SOLID Principles

SOLID is a set of five design principles that reduce coupling, increase cohesion, and make software easier to maintain and extend. Apply them when writing new code and check for violations when reviewing code.

S — Single Responsibility Principle (SRP)

A module, class, or function should have one reason to change.

A "reason to change" means one actor or stakeholder whose requirements could change the code. Code with multiple responsibilities becomes a tangled web where changing one concern breaks another.

Code smells indicating SRP violation

• A class or function with more than one clear purpose (e.g., UserService

that validates input, persists to the database, AND sends emails)

• Functions longer than ~30 lines — length often signals multiple concerns
• and in a function name: validateAndSave(), parseAndFormatDate()
• Multiple, unrelated tests for a single unit

Applying SRP

• Separate concerns into distinct modules: parsing, validation, persistence,

notification each belong in separate classes or functions

• A function should do one thing and name that thing clearly
• If you cannot name a function without using "and", it has too many jobs

O — Open/Closed Principle (OCP)

Software entities should be open for extension but closed for modification.

Adding new behavior should not require changing existing, tested code. Achieve this through abstraction: program to interfaces, not implementations.

Code smells indicating OCP violation

• A long if/else or switch on a type field that must be modified every

time a new type is added

• Hardcoded lists of variants: if type === 'admin' ... else if type === 'user'
• Tests that break whenever a new variant is added to an existing structure

Applying OCP

• Define interfaces or abstract base types for behavior that varies
• Add new behavior by adding new implementations, not modifying existing ones
• Use strategy pattern, plugin pattern, or polymorphism to swap behavior

L — Liskov Substitution Principle (LSP)

Subtypes must be substitutable for their base types without altering program correctness.

If code works correctly with a Shape, it must work correctly with a Rectangle that extends Shape, without the caller knowing which it got.

Code smells indicating LSP violation

• Overriding a method to throw NotImplementedError or do nothing
• Callers checking instanceof before calling methods: if (x instanceof Square)
• Subclasses that weaken preconditions or strengthen postconditions beyond

what the base type promises

• Tests that cannot be run on both the base type and the subtype

Applying LSP

• Design inheritance hierarchies based on behavior, not taxonomy
• Prefer composition over inheritance when the "is-a" relationship does not

hold behaviorally

• A subclass may restrict behavior (e.g., ReadOnlyList cannot mutate) but

must fulfill all contracts the base type advertises

I — Interface Segregation Principle (ISP)

Clients should not be forced to depend on interfaces they do not use.

Fat interfaces force clients to implement or depend on methods they do not need, creating unnecessary coupling.

Code smells indicating ISP violation

• Interface with 10+ methods implemented by multiple classes, each of which

uses only 3 of them

• Classes that implement an interface by throwing UnsupportedOperationException

for several methods

• Test doubles that must stub many irrelevant methods to satisfy an interface

Applying ISP

• Split large interfaces into smaller, focused ones
• A client should only know about the methods it actually calls
• Compose multiple small interfaces when a concrete type needs to satisfy

several contracts

D — Dependency Inversion Principle (DIP)

High-level modules should not depend on low-level modules. Both should depend on abstractions. Abstractions should not depend on details.

Business logic should not import database drivers, HTTP clients, or file system APIs directly. It should depend on interfaces that those details implement.

Code smells indicating DIP violation

• Business logic classes that instantiate their own dependencies with new
• import DatabaseClient from './database' inside a domain service
• Static method calls into infrastructure (Database.query(...),

Clock.now(), Config.get(...)) — static calls have no seam, so tests cannot substitute them

• Singletons fetched from inside business code (Registry.getInstance()) —

the dependency is real but invisible in the signature, so the class lies about what it needs

• Tests that cannot run without real databases, network calls, or file system

access

• Difficult to test without mocking entire subsystems

Applying DIP

• Inject dependencies through constructors or function parameters
• Define interfaces in the domain layer; implement them in the infrastructure

layer

• Pass in collaborators as arguments rather than instantiating them inside

the function

• Construct with collaborators; call with work. The constructor takes

the long-lived collaborators that define what the object IS (its clients, its loggers, its clock, its database handle). Methods take the per-call work parameters. A ReportGenerator(reportingDb, clock) can serve many date ranges via generate(startDate, endDate). A ReportGenerator(reportingDb, clock, startDate, endDate) creates a new instance per query and conflates identity with work.

Applying SOLID in the Implementer Role

When writing new code:

1. Before writing: Ask "what is this unit's single responsibility?" If the

answer contains "and", split it.

1. When adding behavior: Ask "can I add this without modifying existing

tested code?" Use abstractions where extension is expected.

1. When using inheritance: Verify that subtypes honor the base type's

contracts. Prefer composition when uncertain.

1. When defining interfaces: Define only what callers need. Split when

multiple distinct clients use the same interface differently.

1. When using dependencies: Inject them. Do not instantiate infrastructure

inside domain logic.

Applying SOLID in the Reviewer Role

When reviewing code for SOLID violations:

• Flag SRP violations by name: `issue: SRP violation — this function handles

both input validation and database write.`

• Flag OCP violations when new behavior requires modifying an existing

switch or if/else chain.

• Flag LSP violations when subtypes override methods to throw or do nothing.
• Flag ISP violations when interfaces force clients to depend on methods they

do not use.

• Flag DIP violations when business logic instantiates its own dependencies.

Every SOLID finding should cite the specific file and line, name the principle, and explain the consequence: why does this violation matter for this codebase right now?