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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsCommon object-oriented design mistakes are warning signs—not automatic proof of a bug. A class that changes for unrelated reasons, duplicated rules, or objects that know too much about one another may point to low cohesion or harmful coupling. Confirm the maintenance problem first, then make the smallest behavior-preserving change that addresses it.
What counts as an object-oriented design mistake?
A code smell is a symptom worth investigating, not a diagnosis. Martin Fowler defines it as “a surface indication that usually corresponds to a deeper problem in the system” (Code Smell). A smell is not necessarily a bug, and it may be harmless in a particular context.
Many familiar smells relate to two design qualities: cohesion—whether a class’s responsibilities belong together—and coupling—how much one part depends on another. The practical question is whether a current change or maintenance task is harder because of the design. Microsoft’s archived discussion of cohesion and coupling describes divergent change as a class changing for different reasons, which can suggest that separate responsibilities should live apart (Microsoft Learn: Patterns in Practice).
Why is one class doing too much?
A large class may handle several unrelated tasks, or its code may change for distinct reasons. That can make an update in one area collide with or complicate work in another. Class size alone does not establish a problem: do not split a class just to meet an arbitrary line-count target.
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Instead, identify the reasons it changes. If, for example, a class both calculates an order total and formats a report, and those concerns change independently, consider extracting one responsibility into a focused collaborator. Keep the collaboration and interfaces understandable; the goal is a clearer boundary, not more classes for their own sake. (See Microsoft Learn and IBM’s overview of code smells.)
How can you reduce coupling without adding unnecessary layers?
Tight coupling becomes visible when an object relies heavily on another object’s internal data or implementation. A change to that detail can ripple into dependent code, making independent testing or reuse harder. One related smell, feature envy, occurs when a method seems more interested in another object’s data than its own.
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When the behavior naturally belongs with the information it uses, consider moving it there. If there is a real point where implementations need to vary, define a smaller boundary around that variation. Compare possible fixes by asking which one addresses the actual reason for change, improves responsibility clarity, removes rather than adds coupling, avoids unnecessary indirection, and remains easy to test. Do not add interfaces everywhere without a demonstrated need. (See IBM and Microsoft Learn.)
When should duplicated code be consolidated?
Duplicated logic creates a risk that a future correction will be made in one place but missed elsewhere. Consolidation is useful when the repeated code represents the same rule and is likely to change together. Similar-looking code may implement different behavior, so first compare what each copy means and how it is expected to evolve.
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If the copies do represent one rule, factor that rule into a suitable shared abstraction. If they do not, keeping them separate can be clearer than forcing unrelated behavior together. The Object-Oriented Reengineering Patterns reference identifies duplication as a smell and recommends factoring common parts into suitable abstractions; the suitability of the abstraction is the important qualification.
What if inheritance does not fit a subtype’s behavior?
A subclass that does not use inherited behavior may exhibit refused bequest: the hierarchy promises a relationship or capability the subtype does not need. This is a reason to examine the model, not proof that inheritance must be removed.
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Ask whether the subtype genuinely belongs in the hierarchy and whether it needs the inherited contract. Depending on the answer, composition or a narrower contract may describe the behavior better. Those are alternatives to evaluate in context, not universal remedies. IBM includes refused bequest among code smells (IBM: What Are Code Smells?).
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Are repeated switches or temporary fields always a problem?
No. A switch is not automatically bad, and conditionals do not all need to become inheritance. Repeated switches on the same type may indicate that behavior is in the wrong place, while a temporary field—one that is meaningful only in certain situations—may suggest that state belongs to a more specific abstraction.
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Check whether the branches vary according to a stable domain type and whether moving the behavior behind a polymorphic contract would make the design clearer. If the cases are naturally handled in one place and remain understandable, a switch may be the simpler choice. The same test applies to temporary state: change the design only when its conditional use creates real confusion or maintenance difficulty. IBM describes these as smells to recognize, not blanket commands to redesign (IBM).
How do you fix a smell without overengineering?
Patterns and principles are tools for solving a present design problem, not goals to satisfy preemptively. Speculative abstractions add concepts and indirection that maintainers must understand without providing a current benefit. UK Home Office guidance advises keeping code simple, using patterns when appropriate, and refactoring for new use cases when they arise (Write maintainable, reusable and evolutionary code).
Use a short repair loop:
- Name the concrete pain. For example, unrelated changes collide in one class, duplicated rules drift, or an object reaches into another object’s data.
- Protect the behavior that must remain stable. Identify the relevant behavior and make sure tests can check it before restructuring.
- Make one small structural change. Extract a responsibility, move behavior toward the information it uses, consolidate genuinely shared logic, or narrow an oversized contract.
- Run the relevant tests and inspect the result. Confirm the intended behavior remains intact and that the change has addressed the maintenance problem. Stop rather than continuing to abstract for its own sake.
Refactoring means improving the design of existing code without changing system behavior, as the OpenUP/EPF refactoring guideline puts it. A full set of developer tests is needed to apply refactoring safely; tests help check that the intended behavior remains intact. Martin Fowler’s Refactoring book page also emphasizes behavior-preserving transformations and the role of tests.
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