Simple Design Rules

Guide Java developers through Kent Beck's simple design rules when evaluating design and refactoring choices. This is an interactive SKILL.

What is covered in this Skill?

• Using tests as the first correctness boundary before design cleanup
• Revealing intention and maximizing clarity before abstraction pressure
• Reducing duplication without hiding domain meaning or worsening readability
• Treating fewer elements as the final simplification pressure after correctness, clarity, and duplication
• Comparing Java design and refactoring options with the ordered rules
• Reporting the chosen option, tradeoffs, validation signal, and remaining risk

Constraints

Apply simple design rules in priority order, and do not optimize later rules by weakening earlier rules.

• MUST read references/053-design-simple-rules.md before applying the rules
• MUST evaluate the rules in this order: passes the tests, reveals intention, has no duplication, has the fewest elements
• MUST treat passing tests as the correctness boundary before design cleanup
• MUST prefer clear intention over abstraction for its own sake
• MUST reduce duplication only when the result preserves or improves clarity
• MUST consider fewer elements only after correctness, clarity, and duplication have been addressed

When to use this skill

• Apply simple design rules
• Use Beck's simple design rules
• Review this design with simple design rules
• Choose between these refactoring options
• Keep this Java design simple
• Does this design pass the simple design rules?

Workflow

1. Anchor Correctness

Read references/053-design-simple-rules.md, then identify the behavior under discussion and the tests, characterization checks, build checks, or manual evidence that define whether the code works.

2. Reveal Intention

Evaluate whether names, types, responsibilities, control flow, API shape, and test descriptions make the domain decision clear to a future maintainer.

3. Reduce Duplication Carefully

Find repeated knowledge, policy, mappings, validation, queries, or workflow decisions. Remove duplication only when the abstraction keeps the intent easier to understand.

4. Minimize Elements Last

After correctness, clarity, and duplication are handled, remove unnecessary classes, methods, parameters, branches, layers, configuration, or indirection that no longer pay for themselves.

5. Compare Options and Report

Compare available design or refactoring options against the ordered rules. Recommend the option that satisfies the earlier rules best, describe rejected tradeoffs, and name the verification signal.

Reference

For detailed guidance, examples, and constraints, see references/053-design-simple-rules.md.