SOLID= Single responsibility (class must be responsible for singe skill in solution scale), Open for extensions, closed for modifications, Liskov principle: behavior of a child class must not affect expected behavior of parent when parent is substituted by child, I-interface segregation: divide functionality by interfaces with limited method set, rather than obligate API user to implement unnecessary methods (the ugly invention of default methods in Java 8 decreases important mean of the principle., D depend on abstraction rather than on concrete class