Design Principles Fundamentals
Fast recall
Quick revision
Core answer
Interview-ready guide
Interview answer
Design principles are guidelines that help a team make code easier to understand and change. SOLID covers responsibilities, extension points, subtype contracts, focused interfaces, and dependency direction. DRY addresses duplicated knowledge, while KISS and YAGNI challenge unnecessary complexity and speculative functionality.
For example, repeated payment branches may suggest Strategy and OCP, but an interface for a behavior with one stable implementation may add no value. Likewise, extracting two similar lines may satisfy a shallow view of DRY while coupling two business rules that need to evolve separately.
The principles are not laws and do not replace tests or domain understanding. I apply them to a concrete change: identify what varies, who owns the rule, which contract clients rely on, and whether an abstraction reduces the real cost of change. The best design is the simplest one that preserves the required behavior and leaves known changes manageable.
Design Principles Fundamentals
Direct Answer
Design principles such as SOLID, DRY, KISS, and YAGNI are guidelines for placing responsibilities, dependencies, duplication, and complexity. They help developers discuss why code is hard to change and choose safer boundaries. They are context-dependent heuristics, not laws or proof that a design is good.
Check your understanding
Answer these without looking back at the article.
- What does SOLID stand for, and what does each principle prevent?
- How do OCP and DIP work together to add a new feature without editing existing code?
- When does DRY conflict with KISS, and how do you resolve it?
- What does YAGNI prevent, and when is it safe to build for a future requirement?
- What is a god-class, and how does SRP fix it?
Looking for a structured roadmap?
Every module on this page is also part of the full Java backend interview track.