difference between upcasting and downcasting

What is the difference between up-casting and down-casting?
Casting in java means converting from type to type. When It comes to the talking about upcasting and downcasting concepts we are talking about converting the objects references types between the child type classes and parent type class. Suppose that we have three classes (A,B,C). Class B inherit from class A. Class C inherit from class B. As follows :
java code

interface A 
{
  void display();
}
class B implements A
{
    public void display() {
        System.out.println("Am in class B");
    }

}
class C extends B
{
    @Override
    public void display() {
        System.out.println("Am in class C");
    }
}

Note that we have declared the class A as interface but this will not make any changes in illustrating the upcasting/downcasting ideas.. However, It only restricts you from creating an object instance directly from type A.

Applying Up-casting

The upcasting is casting from the child class to base class. The upcasting in java is implicit which means that you don't have to put the braces(type) as a indication for casting. Below is an example of upcasting where we create a new instance from class C and pass it to a reference of type A. Then we call the function display.
java code

public static void main(String[] args) {

        // Upcasting from subclass to super class.
        A aRef=new C();

        aRef.display();//Am in class C
    

    }

Applying Down-casting

The downcasting is the casting from base class to child class. Below we continue on the previous upcasting snippet by adding to lines for downcasting where we down cast the aRef reference from type A to type B. Downcasting is explicit note the usage of braces(type) in the example below.
java code

public static void main(String[] args) {

        // Upcasting from subclass to super class.
        A aRef=new C();

        aRef.display();//Am in class C
        //Downcasting of reference to subclass reference.
        B bRef=(B) aRef;
        bRef.display();//Am in class C

    }

The output of the code is :

Note the display function of class C is called because the type of object is class C

Functions and variables after casting

After casting, you will have access only to the current reference type class members even your object is from type C and your reference of type A. Any unique class member in class C will not be visible to you. For instance :
java code

public class ExampleClass {

    public static void main(String[] args) {

        // Upcasting from subclass to super class.
        A aRef=new C();
        aRef.setX(43);// Compile Error

    }
}

interface A 
{
  void display();
}

class B implements A
{

    public void display() {
        System.out.println("Am in class B");
    }

}

class C extends B
{
    private int x;
    @Override
    public void display() {
        System.out.println("Am in class C");
    }

    public void setX(int x) {
        this.x = x;
    }    
}

Common mistake in casting usage :

In downcasting you can't down cast to a inheritance hierarchy level less than your object instance level at creation time. For instance :
java code

public class ExampleClass {

    public static void main(String[] args) {

        // Upcasting from subclass to super class.
        A aRef=new B();

        aRef.display();//Am in class C
        //Downcasting of reference to subclass reference.
        C bRef=(C) aRef; //ERROR
        bRef.display();
    }
}

interface A 
{
  void display();
}

class B implements A
{

    public void display() {
        System.out.println("Am in class B");
    }

}

class C extends B
{
    @Override
    public void display() {
        System.out.println("Am in class C");
    }
    
}

In case above your code will compile correctly but you will get runtime error because a created object instance of type B is passed to a reference of type C which is less than B in level. This means that in down-casting we are limited to the original object instance class type while in up-casting we don't have such similar situations.
The output:

updated.