A design principle is a general guideline for structuring software so it becomes more flexible, reliable, maintainable, and easier to test. It improves a few lines of code, a function, a class, or cooperating classes. section 1.0 section 1.9
Use design principles to address the central problems of software design:
- Handle changing requirements by isolating changes so one modification does not force unrelated code changes. section 1.1 section 1.7
- Control complexity by making tradeoffs between flexibility and the manageability of the design. section 1.7
- Support encapsulation and implementation hiding, limiting public access and reducing dependencies between classes. section 5.0 section 1.9
- Provide the foundation for design patterns, which address common software architecture problems at a higher level. section 1.0 section 5.0
Refactoring toward a principle
Start with a before-design program, identify how a requirement change or class dependency could spread through the code, and then refactor incrementally. Compare the after-design version to see how implementation hiding, loose coupling, or another named principle limits the change. If a design decision proves poor, backtrack and continue iterating rather than expecting a single final coding pass. section 1.2 section 2.0 section 5.0
Good design principles are applied during requirements analysis and repeated development iterations; they are practical tools rather than rigid rules. Their value lies in balancing flexibility, complexity, and the need to recover from bad decisions. section 1.7 section 1.9 section 2.0
