Explain the Open/Closed Principle (OCP). How do you achieve it without modifying existing code?
Open/Closed Principle
The Open/Closed Principle says software entities should be open for extension but closed for modification. In practice, I achieve this by identifying stable abstractions and introducing polymorphism, strategies, factories, plugins or configuration-driven behavior. The goal is not literally to never modify existing code; rather, frequently changing requirements should ideally be accommodated through new implementations instead of repeatedly editing stable, tested logic.
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Use abstractions at points where behavior is expected to vary.
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Use polymorphism instead of large conditional chains.
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Strategy and Factory patterns commonly support OCP.
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Avoid blindly creating abstractions for behavior that is unlikely to change.
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OCP reduces regression risk when adding new variants.
Follow-up Questions
- How does Strategy implement OCP?
- Can OCP be achieved without interfaces?
- Does OCP mean existing code can never be changed?
- How can OCP cause over-engineering?