Abstract

✅ 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, Cat class: extends Animal abstract class

public abstract class Animal { //abstract class, parent
}

public class Dog extends Animal{ //child class
}

public class Cat extends Animal{
}

• cannot create an instance of abstract class

Animal animal = new Animal(); //❌ error

Dog dog = new Dog(); //⭕️ create child class that extends Animal class
Animal poly = new Dog(); //⭕️ polymorphism

✅ Abstract method

• 1️⃣ child class MUST override abstract method
  • abstract method is only used in abstract class
• 2️⃣ no body
  • will be overrided by child class

public abstract class Animal {
    public abstract void sound(); //abstract method
    //only in abstract class
    //MUST be overrided by child class
    //abstract method has no body(will be overrided by child class)


    public void move(){ //not abstract method
        System.out.println("animal is moving");
    }
    //override not mandatory
}

public class Cat extends Animal{
    @Override
    public void sound() {
        System.out.println("meow");
    }
    //child class MUST override abstract method
}

• parent class: 메소드를 정의만 하고
• child class: override 해서 메소드 실제 구현

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