Java Patterns

When to use this skill

Use this skill when working on Java services/libs and you want clean layering, safe null handling, predictable build structure (Maven/Gradle), and testable code.

Core patterns

Null handling (be explicit)

Prefer:

• returning empty collections over null
• Optional<T> for return values (not for fields)
• null checks at boundaries (inputs)

Avoid:

• Optional in fields/DTOs as a general rule (depends on ecosystem)

Layering

Keep boundaries clear:

• controllers/handlers (HTTP, RPC)
• services (business logic)
• repositories/DAOs (persistence)

Immutability by default

Prefer:

• final fields
• constructors that enforce invariants
• builders only when needed

Exceptions

Rules:

• throw exceptions with actionable messages
• don’t catch-and-ignore
• wrap lower-level exceptions with context

Tests

Patterns:

• JUnit for unit tests
• integration tests for persistence and wiring
• avoid heavy mocks; mock at boundaries

Atlarix tool notes

• Explore: identify build system (pom.xml vs build.gradle) and entry points.
• Build: run mvn test or gradle test depending on project.
• Debug: inspect stack traces; find first application frame.
• Review: verify layering and that exceptions don’t leak secrets.

Common mistakes to avoid

• God services with too many responsibilities
• Catching Exception broadly and hiding failures
• NPEs due to missing boundary validation

Mini-checklist

• Clear layers
• Nulls handled intentionally
• Tests cover critical paths
• Build commands documented