What's the difference between design patterns and design principles?

Question

I'm new to Ruby on Rails, and I went through these articles.

• Design Patterns in Ruby: Observer, Singleton
• Design Patterns in Ruby

But I couldn't understand the actual difference between design patterns and design principles. Could someone please explain the distinction?

Answer 1

Design Principles:

Design principles are core abstract principles that we are supposed to follow while designing software. Remember they aren't concrete - rather abstract. They can be applied in any language, on any platform regardless of the state as long as we are within the permissible conditions.

Examples:

• Encapsulate what varies.
• Program to interfaces, not to implementations.
• Depend upon abstractions. Do not depend upon concrete classes.

Design Patterns:

They are solutions to real-world problems that pop up time and again, so instead of reinventing the wheel, we follow the design patterns that are well-proven, tested by others, and safe to follow. Now, design patterns are specific; there are terms and conditions only in which a design pattern can be applied.

Examples:

• Singleton Pattern ( One class can only have one instance at a time )

• Adapter Pattern ( Match interface of different classes )

The following analogy will help you understand the difference better:

Principle: We should teach others in order to educate ourselves as well as others, and overall make our nation a progressive nation.

Pattern: In our country, each medical doctor graduate is supposed to teach 6 months in a far-away village to complete his/her degree.

Answer 2

I think the answer from @ArslanAli is worth summarizing.

• Principles apply to all of programming. You should have a very good reason any time you choose not to follow principles.
• Patterns apply to specific, common problems. You should have a very good reason any time you choose to implement a pattern.