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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
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
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.
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
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