why subclass reference can't hold superclass object in java

Question

class One{

}
class Two extends One{

}
class Main{

      public static void main(String[]  args){
       Two t = new One(); // invalid 

}`
}

I am not able to understand the reason behind it, why child class reference could not hold the object of parent, while superclass reference can hold the object of subclass.

Answer 1

Because a dog has all the behaviours of an animal, but something that is only known to be an animal is not guaranteed to have all the behaviours of a dog.

Answer 2

Every Child is a Parent but not every Parent is a Child. Rule of Inheritance.

Answer 3

If we think in terms of set theory, compared to the parent class, the child class is a super set. The child class has all possible properties and methods, compared to the parent class.

So, a parent class object can refer to its child class object, as the child class object includes the parent's methods and properties. Thinking vice versa, since a parent class object does not have all the methods and properties needed by a child class, a child class object cannot refer to a parent class object.