Java OOP & Design Patterns

Java OOP & Design Patterns is a free practice course on Abekus built around 1,600+ MCQs covering the design half of Java — the SOLID principles, the interface-vs-abstract-class decision, inheritance vs composition, every Gang-of-Four pattern (Creational, Structural, Behavioral) in idiomatic Java, plus the anti-patterns and code smells reviewers flag in pull requests.

The format is one question at a time with an explanation that fires the moment you click a wrong answer. No videos, no passive reading — retrieval practice on the pattern-selection and refactoring decisions you make every day in production code. You will pick between Adapter and Decorator under pressure, recognise a Strategy pattern hidden inside a switch statement, and trace why a Singleton with mutable state is a thread-safety landmine.

• Format — 1,600+ free MCQs with instant explanations after every wrong answer.
• Duration — about 17.9 hours of focused practice; most learners finish over 4–5 weeks at 45–60 minutes a day.
• Level — intermediate. Assumes Java fundamentals (classes, inheritance, interfaces, generics, exceptions). No prior design-pattern experience required.
• Cost — free, with a free completion certificate.
• Audience — working Java developers stepping up to senior roles, candidates preparing for product-company Java interviews where OOP-design rounds are standard, engineers preparing for system-design interviews where pattern vocabulary is table stakes, and college students cleaning up the OOP intuition from their semester courses.
• Recommended before this — Java Fundamentals (covers the language ground this one assumes).
• Companion comparison — Design Patterns: Behavioral Patterns & Mastery goes deeper on the behavioral half in a language-agnostic way; this course is the Java-specific complement.

Four audiences specifically. First, working Java developers stepping up to senior roles where the design-round ("design a parking lot", "design a notification service") expects pattern vocabulary on tap — Strategy, Observer, Factory, Builder, Decorator — and the SOLID intuition to apply them without ceremony. Second, candidates preparing for product-company Java interviews where OOP-design rounds are a standard filter and questions like "refactor this nested-if mess" or "why does this Singleton break under concurrency" come up verbatim. Third, engineers preparing for system-design interviews where being able to name and apply a pattern correctly is the difference between sounding senior and sounding cargo-culted. Fourth, engineering students who learned OOP in a semester course and want to convert dictionary-definition recall ("polymorphism is when subclass...") into production intuition.

This course assumes Java basics. If you do not yet know how interfaces and abstract classes differ syntactically, or what an exception chain looks like, work through Java Fundamentals first. The first topic introduces OOP and design patterns at a conceptual level, but the snippets from Topic 2 onwards expect comfort reading 20-line Java classes.

Foundations — principles before patterns

• Introduction to OOP & Design Patterns — OOP principles overview (encapsulation, inheritance, polymorphism, abstraction) and the design-patterns landscape (Gang of Four, when patterns help, when they hurt).
• OOP Principles: SOLID — Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, Dependency Inversion. Each principle drilled with code that violates it and code that doesn't.
• Interfaces, Abstract Classes & Polymorphism — interface vs abstract class (and the default-method era after Java 8), polymorphism deep dive (dynamic dispatch, covariant returns), inheritance vs composition.

The pattern catalogue — every GoF pattern in idiomatic Java

• Creational Patterns — Singleton (and when to avoid it; the enum singleton; thread-safety variants), Prototype, Factory Method, Abstract Factory, Builder (including the modern fluent-API Builder).
• Structural Patterns I — Adapter (wrap incompatible interfaces), Decorator (add behaviour without inheritance), Proxy (controlled access, lazy loading, remote stubs).
• Structural Patterns II — Facade (subsystem simplification), Composite (tree-of-uniform-nodes), Bridge (decouple abstraction from implementation), Flyweight (sharing for memory efficiency).
• Behavioral Patterns I — Strategy (algorithm selection at runtime, often replaced by lambdas in modern Java), Observer (event broadcast and the modern Reactive variants), Template Method (skeleton algorithm with hooks).
• Behavioral Patterns II — Command (encapsulated requests, undo/redo), State (state machines without if/else chains), Chain of Responsibility (handler pipelines for filters, middleware).
• Behavioral Patterns III — Iterator (already part of the language), Mediator and Memento (controlled state and rollback), Visitor (double dispatch), Interpreter (small DSLs).

