Polymorphism

• ⭐️ extends
• ⭐️ method overriding

✅ Polymorphism: parent can become child

• parent class
• child class extends parent class
• 1️⃣ Polymorphism: parent CAN become child instance
• child CAN NOT become parent instance

Parent parent = new Parent();
Parent parentChild = new Child(); //🟢 parent CAN become child instance = Polymorphism
Parent parentGrandson = new Grandson();

//Child child = new Parent(); //🔴 child CAN NOT become parent instance

• 2️⃣ can only call its type method
• creating new Child() instance will create both child instance and parent instance(as child extends parent)
• however as parentChild is Parent type, can only call Parent field and method

Parent parentChild = new Child();
parentChild.parentMethod(); //parentChild is Parent type, can call parent method
//parentChild.childMethod(); // parentChild is Parent type, CAN NOT call child method

• new Child(): 자식 클래스를 생성하면 child class ➕ parent class 둘 다 생성됨
• 그러나 Parent parentChild 이므로 parentChild는 Parent 타입이다
• 따라서 parentChild는 Parent의 field, method만 사용 가능하다

✅ Downcasting

• I am parent type in child instance, 👎🏻 so I can only call parent field and methods

• 🤨 But I want to call child field and methods

• ➡️ Downcasting: change parent type to child type
• CAN NOT omit type

public class CastingMain1 {
    public static void main(String[] args) {
        Parent parentPoly = new Child(); // parentPoly is Parent type, but Child instance
        //parentPoly.childMethod(); // 🔴 compile error, 👎🏻 only can call type method

        Child childPoly = (Child) parentPoly; //💡 Downcasting //change parent type to child type
        childPoly.childMethod(); //now can call childMethod();
        //parentPoly type: parent
        //childPoly type: child
    }
}

• 🤨 why is downcasting possible?
  • parent type is created child instance,
  • in memory, both parent instance and child instance is created
  • thus, can change type from parent type to child type

ublic class CastingMain3 {
    public static void main(String[] args) {
        Parent parent1 = new Child();
        Child child1 = (Child) parent1; //🟢downcasting

        Parent parent2 = new Parent();
        Child child2 = (Child) parent2; //🔴error, parent2 is parent instance thus downcasting not possible
    }
}

• Parent parent1 = new Child();를 생성하면 type은 parent이지만 instance가 child이므로 parent instance, child instance가 모두 생성된다
• 따라서 downcasting 가능 ⭕️
• 그러나 Parent parent2 = new Parent();하면 instance가 parent이므로 parent instance만 생성
• 따라서 downcasting 불가능 ❌

✅ Upcasting

• Upcasting: changing child type to parent type
• can omit type

public class CastingMain2 {
    public static void main(String[] args) {
        Child child = new Child();
        Parent parent1 = (Parent) child; //upcasting
        Parent parent2 = child; //can omit (Parent)

        parent1.parentMethod();
    }
}

Downcasting 🆚 Upcasting

• Downcasting: 부모 ➡️ 자식

• Upcasting: 자식 ➡️ 부모

• Upcasting은 자식 instance를 생성하므로 부모 instance도 모두 항상 생성됨, upcasting하는 type을 omit하고 사용 가능
• Downcasting은 부모 instance만 생성되므로 downcasting하는 type을 명시해주어야 한다

✅ InstanceOf

• what is my instance?
• can I downcast to this instance? check instance

public class CastingMain5 {
    public static void main(String[] args) {
        Parent parentParent = new Parent();
        parentParent instanceOf Parent //true
        parentParent instanceOf Child //false, downcast NOT possible

        Parent parentChild = new Child();
        parentChild instanceOf Parent //true
        parentChild instanceOf Child //true, downcast possible

    }
}

✅ Polymorphism and overriding

• method: overrided method always has priority
• field: field is not overrided

Parent poly = new Child();
poly.value //field: parent field, field is NOT override
poly.method(); //method: child method, method IS override

• poly type is Parent, instance is Child
• when field is called, go to Parent and find, return Parent field
• when method is called, look for override
• if there is override in child, return Child method

☑️ 결론

• 1️⃣ polymorphism: 부모 type을 자식 instance로 생성할 수 있다(부모는 자식을 품을 수 있음)
• 2️⃣ 자식 instance로 생성하면 자식, 부모 instance를 모두 생성한다
  • Animal animal = new Dog();하면 animal, dog인스턴스 모두 생성됨
• 3️⃣ Downcasting: 부모 ➡️ 자식으로 타입을 바꿈, 내가 명시한 instance일때만 그 type으로 downcast 가능
• 4️⃣ Upcasting: 자식 ➡️ 부모로 타입을 바꿈, 항상 가능
• 5️⃣ 내가 무슨 instance인지 확인하기 위해 instanceOf를 사용할 수 있다
• 6️⃣ overriding한 method가 항상 우선순위를 가진다(자식 인스턴스)
• method is override, field is not overrided