java-design-pattern— agent skill

Either detect which design patterns are used in the provided code, or recommend the right pattern for a described problem. Tailor all examples to the detected Java version (Java 8+).

Mode A — Detect patterns in existing code

Scan the code for these common patterns and name them explicitly:

Creational:

• Singleton — private constructor + static instance (flag if not thread-safe without volatile or enum)
• Factory Method — static create() or of() returning an interface type
• Builder — inner static Builder class with fluent setters and build()
• Abstract Factory — factory that creates families of related objects

Structural:

• Decorator — wraps another object implementing the same interface to add behaviour
• Proxy — Spring @Transactional, @Cacheable, @Async are all proxies; flag manual proxies
• Adapter — converts one interface to another (often with *Adapter or *Wrapper class name)
• Facade — simplifies a complex subsystem; often a service that orchestrates multiple repos/clients
• Composite — tree structure where leaf and composite implement the same interface

Behavioral:

• Strategy — interface with multiple implementations selected at runtime
• Observer — Spring ApplicationEvent / @EventListener or manual listener list
• Template Method — abstract class with final algorithm method calling overridable steps
• Command — encapsulates a request as an object (common in undo/redo, queuing)
• Chain of Responsibility — Spring Security filter chain, Servlet filters
• State — object behaviour changes based on internal state enum/object

For each detected pattern: name it, show the key code that reveals it, explain how it's used here.

Mode B — Recommend a pattern for a problem

When the user describes a problem, recommend the most appropriate pattern:

Show a minimal Java implementation for the recommended pattern, appropriate for the detected Java version:

• Java 8+: use functional interfaces (Strategy can be a lambda Function<T,R>)
• Java 17+: use sealed classes for State pattern
• Java 16+: use records for value objects in patterns

Output format

Mode A: List each detected pattern with: name + location + explanation + any concerns (e.g., non-thread-safe Singleton) Mode B: Recommend pattern + minimal before/after code + when to use vs alternatives

Next Steps

• After identifying patterns → run /java-solid to check if patterns are applied correctly
• For implementation help → ask the java-architect agent for full design guidance