Why can't reference to child Class object refer to the parent Class object?

Question

I was explaining OOP to my friend. I was unable to answer this question.

I just escaped by saying, since OOP depicts the real world. In real world, parents can accommodate children but children cannot accommodate parents. same is the case in OOP.

class Parent
{
  int prop1;
  int prop2;
}

class Child : Parent // class Child extends Parent  (in case of Java Lang.)
{
  int prop3;
  int prop4;
  
  public static void Main()
  {
     Child aChild = new Child();
     Parent aParent = new Parent();
     aParent = aChild;// is perfectly valid.
     aChild = aParent;// is not valid. Why??
 
  }
}

Why isn't this statement valid?

 aChild = aParent;// is not valid. Why??

since aChild's members are superset of aParent's members. Then why can't aChild accommodate a parent.

Answer 1

Exactly because aChild is a superset of aParent's abilities. You can write:

class Fox : Animal

Because each Fox is an Animal. But the other way is not always true (not every Animal is a Fox).

Also it seems that you have your OOP mixed up. This is not a Parent-Child relationship, because there's no composition/trees involved. This is a Ancestor/Descendant inheritance relation.

Inheritance is "type of" not "contains". Hence it's Fox is a type of Animal, in your case it doesn't sound right -- "Child is a type of Parent" ? The naming of classes was the source of confusion ;).

class Animal {}
class Fox : Animal {}
class Fish : Animal {}

Animal a = new Fox(); // ok!
Animal b = new Fish(); // ok!
Fox f = b; // obviously no!

Answer 2

If it was valid, what would you expect when you read aChild.prop3? It is not defined on aParent.