Upcasting and Downcasting in Java

A process of converting one data type to another is known as Typecasting and Upcasting and Downcasting is the type of object typecasting. In Java, the object can also be typecasted like the datatypes. Parent and Child objects are two types of objects. So, there are two types of typecasting possible for an object, i.e., Parent to Child and Child to Parent or can say Upcasting and Downcasting.

In Java, the object can also be typecasted like the datatypes. Parent and Child objects are two types of objects. So, there are two types of typecasting possible for an object, i.e., Parent to Child and Child to Parent or can say Upcasting and Downcasting.

Typecasting is used to ensure whether variables are correctly processed by a function or not. In Upcasting and Downcasting, we typecast a child object to a parent object and a parent object to a child object simultaneously. We can perform Upcasting implicitly or explicitly, but downcasting cannot be implicitly possible.

Let's dive into deep of both these type of object casting:

1) Upcasting

Upcasting is a type of object typecasting in which a child object is typecasted to a parent class object. By using the Upcasting, we can easily access the variables and methods of the parent class to the child class. Here, we don't access all the variables and the method. We access only some specified variables and methods of the child class. Upcasting is also known as Generalization and Widening.

UpcastingExample.java

class  Parent{
   void PrintData() {
      System.out.println("method of parent class");
   }
}

class Child extends Parent {
   void PrintData() {
      System.out.println("method of child class");
   }
}
class UpcastingExample{
   public static void main(String args[]) {
      
	  Parent obj1 = (Parent) new Child();
	  Parent obj2 = (Parent) new Child(); 
	  obj1.PrintData();
	  obj2.PrintData();
   }
}

Output:

2) Downcasting

Upcasting is another type of object typecasting. In Upcasting, we assign a parent class reference object to the child class. In Java, we cannot assign a parent class reference object to the child class, but if we perform downcasting, we will not get any compile-time error. However, when we run it, it throws the "ClassCastException". Now the point is if downcasting is not possible in Java, then why is it allowed by the compiler? In Java, some scenarios allow us to perform downcasting. Here, the subclass object is referred by the parent class.

Below is an example of downcasting in which both the valid and the invalid scenarios are explained:

DowncastingExample.java

//Parent class
class Parent { 
    String name; 
  
    // A method which prints the data of the parent class 
    void showMessage() 
    { 
        System.out.println("Parent method is called"); 
    } 
} 
  
// Child class 
class Child extends Parent { 
    int age; 
  
    // Performing overriding
    @Override
    void showMessage() 
    { 
        System.out.println("Child method is called"); 
    } 
} 
  
public class Downcasting{
  
    public static void main(String[] args) 
    { 
		Parent p = new Child();
		p.name = "Shubham";
		
		// Performing Downcasting Implicitly 
        //Child c = new Parent(); // it gives compile-time error 
		
        // Performing Downcasting Explicitly 
        Child c = (Child)p; 
  
        c.age = 18; 
        System.out.println(c.name); 
        System.out.println(c.age); 
        c.showMessage(); 
    } 
}

Output:

Why we need Upcasting and Downcasting?

In Java, we rarely use Upcasting. We use it when we need to develop a code that deals with only the parent class. Downcasting is used when we need to develop a code that accesses behaviors of the child class.

Difference between Upcasting and Downcasting

These are the following differences between Upcasting and Downcasting:

S.No	Upcasting	Downcasting
1.	A child object is typecasted to a parent object.	The reference of the parent class object is passed to the child class.
2.	We can perform Upcasting implicitly or explicitly.	Implicitly Downcasting is not possible.
3.	In the child class, we can access the methods and variables of the parent class.	The methods and variables of both the classes(parent and child) can be accessed.
4.	We can access some specified methods of the child class.	All the methods and variables of both classes can be accessed by performing downcasting.
5.	Parent p = new Parent()	Parent p = new Child() Child c = (Child)p;