principle-solidlisted

# SOLID Five heuristics for OO design. Guidelines, not laws. ## S — Single Responsibility *A module should have one reason to change.* Usually that means one *actor* it serves. - **Good:** `InvoiceCalculator` computes totals; `InvoiceRenderer` formats output. - **Bad:** `Invoice` that calculates, renders, emails, and persists. - **Misapplied:** splitting a 20-line class into five 4-line classes. SRP is about change vectors, not line count. ## O — Open-Closed *Open for extension, closed for modification.* New behavior should not require editing stable code. - **Good:** a payment processor iterates `PaymentMethod` implementations; new methods are new files. - **Bad:** a giant `switch` on `paymentType` that every new method edits. - **Misapplied:** introducing a plugin framework for three enum values that will never grow. ## L — Liskov Substitution *Subtypes must be usable wherever the base type is expected, without surprising callers.* - **Good:** `ReadOnlyList` is not a subtype of `MutableList` — don't force it. - **Bad:** `Square extends Rectangle` overriding `setWidth` to also set height. - **Misapplied:** forbidding inheritance entirely. LSP is about honoring contracts, not avoiding hierarchies. ## I — Interface Segregation *Clients should not depend on methods they do not use.* - **Good:** `Reader`, `Writer`, `Closer` composed as needed. - **Bad:** one `IFileSystem` with 40 methods every consumer imports. - **Misapplied:** one-method interfaces for every action, creat