inheritance - In java , can we pass superclass Object to subclass reference?

Question

In java, can we pass superclass Object to subclass reference ?

I know that it is a weird question/practically not viable, but I want to understand the logic behind this Why is it not allowed in java.

class Employee {
    public void met1(){
        System.out.println("met1");
    }
}


class SalesPerson extends Employee 
{
    @Override
    public void met1(){
    System.out.println("new met1");
    }


    public void met2(){
        System.out.println("met2");
    }

}

public class ReferenceTest {
    public static void main(String[] args) {

        SalesPerson sales = new Employee(); // line 1

        sales.met1();  // line 2

        sales.met2();  // line 3
    }
}

What would have happened if Java allowed compilation of line 1? Where would the problem arise?

Any inputs/link are welcomes.

Answer 1

If your SalesPerson sales = new Employee(); statement was allowed to compile, this would have broken the principles of Polymorphism, which is one of the features that the language has.

Also, you should get familiar with that does compile time type and runtime type mean:

The compile-time type of a variable is the type it is declared as, while the runtime type is the type of the actual object the variable points to. For example:

Employee sales = new SalesPerson();  

The compile-time type of sales is Employee, and the runtime type will be SalesPerson. The compile-time type defines which methods can be called, while the runtime type defines what happens during the actual call.

Let's suppose for a moment that this statement was valid:

SalesPerson sales = new Employee();

As I said, the compile-time type defines which methods can be called, so met2() would have been eligible for calling. Meanwhile, the Employee class doesn't have a met2() and so the actual call would have been impossible.