interface CashPayment { void payWithCash(); } interface CryptoPayment { void payWithCrypto(); } [OK]

Hint: Split interfaces by payment method for flexibility [OK]

Common Mistakes:

• Combining all payment methods in one interface
• Forcing classes to implement unused methods
• Ignoring interface segregation benefits