Design Patterns Fundamentals
Fast recall
Quick revision
Core answer
Whether you understand patterns as named, force-driven solutions to recurring design problems — and can name the force each resolves and when NOT to apply it.
Memorizing the 23 GoF patterns and applying them everywhere ('I'll use a Singleton') without naming the force the pattern resolves or the cost it adds.
Group patterns by intent — creational (how to make objects), structural (how to compose them), behavioral (how to assign responsibility) — and for each, name the force it resolves and the cost it adds, so you can also say when NOT to apply it.
Interview-ready guide
Interview answer
Design patterns are reusable, named solutions to recurring problems in software design, and they exist so a team can communicate at a higher level — 'use a Strategy here' — instead of re-deriving the design every time.
I group them by intent: creational for how objects are made, structural for how they're composed, and behavioral for who has responsibility.
The thing I always stress is that every pattern resolves a specific force and adds a cost — a Singleton buys a single instance but adds global state, a Decorator buys composable behavior but adds layers — so the decision is whether the force is real enough to justify the cost. And underneath them are the SOLID principles; Strategy is Open/Closed in action, dependency injection is Dependency Inversion in action.
The most common mistake I see is over-applying patterns just because they're famous, which adds indirection that makes code harder, not easier, to change.
Design Patterns Fundamentals
Direct Answer
Design patterns are reusable, named solutions to common problems in object-oriented design — proven arrangements of classes and objects that resolve a recurring force (creating objects, decoupling behavior, varying algorithms). They exist so a team can communicate design at a higher level ('use a Factory here', 'this is a Strategy') instead of re-deriving each solution from first principles, and so that proven, reviewed structures are preferred over ad-hoc ones that look fine but fail under extension.
Common Mistakes & Pitfalls
Memorizing the 23 GoF patterns and applying them everywhere ('I'll use a Singleton') without naming the force the pattern resolves or the cost it adds.
Check your understanding
Answer these without looking back at the article.
- When does the Strategy pattern justify its cost over a single method with a switch statement?
- How does the Decorator pattern realize the Open/Closed principle, and when does it violate simplicity?
- Why is a Singleton often an anti-pattern, and what's the modern alternative for a single instance?
- How do SOLID principles and design patterns relate — which principle does each pattern implement?
- What is the cost of mislabeling a static factory method as a 'Factory pattern' in a codebase?
Related interview prep categories
2Want a guided curriculum order?
These modules are also sequenced in our flagship backend interview track — open it when you prefer an ordered path instead of browsing by topic.