Class Type Casting in Java

Typecasting is the assessment of the value of one primitive data type to another type. In java, there are two types of casting namely upcasting and downcasting as follows:

1. Upcasting is casting a subtype to a super type in an upward direction to the inheritance tree. It is an automatic procedure for which there are no efforts poured in to do so where a sub-class object is referred by a superclass reference variable. One can relate it with dynamic polymorphism.
   • Implicit casting means class typecasting done by the compiler without cast syntax.
   • Explicit casting means class typecasting done by the programmer with cast syntax.
2. Downcasting refers to the procedure when subclass type refers to the object of the parent class is known as downcasting. If it is performed directly compiler gives an error as ClassCastException is thrown at runtime. It is only achievable with the use of instanceof operator The object which is already upcast, that object only can be performed downcast.

In order to perform class type casting we have to follow these two rules as follows:

1. Classes must be “IS-A-Relationship “
2. An object must have the property of a class in which it is going to cast.

Implementation: 

(A) Upcasting

Example 1

Java

// Importing input output classes
import java.io.*;

// Class 1
// Parent class
class Parent
{

  // Function
  void show()
  {

    // Print message for this class
    System.out.println("Parent show method is called");
  }
}

// Class 2
// Child class
class Child extends Parent
  {

    // Overriding existing method of Parent class
    @Override

    // Same Function which will override
    // existing Parent class function
    void show()
    {

    // Print message for this class
    System.out.println("Child show method is called");
    }

  }

// Class3
// Main class
class GFG
{

    // Main driver method
    public static void main(String[] args)
    {
      // Creating a Parent class object
      // but referencing it to a Child class
       Parent obj = new Child();

      // Calling the show() method to execute
       obj.show();
    }
}

Output

Child show method is called

Output explanation: Here parent class object is called but referred to the child’s class object. Hence, one can relate this with dynamic polymorphism or function overriding. 

(B) Downcasting

Example 2

Java

// Java Program to illustrate Downcasting

// Importing input output classes
import java.io.*;

// Class 1
// Parent class
class Vehicles {
}

// Class 2
// Child class
class Car extends Vehicles {
    static void method(Vehicles v)
    {

        //
        if (v instanceof Car) {

            // Downcasting
            Car c = (Car)v;

            // Display message
            System.out.println("Downcasting performed");
        }
    }
    // Main driver method
    public static void main(String[] args)
    {
        // Creating an object of Vehicle class
        // and referring it to Car class
        Vehicles v = new Car();
        Car.method(v);
    }
}

Output

Downcasting performed

NOTE : Without perform upcast if we try to downcast , ClassCastException will be thrown.

• It is a runtime exception or unchecked exception.
• It is class, present in java.lang package.
• It can be avoided by using a operator known as ‘instanceof’.

Example 3

Java

// Java Program showing ClassCastException

// Importing input output classes
import java.io.*;

// Class 1
// Parent class/ Member class
class Member {

    // Member variable of this class
    String name;
    long phone;

    // Member function of this class
    void chat()
    {

        // Print message of Member/ Child class
        System.out.println(
            name + " : chatting in whatsapp group");
    }
}

// Class 2
// Child class/ Admin class
class Admin extends Member {

    // Member function of this class
    void addUser()
    {

        // Print message of Admin/ Parent class
        System.out.println(
            name
            + " : adding a new user in whatsapp group");
    }
}

// Class3 - Main class
class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating an object Ad
        Member mem = new Admin();

        // Upcasting access only general property of
        // superclass

        // Custom entry for Member class
        mem.name = "Sneha";
        mem.phone = 9876543210l;
        // Calling function
        mem.chat();

        Admin ad = (Admin)mem;

        // Downcast to access specific property of subclass
        ad.addUser();
    }
}

Output

Sneha : chatting in whatsapp group
Sneha : adding a new user in whatsapp group