Can we assign parent class object to a child class variable in Java?

Question

I'm a beginner in Java and I was playing with some Java code. However I couldn't sort out why is it giving runtime error when I assign the reference of a parent class object to a child class variable. I've done explicit typecasting too. Is there any way possible to resolve it?

class Employee{
    int e;//creating empty class
}

class Teacher extends Employee{
    int f;//creating empty class
}

class run{
    public static void main(String args[]){
        Employee e;
        e= new Employee();
        Teacher t;
        t= (Teacher)e; //giving runtime error
    }
}

Does this answer your question? [Can't not downcast in java at runtime](https://stackoverflow.com/questions/17499281/cant-not-downcast-in-java-at-runtime) — maloomeister, Oct 12 '21 at 07:43

score 2 · Answer 1 · answered Oct 12 '21 at 07:46

2

We can assign child class object to parent class reference variable but we can't assign parent class object to child class reference variable.

answered Oct 12 '21 at 07:46

Kishor Patil

29
5

score 2 · Answer 2 · answered Oct 12 '21 at 09:03

A Teacher is an Employee, but not every Employee is a Teacher.

So, an Employee variable can hold objects of type Employee itself, as well as its subclasses like Teacher, Worker or whatever. But if you have a Teacher variable, it can only hold Teacher instances (and subclasses of Teacher), but not Employee or Worker instances.

With

t = (Teacher) e;

you have e, an Employee variable. Generally, this e does not "fit into" a Teacher variable, only in the possible case that the Employee variable e holds an Employee subclass instance, namely a Teacher instance.

So, without the cast, the compiler forbids the assignment. Writing the cast, you as developer tell the compiler

Hey, I know that generally the assignment isn't allowed, but I promise that, at runtime, the instance in e will be a Teacher. So, please compile that line of code despite the risk, and give me an exception at runtime if my promise doesn't hold.

And that's what happens.

Can we assign parent class object to a child class variable in Java?

2 Answers2