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.
Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program." See also design by contract.
Many client-specific interfaces are better than one general-purpose interface.
We should depend upon abstractions, not concretions.