ChildClass childClass=new ParentClass()

[Raj Srimandal]

I have a concrete Parent class(ParentClass) and Child Class(ChildClass) which inherits from ParentClass.I cannot say

ChildClass childClass=new ParentClass() ..i.e this is a compile time check
Does anyone know WHY?

[Christophe Verré]

Does anyone know WHY?

Does anybody doesn't ? You have a "public class ChildClass extends ParentClass". ChildClass may have some methods that ParentClass doesn't. How could you instantiate the parent and assign it to a child ? You probably want to do the opposite :

ParentClass childClass = new ChildClass();
or
ChildClass childClass = new ChildClass();

[Christophe Verré]

And welcome to the ranch
You may be interested to read this tutorial about inheritance.

[fred rosenberger]

public class Parent
{
    methodA();
}

public class Child extends Parent
{
    methodB();
}

if you wrote this:

Child c = new Parent();

What would happen when you then called

c.methodB();

The object that is actually created has NO IDEA what methodB() is - it doesn't exist. But the reference says that you can call it.

It would be like saying you have a "class Animal" and a ''class fish extends animal". You can refer to any fish as an animal, but you can't refer to any animal as a fish.

[Raj Srimandal]

Thanks Chris and Fred..

How is the JVM is letting me do this

Child c =(Child) new Parent();