4 Design Principles

by Kent Beck

"Dady" Kent Beck

• Extreme Programming
• Agile Manifesto
• Test-Driven Development

The 4 Design Principles

1. Passes the tests
2. Reveals intention
3. No duplication
4. Fewest elements

I. Passes the Tests

• your code must work as intended
• the tests make sure that's always true

II. Reveals Intention

• the code must be easy to understand
• always remember that:
  • you spend most of your time reading/browsing code
  • if you can't understand, you can't modify

III. No duplication (''aka.'' D.R.Y.)

• Don't Repeat Yourself
• say things "Once and only once"
• create abstractions

IV. Fewest Elements (''aka.'' no over-engineering)

• don't design future requirements
• you don't know what you will need tomorrow, don't design for it today
• this only makes code harder to change

The 4 Design Principles

1. Passes the tests
2. Reveals intention
3. No duplication
4. Fewest elements