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)
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.
Interface segregation principle
many client-specific interfaces are better than one general-purpose interface.
Dependency inversion principle
Depend upon Abstractions. Do not depend upon concretions. Dependency injection is one method of following this principle.