Production wisdom — anti-patterns and applied judgement

• Anti-patterns & Code Smells — God Object, Spaghetti Code, Premature Optimisation, Long Method, Feature Envy, Shotgun Surgery; the canonical code smells from Fowler's Refactoring book; mechanical refactoring toward the right pattern.
• Java OOP & Design Patterns in Practice — concept comparisons (Strategy vs State; Adapter vs Facade; Builder vs Factory), pattern selection (which one for which problem), and OOP-and-patterns in real production code.

Both, but in the order this course takes them — SOLID first, patterns second. SOLID is a set of design principles that tell you what good OOP looks like (single responsibility, open for extension, substitutable subtypes, segregated interfaces, depend on abstractions). They are the why. Design patterns are repeatable solutions to specific design problems — Strategy, Observer, Decorator, etc. They are the how. Apply patterns without SOLID intuition and you ship over-engineered code; apply SOLID without pattern vocabulary and your code is fine but you cannot communicate it in a code review or a senior interview. Both are tools; this course teaches both with the principles first so the patterns land on the right intuition.

Active recall on real refactoring examples, not skimming the GoF book once. Most working Java developers can recite "Singleton ensures one instance" but cannot recognise a Strategy pattern hidden inside a switch statement, refactor a nested-if mess into a State machine, or pick between Adapter and Facade when both could work. MCQ practice forces production every question: identify the pattern, refactor the smell, pick between two patterns under a specific constraint.

Spacing matters too. 45–60 minutes a day for 4–5 weeks beats one cram weekend, because design-pattern selection has enough subtle distinctions (Strategy vs State, Adapter vs Decorator, Bridge vs Adapter) that each one only sticks after multiple rephrasings.

One question at a time. You read a snippet (often a 10–20 line class hierarchy or a refactoring before/after), pick the option matching the pattern, the violation, or the right refactor, and submit. Wrong answers trigger an explanation that walks through the underlying principle — the SOLID violation, the pattern's intent, the trade-off the pattern carries, the simpler alternative when the pattern is overkill.

The Abekus AI guide watches which topics you keep getting wrong and surfaces them more often. If you nail Creational patterns but keep mis-identifying the difference between State and Strategy, the next session leans on Behavioral patterns. The certificate is gated on completing every top-level topic, so the depth signal is honest.

Plan on roughly 17.9 hours of focused practice. The math: 1,613 active questions × ~40 seconds per question (reading the snippet, picking an answer, skimming the explanation when wrong) = ~1,075 minutes = ~17.9 hours.

Most learners spread that across 4–5 weeks at 45–60 minutes a day. Some compress it into two long weekends; that works for refresh mode but retention is weaker. Trust the spacing — pattern-selection distinctions (Strategy vs State, Adapter vs Bridge, Builder vs Factory) only stick after seeing them rephrased multiple ways.

This course assumes Java Fundamentals — classes, inheritance, interfaces, generics, exceptions. If you have not done Fundamentals yet, do that first and come back. After this course, the rest of the Java Mastery track on Abekus continues with Collections, Streams, Concurrency, and JVM internals; the senior-interview overlay is Java Interview Mastery, which revisits OOP and patterns as trap questions.

For learners interested in design patterns from a more language-agnostic angle, Design Patterns: Behavioral Patterns & Mastery is a useful adjacent — it goes deeper on the behavioral half without tying examples to Java syntax, which sharpens pattern intuition you can carry across languages. Most senior engineers benefit from both this Java-specific course and the cross-language one taken together.