The purpose of design is to allow you to do design later, and it's primary goal is to reduce the cost of change.
- Single Responsibility Principle: a class should have only a single responsibility
- Open-Closed Principle: Software entities should be open for extension, but closed for modification (inherit instead of modifying existing classes).
- Liskov Substitution: Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.
- Interface Segregation: Many client-specific interfaces are better than one general-purpose interface.
