Is lesson me hum seekhenge:

• Object reference kya hota hai
• Java me Object class kya hoti hai
• Object sab classes ka parent kaise hai
• Upcasting kya hota hai
• Downcasting kya hota hai
• Type casting in objects

Java me jab hum object create karte hain:

Student s = new Student();

Yahan:

Student → class
s → object reference
new Student() → actual object

Diagram:

s  --------->  Student Object in Heap

Object reference sirf:

memory address store karta hai

Java me ek built-in class hoti hai:

java.lang.Object

Ye sab classes ki topmost parent class hai.

Example:

class Student {

}

Actually internally hota hai:

class Student extends Object {

}

Iska matlab:

Har class indirectly Object ko inherit karti hai

Isliye har object ke paas ye methods hote hain:

toString()
equals()
hashCode()
getClass()
wait()
notify()
clone()

Example

class Test {

    public static void main(String[] args){

        String s = "Java";

        System.out.println(s.getClass());
    }
}

Output

class java.lang.String

Object reference kisi bhi object ko store kar sakta hai.

Example

public class Test {

    public static void main(String[] args){

        Object obj = new String("Hello");

        System.out.println(obj);
    }
}

Output

Hello

Yahan:

Object reference → String object store kar raha hai

Isse kehte hain:

Upcasting

Jab child object ko parent reference me store karte hain.

Animal a = new Dog();

Rules:

✔ automatic hota hai
✔ safe hota hai
✔ compile time par allowed hota hai

Jab parent reference ko child type me convert karte hain.

Animal a = new Dog();

Dog d = (Dog) a;

Yahan cast karna padta hai:

(Dog)

class Animal {

    void sound(){
        System.out.println("Animal sound");
    }
}

class Dog extends Animal {

    void bark(){
        System.out.println("Dog bark");
    }
}

public class Test {

    public static void main(String[] args){

        Animal a = new Dog(); // upcasting

        a.sound();
    }
}

Output

Animal sound

class Animal {

}

class Dog extends Animal {

    void bark(){
        System.out.println("Dog bark");
    }
}

public class Test {

    public static void main(String[] args){

        Animal a = new Dog();

        Dog d = (Dog) a; // downcasting

        d.bark();
    }
}

Output

Dog bark

Animal a = new Animal();

Dog d = (Dog) a; // Runtime Error

Error:

ClassCastException

Isliye safe casting ke liye:

if(a instanceof Dog){
    Dog d = (Dog) a;
}

Use hota hai:

polymorphism
method overriding
collection framework
generic programming

Example:

Object[] arr = new Object[3];

arr[0] = "Java";
arr[1] = 10;
arr[2] = new Student();

1️⃣ Java me Object class kya hoti hai?
2️⃣ Kya har class Object ko inherit karti hai?
3️⃣ Object reference kya hota hai?
4️⃣ Upcasting aur downcasting me difference kya hai?
5️⃣ ClassCastException kab aata hai?
6️⃣ instanceof operator ka use kya hai?

Is lesson me humne seekha:

✔ Object reference concept
✔ Object class as parent
✔ Upcasting
✔ Downcasting
✔ Type casting rules
✔ instanceof safety check

Java OOP ka ye foundation concept hai jo polymorphism aur collections samajhne ke liye zaruri hai.