A class should only have a single responsibility, that is, only changes to one part of the software’s specification should be able to affect the specification of the class.
“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, [not] concretions.”