How do you upcast and downcast the same object in Java?

Converting one data type to others in Java is known as casting.

Up casting − If you convert a higher datatype to lower datatype, it is known as narrowing (assigning higher data type value to the lower data type variable).

Example

 Live Demo

import java.util.Scanner;
public class NarrowingExample {
   public static void main(String args[]){
      char ch = (char) 67;
      System.out.println("Character value of the given integer: "+ch);
   }
}

Output

Character value of the given integer: C

Down casting − If you convert a lower datatype to a higher data type, it is known as widening (assigning lower data type value to the higher data type variable).

Example

 Live Demo

public class WideningExample {
   public static void main(String args[]){
      char ch = 'C';
      int i = ch;
      System.out.println(i);
   }
}

Output

Integer value of the given character: 67

upcasting and downcasting the same object

Similarly, you can also cast/convert an object of one class type to others. But these two classes should be in an inheritance relation. Then,

• If you convert a Super class to subclass type it is known as narrowing in terms of references (Subclass reference variable holding an object of the superclass).

Sub sub = (Sub)new Super();

• If you convert a Subclass to Superclass type it is known as widening in terms of references (superclass reference variable holding an object of the subclass).

Super sup = new Sub();

Example

The following Java program demonstrates how to upcast and down cast the same object.

 Live Demo

class Person{
   public String name;
   public int age;
   Person(){}
   public Person(String name, int age){
      this.name = name;
      this.age = age;
   }
   public void displayPerson() {
      System.out.println("Data of the Person class: ");
      System.out.println("Name: "+this.name);
      System.out.println("Age: "+this.age);
   }
}
public class Student extends Person {
   public String branch;
   public int Student_id;
   Student(){}
   public Student(String name, int age, String branch, int Student_id){
      super(name, age);
      this.branch = branch;
      this.Student_id = Student_id;
   }
   public void displayStudent() {
      System.out.println("Data of the Student class: ");
      System.out.println("Name: "+super.name);
      System.out.println("Age: "+super.age);
      System.out.println("Branch: "+this.branch);
      System.out.println("Student ID: "+this.Student_id);
   }
   public static void main(String[] args) {
      Person person = new Person();
      Student student = new Student("Krishna", 20, "IT", 1256);
      //Up casting
      person = student;
      person.displayPerson(); //only super class methods
      //Down casting
      student = (Student) person;
      student.displayPerson();
      student.displayStudent();
   }
}

Output

Data of the Person class:
Name: Krishna
Age: 20
Data of the Person class:
Name: Krishna
Age: 20
Data of the Student class:
Name: Krishna
Age: 20
Branch: IT
Student ID: 1256