oop

1. OOP and its 4 principles?
   • is object-oriented programming -> focus on objects and builds things upon them
   • 4 principles
     • encapsulation
       • wraps things into a unity, wraps methods and fields into a class, classes into a package
     • abstraction
       • only care about public fields, attributes, methods, and needn’t care about its private stuff and how they are implemented
     • inheritance
       • allow a class to inherit the properties and characteristics from other classes
     • polymorphism
       • uses same types and functions, but their actions depend on its entity (eg. like circle, rectangle, …)
2. access modifiers?
   • private: can be accessed by things in same class (object?)
   • default: can be accessed in the same package
   • protected: can be accessed in the same package + outside things that have inheritance relationship
   • public: can be accessed by everything
3. interface and abstract class?
   • the difference is the relationship between it and the classes that extend or implement it
   • class A extends abstract class B -> A is B
   • class A implements interface C -> A has C’s characteristics
4. SOLID
   • Single responsibility principle
     • a class should be focused on a single functionality -> easier testing, lower coupling
   • Open/closed principle
     • software entities should be open for extension, but closed for modification -> reduce risk of introducing bugs into existing code
   • Liskov substitution principle
     • objects of a super class should be replaceable with objects of its subclass -> ensure that inheritance hierarchies are logically sound
   • Interface segregation principle
     • no client should be forced to depend on methods that it doesn’t use -> prevent polluting classes with unused methods
   • Dependency inversion principle
     • depends upon abstractions, not concretions -> make system pluggable