bostonaholic SOLID Principles
Apply when writing new code and reviewing diffs.
S — Single Responsibility
One reason to change: one actor or stakeholder. Smells: multiple purposes;
and in validateAndSave(); functions past ~30 lines; unrelated tests for one unit.
If you cannot name a function without "and", it has too many jobs.
O — Open/Closed
Open for extension, closed for modification. Smells: switch/if-else on
types, hardcoded variants, or tests broken by a new variant. Define an interface
for varying behavior; add implementations instead of branches.
L — Liskov Substitution
Subtypes substitute for base types. Smells: NotImplementedError, no-op
overrides, caller instanceof, weaker preconditions, stronger postconditions,
or tests that cannot cover both. Model behavior, prefer composition when “is-a”
fails, and keep every advertised contract, including for ReadOnlyList.
I — Interface Segregation
No client depends on unused methods. Smells: a 10-method interface whose
implementers use three, UnsupportedOperationException, or irrelevant stubs.
Split interfaces; compose small contracts.
D — Dependency Inversion
High- and low-level modules depend on abstractions. Business logic never
imports database, HTTP, or filesystem APIs directly. Smells: constructing
dependencies, infrastructure imports, required real services, static
Database.query(...)/Clock.now()/Config.get(...), or hidden
Registry.getInstance() singletons.
Construct with collaborators. Call with work. Inject long-lived clients,
loggers, clocks, and DB handles; pass per-call inputs to methods. Prefer
ReportGenerator(reportingDb, clock).generate(startDate, endDate) over putting
the date range in the constructor.
Reuse ReportGenerator(reportingDb, clock) across calls; constructing ReportGenerator(reportingDb, clock, startDate, endDate) per range is the smell.
In the reviewer role
Name the principle, cite file:line, and state the current consequence:
issue: SRP violation — this function handles both input validation and the database write.
A finding that names no principle and no consequence is not actionable.