SCNA 2009 - Grand Unified Theory of Software Design: Jim Weirich

EdgeCase, LLC

Tech Interview

When interviewing over the phone, what do you look for? Passion?

"What do you look for in a good design?" Criteria?

Some principles:

• S.O.L.I.D.
• Law of Demeter
  • Doesn't always apply - Fluent Interfaces
• DRY
• Small Methods
• Design by Contract

Coupling and Cohesion - Myers

7 types of coupling - less is good.

Control Coupling

• Method has "flag" parameter that controls what algorithm to use
• Description includes "or" or "otherwise"

Connascence

Two pieces of software share connascence when a change in one requires a corresponding change in the other.

From What Every Programmer Should Know About Object-Oriented Design (book). Section 2 predated UML - irrelevant. Updated version is: Fundamentals of Object-Oriented Design in UML.

Locality matters when looking at connascence. As the distance between pieces increases, you want weaker connascence.

Connascence of Name (CoN)

• Names (type, etc.) must agree

Connascence of Position (CoP)

• Ordering matters
• Number of elements is relevant (more than 2-3)
• CoP -> CoN
• CoN < CoP (matter of degree)
• Basic OO - named properties OK, long method signatures not so good, omittable keywords OK

Connascence of Meaning (CoM)

• Enums, constants take CoM -> CoN (good)

Contranascence

Things can't conflict with each other.

• Naming conflicts (namespaces)
• Two enums with same value
• Extension methods with same name

Connascence of Algorithm (CoA)

• Checksum generation code and checksum checking code
• Move to common method; share code (when possible)
• DRY

Connascence of Timing (CoT)

• Race conditions
• Convert to atomic operations

Connascence of Execution (CoE)

• Ordering of steps in algorithm relevant

Connascence of Identity (CoI)

• Objects must be same object (uniquing)

Connascence of Value (CoV)

• Values are related; constraints

Other Books

• Object-Oriented Software Construction (Meyer)
• The Pragmatic Programmer: From Journeyman to Master