S SRP[4] Single responsibility principle a class should have only a single responsibility (i.e. only one potential change in the software's specification should be able to affect the specification of the class) O OCP[5] Open/closed principle “software entities … should be open for extension, but closed for modification.” L LSP[6] 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. I ISP[7] Interface segregation principle “many client-specific interfaces are better than one general-purpose interface.”[8] D DIP[9] Dependency inversion principle one should “Depend upon Abstractions. Do not depend upon concretions.”[8]