Mediator, Iterator & Null Object — low level design interview guide

Mediator, Iterator & Null Object is a depth lesson in the Observer, Decorator & Behavioral family of the Thita low level design course.

#3 most tested pattern: Observer for event systems, Decorator for middleware chains, Command for undo/redo.

It sits on the Deep Dive path — worth reading once the Interview Soon lessons are solid, or when a role names this area explicitly.

The rest of Observer, Decorator & Behavioral

• Observer Pattern — Core lesson.
• Decorator & Chain of Responsibility — Core lesson.
• Case Study: Command Pattern & Text Editor — Core lesson, worked case study, about 45 minutes, asked at Google, Microsoft, Adobe.
• Mediator, Iterator & Null Object — Depth lesson.

Other low level design pattern families

• OOP Foundations & Class Design — OOP pillars, SOLID principles, entity modeling and class extraction. 3 lessons · about 2.5 hours · Interview Soon.
• Strategy & Factory Patterns — #1 and #4 most tested patterns: eliminate if/else chains with Strategy and create objects without specifying exact classes with Factory. 4 lessons · about 2 hours · Interview Soon.
• State Pattern & State Machines — #2 most tested pattern: state transition tables, vending machine case study embedded. 3 lessons · about 2 hours · Interview Soon · 1 worked case study.
• Concurrency Patterns — Race conditions, locks, BlockingQueue, thread pools, and pool sizing. 4 lessons · about 2 hours · Interview Soon.
• Machine Coding & Interview Execution — Three LLD round formats, time budgets, project structure, and pattern recognition. 2 lessons · about 1 hour · Interview Soon.
• Case Studies — Foundational — Top 6 most-asked multi-pattern LLD problems: Parking Lot, Elevator, BookMyShow, Splitwise, Snake & Ladder, Rate Limiter. 6 lessons · about 5 hours · Interview Soon · 6 worked case studies.
• Case Studies — Applied — Advanced combos and data structure design: LRU Cache, Notification System, Chess, Food Delivery. 4 lessons · about 3 hours · Interview Soon · 4 worked case studies.
• Advanced Case Studies — Thread-safe designs, distributed patterns, and domain-heavy problems for SDE-2+ interviews. 8 lessons · about 8 hours · Deep Dive · 8 worked case studies.
• Domain-Specific Patterns — Financial systems, progressive design (Coinbase-style), and game engine patterns. 3 lessons · about 3 hours · Deep Dive.
• Machine Coding Mastery — Company-specific machine coding prep and advanced architectural patterns. 2 lessons · about 2 hours · Deep Dive.
• Architecture Teardowns — Deep dives into Spring IoC, Java Collections, and Python data model internals. 3 lessons · about 3 hours · Deep Dive.

Related interview preparation

• Looking for the practice sheet? — Track your progress against every family as a checklist, with company tags and AI interview practice.
• System Design course — The architecture-level companion: APIs, storage, caching, queues and scaling.
• DSA patterns course — The coding round: the algorithmic patterns behind the problems asked alongside design.
• AI mock interviews — Practise explaining a class diagram out loud against an interviewer that asks follow-ups.