Principles to program by ->
Single responsibility principle
An object should have only a single responsibility.
“Software entities … should be open for extension, but closed for modification”.
Liskov substitution principle
“Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program”. See also design by contract.
Interface segregation principle
“Many client-specific interfaces are better than one general-purpose interface.”
Dependency inversion principle
One should “Depend upon Abstractions. Do not depend upon concretions.”
Dependency injection is one method of following this principle.