Help with java inheritance !!!

[vikkyhacks]

class A { int a = 10; }
class B extends A { int a = 20; }


class C {
	
	public static void main(String[] args) {
		A a = new B(); // why does this work ??
		B b = new A(); // why does this not ??
	}
	
}

I just can't understand why

BASE_CLASS <var_name> = new DERIVED_CLASS (will work)
DERIVED_CLASS <var_name> = new BASE_CLASS (will not work)

HELP !!!!!!!

[diesch]

If b is a variable of Type B it needs to ysupport all the operations that type B supports. This is only guaranted if its value is an instance of B or one of its subclasses, so you'll get a compile time error if you try to assign a value to b that is not such an instance.

It may be more obvious if you define B as

class B extends A { int b = 20; }

Then if b is of type B it has to have the class variables a (inherited from A) and b.
As doesn't have b if it is an instance of A it is not allowed to be such an instance.

[slickymaster]

Java Inheritance defines an is-a relationship between a superclass and its subclasses. This means that an object of a subclass can be used wherever an object of the superclass can be used. The inheritance relationship is transitive: if class x extends class y, then a class z, which extends class x, will also inherit from class y.

[Vaphell]

to visualize the problem better

class HasFourLegs {} // base class
class Dog extends HasFourLegs { <dog stuff> }
class Table extends HasFourLegs { <furniture stuff> }

HasFourLegs a = new Dog(); // makes sense
Dog b = new HasFourLegs(); // doesn't make sense, especially because it would also mean that Dog is compatible with Table

in other words inheritance is a one way street. Every DerivedClass object is of BaseClass type at the same time, but not every object under the umbrella of BaseClass belongs to DerivedClass