✅ Why do we need Enum? Limit field to certain value User role can be only BASIC, GOLD, DIAMOND 👎🏻 how can we prevent entering basiiiic(오타) or diamond(소문자) or VIP(존재하지 않는 값)? 💊 ENU...

✅ Class 클래스 Class 클래스를 통해서 get metadata about a class 할 수 있다 ✔️ get class type information: class name, super class, interface, access modifier… ✔️ reflection: get field, method, co...

✅ String is reference String is reference value can create String as literal or as instance String str1= "hello"; //literal String str2 = new String("hello"); //instance 🟰 two code is...

✅ Summary Immutable: cannot change reference value 1️⃣ final field, 2️⃣ no setter ✅ Primitive 🆚 Reference 🆚 Primitive int a = 10; int b = a; b = 20; ❓ 이제 a의 값은? 여전히 10 ❓ 이제 b의...

✅ Object class Object class is parent of all Java classes highest parent of all Java class 자바의 최상위 부모 클래스 implicitly imports object public class Parent extends Object (extends Obje...

✅ OCP Open for extension: new code should be able to be extended 확장에는 열려있고 Closed for modification: old code must not be modified 변경에는 닫혀있어야 한다 ✅ How t...

✅ Summary interface is like pure abstract class cannot create instance MUST be overrided by child class implements can implements several interface ✅ Interface ...

✅ Abstract class Parent class, but can NOT be created as instance 동물이라는 개념 안에 강아지, 고양이 등이 있지만 "동물"이라고 하는 동물은 없음 따라서 동물은 인스턴스를 생성하면 안 됨 Animal class: abstract class Dog class, Ca...

⭐️ extends ⭐️ method overriding ✅ Polymorphism: parent can become child parent class child class extends parent class 1️⃣ Polymorphism: parent CAN become child instance child CAN N...

✅ Final final: can not change after initialization final value can not be altered after initializatoin 처음 값 할당 후에는 변수의 값을 변경하지 못하게 막아버린다 👎🏻 without final int data0; data0 = 10; ...