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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallWhat are the SOLID principles in low-level design? They are five object-oriented design principles that help you decide how to divide responsibilities, handle change, preserve behavior, scope interfaces, and direct dependencies. Their value is not in creating more classes; it is in making the consequences of a design easier to reason about as code evolves.
That shifts the starting question from “Which class should I create?” to “What is changing, who owns that behavior, and what should its collaborators be allowed to rely on?”
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What SOLID changes about low-level design
Low-level design is often treated as a naming exercise: identify nouns, turn them into classes, and connect the classes. Responsibility-driven design asks a more useful set of questions: what should each object do, and which collaborators does it need? The goal is to distribute responsibilities so related behavior stays together without making one object responsible for unrelated work. A University of Bern lecture presents design methods as guidelines, not fixed rules, and places SOLID within object-oriented design: University of Bern lecture materials.
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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteSOLID names five pressures to consider when choosing those boundaries. It is most useful when a system changes over time, different groups drive different changes, or dependencies need to be substituted for testing. It is not a checklist that every small class must visibly satisfy.
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What the five SOLID principles mean
Single Responsibility: group behavior around a coherent reason to change
Single Responsibility (SRP) is commonly summarized as: “A module should have one, and only one, reason to change.” — Robert C. Martin, as quoted by the SE Book. The important word is not “one” in isolation, but “reason.” A class with many methods can still have one coherent responsibility; a class with only a few methods can mix unrelated concerns.
Consider an order workflow that validates a purchase, calculates its total, saves it, and sends a receipt. If tax rules change for one business reason while receipt formatting changes for another, putting both concerns in a single workflow object can make unrelated changes collide. SRP asks whether those changes come from distinct actors or needs, and whether the behavior belongs together—not whether every operation deserves its own class.
Open/Closed: make likely variation possible without repeatedly rewriting stable policy
Open/Closed (OCP) is phrased as: “Software entities should be open for extension, but closed for modification.” — Robert C. Martin, as attributed by Design Principles. In practice, this means identifying a real variation point and giving new behavior a place to fit, rather than adding conditionals throughout stable code every time a new case appears.
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For example, if an order workflow will support different receipt-delivery methods, a focused delivery abstraction may let another method be introduced without rewriting purchase validation. OCP does not mean code can never be changed; it means avoid making likely extensions depend on risky edits to policy that is already stable.
Liskov Substitution: preserve what callers expect
Liskov Substitution (LSP) concerns whether one implementation can stand in for another without breaking the correctness callers rely on. Martin’s attributed formulation is: “Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.” — Design Principles.
A type relationship is not sound merely because one class inherits from another. If a caller expects a storage implementation to save an order and retrieve it under the documented conditions, an alternate implementation must preserve those expectations. A subtype that silently rejects valid inputs, changes the meaning of a successful result, or requires callers to know special rules is not a safe substitute.
Interface Segregation: expose only what a client needs
Interface Segregation (ISP) says clients should not be forced to depend on operations they do not use. Martin’s attributed shorthand is: “Many client-specific interfaces are better than one general-purpose interface.” — Design Principles.
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Dependency Inversion: keep policy from depending directly on infrastructure
Dependency Inversion (DIP) directs dependencies toward abstractions rather than making core policy depend on concrete low-level details. Martin’s attributed summary is: “One should depend upon abstractions, rather than concrete implementations.” — Design Principles.
In the order example, the workflow can depend on a small persistence contract instead of constructing a particular database adapter itself. The database implementation then fulfills that contract. This separates business policy from storage mechanics and can make substitution for testing possible where that is useful. An abstraction does add indirection, so it should earn its place through a real change, substitution, or testing need.
How to apply SOLID to an order workflow
Use the principles as questions about design pressure, not as a sequence of mandatory refactors. A modest order workflow illustrates how the questions connect:
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- Find distinct reasons for change. Ask which business needs or actors could request changes to those behaviors. If receipt formatting and purchase validation change for unrelated reasons, consider whether they belong in separate responsibilities (SRP).
- Choose a clear owner for each behavior. Keep related rules together, and give each object only the collaborators needed to perform its responsibility. This is responsibility-driven design: objects do work and collaborate rather than serving only as labeled data containers.
- Identify likely variation before adding extension points. If a genuinely expected new delivery method would otherwise require edits to stable workflow policy, define a focused way to supply that behavior (OCP). Do not build a framework for hypothetical variants without a reason.
- Define client-sized contracts. Give the workflow only the persistence or delivery operations it actually uses (ISP). Check that every implementation of a contract honors the behavior its callers expect (LSP).
- Point policy at the contract, not the adapter. Let the workflow depend on the relevant abstraction and provide a concrete storage or delivery implementation at an appropriate boundary (DIP). Keep this indirection only if the flexibility or test seam is valuable.
These choices reinforce one another, but they are not interchangeable. SRP helps decide whether responsibilities belong together; ISP limits what a client sees; DIP controls the direction of dependency; LSP protects behavior when implementations vary; and OCP helps contain the cost of likely extensions.
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How to compare two plausible designs
When deciding whether a design boundary is helping, compare it with the simpler alternative against the pressures that actually matter:
- Responsibility and cohesion: do related changes land together, or do unrelated actors repeatedly edit the same module?
- Change cost: can a likely new behavior fit at a clear extension point, or does it require risky edits through stable policy?
- Substitutability: can callers use another implementation without hidden preconditions or surprising changes in behavior?
- Interface scope: does each client depend only on the operations it needs?
- Dependency direction and testability: does high-level policy rely directly on infrastructure, and is substituting that infrastructure useful for the design or its tests?
- Abstraction cost: is the extra interface or collaborator addressing a real source of change, or only a speculative future?
When SOLID adds more structure than value
SOLID can make code less simple if applied mechanically. The SE Book cautions that the principles can harm simplicity in throwaway code or where only one implementation is expected. A disposable prototype, a one-off script, or a simple value object may not benefit from extra interfaces and collaborators.
Use the smallest design that keeps the current behavior understandable and makes credible changes manageable. Introduce an abstraction when a real variation, dependency boundary, or testing need justifies it; avoid adding one solely to demonstrate a principle. A single implementation is not automatically a reason to reject an interface, but absent a meaningful change or substitution need, the indirection may be unnecessary.
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