What happens if i assign object to another object of different class? [duplicate]

Question

Here is my code :

class A
{
    int x=2,y=3;
}
class B extends A
{
    //Blank 
}
public class Test
{
    public static void main(String args[])
    {
        A a=new A();
        B b;
        b=a;
        System.out.println(b.x);
        System.out.println(b.y);
    }
}

When i'm trying to run this code, "incompatible types: A can't be converted to B" Error is shown.

And also when i'm trying to type-cast "a" into "B", ClassCastException is occured.

Is there any way to assign object to another object of different class in java ?

Answer 1

Technically speaking, a variable of "base class" can be assigned "sub class" but not the other way around. Namely you can have "A a=new B()" but not "B b=new A()".

There is also an intuitive reason. The base class is usually something basic, say "Person", while the subclass is more specific and promises to provide more properties/capabilities e.g. "Teacher" (= person plus additional capabilities to teach). You can't force an ordinary Person to magically call itself a teacher, it will just fail the minute you try to activate any teaching capabilities...

We could start a philosophical argument about occasions where the specific thing has less capabilities than the base (e.g. "IncompetentPerson extends Person") but java inheritance is not generally aiming to cover that.