Why I can initialize extended class in other class as attribute?

Question

I don't understand why this code works.

I have 3 classes: A, B and C.

Class A have class B as attribute. Class C extends class B. I can initialize class C and set is as attribute of class A.

public class A {
    public B b;

    public A(){}
}

public class B {
    public String username;

    public B(){}
}

public class C extends B {
    public String password;

    public C(){}
}

public class Program
{

    public static void main(String[] args) {
        A a = new A();
        B b = new B();
        C c = new C();
        a.b = c;
    }
}

How can Java cast B as C if C is "larger"? If I try to do it normally, you can't, for example:

public class Program
{

    public static void main(String[] args) {
        B b = new C();
        // ERROR cannont find symbol (b.password)
        b.password = "asd";
    }
}

or I try it like this:

public class Program
{

    public static void main(String[] args) {
        B b = new B();
        C c = new C();
        b = c;
        //ERROR cannont find symbol (b.password)
        b.password = "123";
    }
}

Actual problem I have is with HTTP response. I create my own response object that has context. This context can be ResponseContext or ResponseContextPagination that extends ResponseContext. So, my Response object has ResponseContext atribute, and I can set that atribute as ResponseContextPagination and when I return whole response to client, it has valid JSON:

{"context":{"hasMore":false,"isLoggedIn":false}}

So, I can't get hasMore attribute if I try to access it because ResponseContext does not have hasMore attribute but why can I then even set hasMore as ResponseContextPagination object? And it serializes fine into JSON.