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Object-oriented programming concepts (classes, the four pillars, object relationships, SOLID, coupling and cohesion, Law of Demeter), with a reference and runnable examples per concept. Use when a question names an OOP concept or SOLID principle, models a domain with classes and interfaces, compares concepts, or asks to review or refactor class designs.
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Object-Oriented Programming Paradigm
Ground every OOP answer in this skill's reference files, not in improvised definitions. Reuse behavior through composition and interfaces; reserve inheritance for true subtype specialization.
Workflow
For class-design reviews or refactoring, first read review and refactoring for caller-based evidence, invariant checks, and migration boundaries.
- Identify the concept(s) in the catalog below, by name or by symptom: a subclass that breaks where its base is expected, a god class, a deep hierarchy, a leaky abstraction, a train-wreck call chain. Done when every concept the question names or implies is listed, or when the problem is better served by plain functions and data; then answer with those.
- Read each identified reference at
references/<concept>.mdin full. When comparing concepts, read every one compared and contrast their intent, not their structure. - Read the language example at
examples/<concept>/<concept>.<ext>for the user's language. For C, Go, Rust, or another language without an example, use the reference's Language Notes for the idiomatic mapping. - Answer with what the concept means, when it applies, the simpler alternative and when it wins, and code adapted to the user's domain. Write the language's idiomatic form rather than a textbook Java-style class diagram: protocols and duck typing in Python, structs and protocols in Swift, closures where a one-method class would be ceremony. When reviewing code, name the violated principle and show the minimal refactor of the user's code. Done when every identified concept has all four parts.
Concept Catalog
Building Blocks
The vocabulary of object-oriented code: what classes, objects, and contracts are.
| Concept | Intent | Reference |
| Classes & Objects | Bundle state and behavior into a blueprint; instantiate independent objects from it | references/classes-and-objects.md |
| Interfaces & Abstract Classes | Separate a contract from its implementations; share partial implementation safely | references/interfaces-and-abstract-classes.md |
The Four Pillars
The defining properties of the paradigm.
| Concept | Intent | Reference |
| Encapsulation | Hide internal state behind a controlled interface that preserves invariants | references/encapsulation.md |
| Abstraction | Expose a component's essential model while hiding operational detail | references/abstraction.md |
| Inheritance | Derive specialized classes from a base, reusing and extending its behavior | references/inheritance.md |
| Polymorphism | Let one call site work across many concrete types (subtype, parametric, ad hoc) | references/polymorphism.md |
Object Relationships
How objects reference, own, and assemble each other.
| Concept | Intent | Reference |
| Object Relationships | Distinguish association, aggregation, and composition by ownership and lifetime | references/object-relationships.md |
| Composition over Inheritance | Build behavior by assembling parts instead of extending deep hierarchies | references/composition-over-inheritance.md |
SOLID Principles
Five principles for class design that keep change local.
| Principle | Intent | Reference |
| Single Responsibility | Give each class one reason to change | references/single-responsibility.md |
| Open/Closed | Make code open for extension without modifying what already works | references/open-closed.md |
| Liskov Substitution | Ensure subtypes are usable anywhere their base type is expected | references/liskov-substitution.md |
| Interface Segregation | Prefer many small client-specific interfaces over one fat interface | references/interface-segregation.md |
| Dependency Inversion | Make high-level policy depend on abstractions, not concrete implementations | references/dependency-inversion.md |
Design Qualities
Cross-cutting qualities used to judge an object-oriented design.
| Concept | Intent | Reference |
| Coupling & Cohesion | Minimize dependence between modules and maximize relatedness within them | references/coupling-and-cohesion.md |
| Law of Demeter | Talk only to immediate collaborators instead of reaching through object graphs | references/law-of-demeter.md |
Examples
Examples are self-contained and runnable, in the languages with first-class classes: Python, C++, Java, C#, JavaScript, PHP, and Swift.
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