SOLID Principles in Java — Practical Application and Trade-offs

See full structured answer at https://interviewexplainer.com/design-patterns-interview-questions/solid-principles-java

Singleton Pattern — Thread-Safe Implementations (DCL vs Enum)

See full structured answer at https://interviewexplainer.com/design-patterns-interview-questions/singleton-thread-safe-java

Factory Method vs Abstract Factory — When to Use Each

See full structured answer at https://interviewexplainer.com/design-patterns-interview-questions/factory-method-vs-abstract-factory-java

Builder Pattern in Java — Fluent API Design

See full structured answer at https://interviewexplainer.com/design-patterns-interview-questions/builder-pattern-fluent-api-java

Decorator Pattern — Adding Behavior Without Subclassing

See full structured answer at https://interviewexplainer.com/design-patterns-interview-questions/decorator-pattern-java-behavior-without-subclassing

Proxy Pattern — How Spring AOP Implements It

See full structured answer at https://interviewexplainer.com/design-patterns-interview-questions/proxy-pattern-spring-aop-java

Observer Pattern — How Spring Events Use It

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Architecture & Design·Interview Q&A

Design Patterns & SOLID interview questions

18+ curated questions with the same answer shape every time: quick recap, deep dive, pitfalls, and follow-ups. Coverage includes solid principles, creational patterns, structural patterns, behavioral patterns, and more.

18questions

9topics

1161difficulty

Open question 1 How to read this module

Suggested path

Read Design Patterns & SOLID from question 1 to 18

Topics follow a teaching order. Each answer ends with a clear Next question control so you can move forward without hunting the list.

1Start with question 1 — it sets up the rest of the module.
2Use Continue · Next question at the bottom of every page (and the arrows in the sticky bar when you scroll).
3Jump to any topic in the full catalog — every row is numbered in reading order (Q1–Q18).

Open question 1 Jump to Q1 in list Scroll to full catalog

About this page

Design-pattern interviews test whether you can apply a pattern to a real problem, not just name it. This module is an elevation arc: it opens with SOLID (the design-principle foundation), then the three GoF categories — creational, structural, behavioral — in order of how often they're asked. From there it moves to refactoring and technical debt (the applied tier: when patterns meet legacy code), then the legacy-patterns and comparison canon. It closes with scenario-based questions where you choose the right pattern for a stated problem.

Reviewed by senior engineers·Updated for 2026·Free · No sign-up required

Part ofLow-Level Design (LLD)Software Architecture & Design Patterns

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Topics are ordered for learning. Within each topic, rows are numbered 1–18 globally so you always know where you are. Collapse topics if you want a lighter view — use Expand all when you need the full list again.

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Design Patterns & SOLID

Numbers 1–18 follow the order we recommend — click any row to open the full answer, or use the sticky Next link at the bottom of each page to go Q1 → Q18.

18 questions·9 topics

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11Refactoring Legacy Java Code: Safety Nets, Techniques, and Refactor-vs-Rewrite DecisionsMedium8m

12Design Patterns as Technical Debt: Over-Engineering, Pattern Soup, and How to Unwind SafelyMedium8m

17GoF Patterns vs Modern Java: Strategy vs Lambdas, Singleton vs DI, Factory vs Builder, Observer vs Reactive, Command vs EventsMedium8m

18Scenario: Refactoring a God Class Using SOLID, Patterns, and StranglerHard8m

Continue your prep

Interviewers rarely ask Design Patterns & SOLID in isolation. These are the modules that most often come up in the same loop.

Most relevant next step52+ Q&A

Java OOP & SOLID Interview Questions

Java OOP interview questions are the single most common opener in any backend interview, from junior to staff. This module is organized as an elevation arc so a learner reads top-to-bottom and feels the level rise. T0 Fundamentals grounds you in what OOP is and why it exists as a response to procedural programming's failures, then walks the four pillars. T1 Core Mechanics covers the daily building blocks - this/super, constructor chaining, overloading vs overriding, static. T2 Internet-Famous is the canon nearly every interview asks: SOLID (the #1 most-asked design-principle set), abstract class vs interface, composition vs inheritance, cohesion and coupling, the Law of Demeter, and the equals/hashCode contract. T3 Applied Patterns moves to code you actually ship - default/static interface methods, the diamond problem, association/aggregation/composition, enums, immutability. T4 Production & Edge Cases separates a working developer from someone who's shipped: shallow vs deep copy, marker interfaces, SerialVersionUID, inner classes, anonymous vs lambda, the Object class methods. Every answer comes with a short code snippet, the trade-off the interviewer is listening for, and the follow-up they are likely to ask next.

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Architecture Patterns & Principles

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Low-Level Design (LLD)

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System Design Fundamentals

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FAQs about Design Patterns & SOLID interview prep

QHow many Design Patterns & SOLID interview questions are on this page?

This module bundles 18+ Design Patterns & SOLID interview questions, grouped by 9 topics. Every answer follows the same structure — one-liner summary, structured deep-dive, common pitfalls, and realistic follow-ups an interviewer might ask.

QIs this Design Patterns & SOLID content up to date for 2026?

Yes. Every answer is reviewed by senior engineers and updated for current best practices, version-specific APIs, and the scenario questions hiring managers are asking today.

QWhich difficulty level are these Design Patterns & SOLID questions?

The set spans all three levels — 1 easy, 16 medium, and 1 hard questions. Junior candidates should focus on the conceptual questions; senior candidates should drill the scenario and trade-off questions in every topic group.

QWhich question should I read first?

Start with question 1 in the ordered list below ("Design Patterns Fundamentals") — it is the recommended entry point. Then use the Next question link at the bottom of each answer page to read straight through to Q18.

QWhat else belongs in a Software Architecture & Design Patterns prep plan?

Design Patterns & SOLID is one module inside the broader Software Architecture & Design Patterns hub — open it to drill the full set of sibling modules with the same structured answer format.

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