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Design Patterns in Java

A study repository for practicing the main Design Patterns, with isolated and executable implementations in Java.

Each pattern lives in its own package, has its logic encapsulated, and is executed from Main.java.

Implemented Patterns

Pattern Category Package
Strategy Behavioral strategy/
Observer Behavioral observer/
Factory Method Creational factorymethod/
Decorator Structural decorator/

Study workflow

This repository uses Claude Code skills to guide the full study cycle of each pattern. Three skills cover the entire flow:

1. /new-pattern <name> — Scaffold a new pattern

Sets up everything needed to study a pattern from scratch:

  • Creates the package with a README.md (intent, structure, roles, when to use)
  • Creates an originalproblem/ subpackage with realistic Java code that shows the pain point before the pattern
  • Creates the demo entry-point class and registers it in Main.java
/new-pattern observer
/new-pattern "Factory Method"
/new-pattern               ← Claude picks the next fundamental pattern

2. /solve-pattern <name> — Clear the solution, keep the problem

Once a pattern has a complete solution committed, use this skill to wipe the solution files and go back to a clean slate — ready for you to implement it yourself.

  • Removes all solution classes (interfaces, abstract classes, concrete implementations)
  • Keeps originalproblem/ and README.md intact
  • Updates the demo class to reference only the original-problem code
  • Verifies the project still compiles
/solve-pattern strategy
/solve-pattern observer

Start here when you want to practice a pattern that already has a reference solution in the repo.

3. /review-pattern <name> — Review your implementation

After you implement the pattern, run this skill to get structured feedback:

  • Pattern correctness: are all GoF roles present?
  • Interface segregation and dependency direction
  • SOLID principles, with class-level observations
  • Java conventions and code quality
  • Pattern-specific best practices and common pitfalls
  • Demo quality: does execute() actually show the pattern's benefit?
/review-pattern strategy
/review-pattern observer

Each dimension gets a verdict (✅ Good / ⚠️ Partial / ❌ Issue) with references to actual class and method names in your code.

Typical session

# 1. Scaffold a new pattern (or pick up one already in the repo)
/new-pattern "Factory Method"

# 2. Read originalproblem/ to understand the pain point
# 3. Read README.md to understand the pattern structure
# 4. Implement the solution inside the package

# 5. (Optional) If the repo already has a reference solution and you want to solve it yourself:
/solve-pattern factory-method

# 6. After implementing, get a review
/review-pattern factory-method

How to run

mvn compile exec:java -Dexec.mainClass="Main"

Requires Java 21+ (uses void main() without a wrapping class — JEP 463, preview feature).