当B扩展A时，为什么B b = new A（）无效而A a = new B（）有效？

Question

I am new to JAVA and want to know that why

A a = new B();

is valid and

B b = new A();

is invalid Considering that:

class A;
class B extends A;

Answer 1

Because B , by extending A , is also an A . We say this in object-orientation terms by saying that a B is-a A . This means that you can use a B anywhere you use an A .

This relationship is not commutative -- B is-a A does not imply that A is-a B . Therefore you cannot use an A anywhere you would use a B .

Consider this case:

class Animal;
class Dog extends Animal;

This makes sense:

Animal animal = new Dog();

Anywhere it makes sense to use an Animal you can also use a Dog . This is intuitive.

Dog dog = new Animal();

This, on the other hand, does not make sense.

Answer 2

Because when B extends A you consider that every B is a more complex A, using the attributes and methods from A and adding some of his own, but an A cannot be a B, there could be methods in B not specified in A, and as it is not extended from B, cannot be instantiated.

Every B is an A , but no every A is a B ( unless you specify that )

Answer 3

class A { 
   int i = 10;
}

class B {
   int i = 10;
   int j = 20;
}

class C {

   public static void main(String args[]){   
      B b;
      B b=new A();
      System.out.println(b.j);
   }
}