Design Patterns & SOLID in Practice

The design patterns used in real Java code and in interviews, applied together with SOLID principles.

This section is being written. There are no lessons in it yet: below is the plan for what it will cover. It opens in the syllabus as soon as its first lessons are ready.

What you will be able to do

Apply SOLID principles when designing new classes, not only when explaining them
Recognise and implement the creational, structural and behavioural patterns used most in Java
Spot these patterns inside the JDK and Spring
Choose a pattern for a reason and avoid forcing one where it does not fit
Solve common low-level design interview problems with clean class designs

What it will cover

19.1SOLID Recap & Design Thinking
A quick refresh of SOLID from OOP, then how principles lead to patterns.
- SOLID in five short examples
- Program to interfaces
- Favour composition over inheritance
- What a design pattern is and is not
- Reading UML class diagrams
19.2Creational Patterns
Control how objects are created so code does not depend on concrete classes.
- Singleton: thread-safe versions and why Spring beans replace it
- Factory Method and simple factories
- Abstract Factory
- Builder for objects with many fields
- Prototype and copying (brief)
19.3Structural Patterns
Combine classes and objects into larger structures without tight coupling.
- Adapter: wrapping an incompatible API
- Decorator: adding behaviour, as in Java I/O streams
- Proxy: lazy loading, access control and Spring AOP
- Facade: one simple entry point
- Composite: trees of objects
19.4Behavioural Patterns
Organise how objects talk to each other and share work.
- Strategy: swapping algorithms, with lambdas
- Observer: events and listeners
- Template Method: fixed steps, custom details
- Command: actions as objects, undo
- Chain of Responsibility: request filters
- State: objects that change behaviour with status
19.5Patterns in the JDK & Spring
See where the patterns you learned already appear in tools you use.
- Iterator and Comparator as patterns
- Builders in HttpClient and Stream.Builder
- Spring beans, factories and dependency injection
- Template classes: JdbcTemplate and RestClient
- Proxies behind @Transactional
- Filter chains in Spring Security
19.6Choosing & Refactoring to Patterns
Use patterns to fix real design problems instead of adding them everywhere.
- Replacing long if-else and switch with Strategy
- Removing duplication with Template Method
- When a pattern makes code worse
- Pattern comparison: Strategy vs State, Adapter vs Decorator vs Proxy
- Design patterns interview questions
19.7Low-Level Design Interview Problems
Practise the class-design questions asked in interviews, from requirements to code.
- A method for LLD rounds: requirements, entities, relationships, code
- Parking lot system
- Movie ticket booking
- Elevator system
- Splitting expenses between friends
- Vending machine with the State pattern
19.8Design Patterns Practice & Review
Apply patterns to ShopLite and review designs as you would in a code review.
- Strategy for discounts and shipping costs
- Observer-style events for order notifications
- Builder for complex order objects
- Review and improve an existing design

Start with what’s live

These sections are ready to study now.

See the whole roadmap

What you will be able to do

Apply SOLID principles when designing new classes, not only when explaining them

Recognise and implement the creational, structural and behavioural patterns used most in Java

Spot these patterns inside the JDK and Spring

Choose a pattern for a reason and avoid forcing one where it does not fit

Solve common low-level design interview problems with clean class designs

What it will cover

19.1SOLID Recap & Design Thinking

A quick refresh of SOLID from OOP, then how principles lead to patterns.

SOLID in five short examples
Program to interfaces
Favour composition over inheritance
What a design pattern is and is not
Reading UML class diagrams

19.2Creational Patterns

Control how objects are created so code does not depend on concrete classes.

Singleton: thread-safe versions and why Spring beans replace it
Factory Method and simple factories
Abstract Factory
Builder for objects with many fields
Prototype and copying (brief)

19.3Structural Patterns

Combine classes and objects into larger structures without tight coupling.

Adapter: wrapping an incompatible API
Decorator: adding behaviour, as in Java I/O streams
Proxy: lazy loading, access control and Spring AOP
Facade: one simple entry point
Composite: trees of objects

19.4Behavioural Patterns

Organise how objects talk to each other and share work.

Strategy: swapping algorithms, with lambdas
Observer: events and listeners
Template Method: fixed steps, custom details
Command: actions as objects, undo
Chain of Responsibility: request filters
State: objects that change behaviour with status

19.5Patterns in the JDK & Spring

See where the patterns you learned already appear in tools you use.

Iterator and Comparator as patterns
Builders in HttpClient and Stream.Builder
Spring beans, factories and dependency injection
Template classes: JdbcTemplate and RestClient
Proxies behind @Transactional
Filter chains in Spring Security

19.6Choosing & Refactoring to Patterns

Use patterns to fix real design problems instead of adding them everywhere.

Replacing long if-else and switch with Strategy
Removing duplication with Template Method
When a pattern makes code worse
Pattern comparison: Strategy vs State, Adapter vs Decorator vs Proxy
Design patterns interview questions

19.7Low-Level Design Interview Problems

Practise the class-design questions asked in interviews, from requirements to code.

A method for LLD rounds: requirements, entities, relationships, code
Parking lot system
Movie ticket booking
Elevator system
Splitting expenses between friends
Vending machine with the State pattern

19.8Design Patterns Practice & Review

Apply patterns to ShopLite and review designs as you would in a code review.

Strategy for discounts and shipping costs
Observer-style events for order notifications
Builder for complex order objects
Review and improve an existing design