The author uses the more complicated definitions of the SOLID principles. These are much simpler to remember: * Single responsibility principle - A class shou

The author uses the more complicated definitions of the SOLID principles. These are much simpler to remember: * Single responsibility principle - A class should have only 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. * Open–closed principle - 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 - One should depend upon abstractions, not concretions. A lot of these principles also tie into other principles such as DRY (Do not repeat yourself).