Generinc

class Box<D>{
    D data;
}
public class GenericEx01 {
    public static void main(String[] args) {
        Box<String> box1 = new Box();
        Box<Integer> box2 = new Box();
        box1.data = "1";
        box2.data = 1;
    }
}

class Box1{
    static Object data;
}
public class GenericEx02 {
    public static void main(String[] args) {
        Box1.data = "1";
        Box1.data = 1;
    }
}

class Dog{
    String name;
    int age;

    void speak(){
        System.out.println("BowWow");
    }
}
class Cat{
    String name;
    int age;

    void speak(){
        System.out.println("naaong");
    }
}
public class GenericEx03 {
    public static void main(String[] args) {
        Object[] arr = new Object[2];
        arr[0]=new Dog();
        arr[1]=new Cat();
        Dog d1 = (Dog) arr[0];
        Cat c1 = (Cat) arr[1];
        d1.speak();
        c1.speak();
    }
}

class Model{
    private static Model instance = new Model();
    public static Model getInstance(){
        return instance;
    }
    private Model(){
    }
    private Object data;
    public void setData(Object data){
        this.data = data;
    }
    public Object getData(){
        return data;
    }
}
class User{
    public int id = 1;
    public String name = "홍길동";
    public String email = "ssar@nate.com";
    public void hello(){
        System.out.println("안녕하세요.");
    }
}
public class GenericEx04 {
    public static void main(String[] args) {
        Model model = Model.getInstance();
        User user = new User();
        model.setData(user);
        User ob = (User) model.getData();
        ob.hello();
    }
}

class Model0<T> {
    private T data;

    public T getData() {
        return data;
    }

    public void setData(T data) {
        this.data = data;
    }
}
 class User0{
    public int id = 1;
    public String name = "홍길동";
    public String email = "ssar@nate.com";
    public void hello(){
        System.out.println("안녕하세요");
    }
 }
public class GenericEx05 {
    public static void main(String[] args) {
        Model0<User0> model0 = new Model0();
        model0.setData(new User0());
        User0 user0 = model0.getData();
        user0.hello();
    }
}

Object를 사용해야 하는 경우

미리 만들어져 있는 static은 new 불가능 (사용자가) ⇒ Object, 즉 Generic으로 사용 불가능 = 사용자가 어떤 타입을 사용할 지 모를 경우에 개발자는 object 사용 → 다운 캐스팅으로 코딩