Question

I am new to java and I have some problem on Casting.
I have a class called Parent, and a class called Children, the Children class is the subclass of Parent.

public class Parent
{
    int age;
    String occupation;

    public void print()
    {
        System.out.println("My age is:"+ age + "and i am a:" + occupation);
    }
}



public class Children extends Parent
{
    int height;

    public static void main(String args[])
    {
        Children c = new Children();
        Parent p = new Parent();

        p=c;
        c=(Children) p;

        /**Here***/



    }

}

My problem is, when I add p.XXX after the casting, I only see the age and occupation accessible for instance p. and when I do c.xxx, I see all age, occupation and height accessible.
I thought when I do p=c, p now are considered to be an instance of a children class isn’t it? If yes, then why i didnt see the height integer accessible?
And when I do c=(Children)p, an instance of Parent class is assigned to an instance of Children class, and since the Parent instance doesn’t have a option, that why we do a casting from parent to children, correct?

Answer 1

I thought when I do p=c, p now are considered to be an instance of a children class isn’t it?

No. The type of the variable p is still Parent. The value will be a reference to an instance of Children, but the compiler doesn’t take that into account. It only uses the declared type of the variable.

And when I do c=(Children)p, an instance of Parent class is assigned to an instance of Children class, and since the Parent instance doesn’t have a option, that why we do a casting from parent to children, correct?

Well you’re not changing the instance at all. You’re casting an expression of type Parent – the value will be a reference. At execution time, the JVM will ensure that the value is actually a reference to an instance of Children or some subtype (or null) – if it isn’t, the JVM will throw an exception.

It’s very important to distinguish between variables, references and objects – they’re three quite different concepts, and making sure you understand the difference between them will make a lot of other things clearer.