About Question enthuware.ocpjp.v7.2.1497 :

[pSteven]

class B {}
class B1 extends B {}
class B2 extends B {}
public class ExtendsTest
{
public static void main(String args[])
{
B b = new B();
B1 b1 = new B1();
B2 b2 = new B2();
// insert statement here
}
}

_______________________

A. b = b1; Not a problem because B1 IS-A B -- I get it.
D. b2 = (B2) b1;
"It will not compile because b1 can never point to an object of class B2."
I thought B2 IS-A B1 and B2 IS-A B. If so, a B1 reference could be assigned to a B2 object, and later transferred to a reference of its own type with the above cast.

Not so?

[admin]

B2 is not a B1. B2 is a B.
-Paul.

[pSteven]

It was late. I must have been tired. I did look at it for a few minutes before I asked, but now it's clear as day. But if the inheritance were thus . . .

class B {}
class B1 extends B {}
class B2 extends B1 {}

Then my question would have been valid. Sorry, and thanks.

[emj211]

I am a bit confused about the explanations for 2 answers here:

(option 3)
b2 = b;
It fails at Compile time as an object referenced by b may not be a B2, so an explicit cast will be needed.

(option 4)
b1 = (B1) b
It will pass at compile time but fail at run time as the actual object referenced by b is not a B1.

Option 3 is implying that if you provide an explicit B2 cast to b then it would work, but that is similar to option 4 and would produce a RuntimeException, right?

[admin]

If you provide an explicit case, you are telling the compiler that at run time the object will be of the specified type, so the compiler lets it compile.
But at runtime, the JVM see that the actual object is not of the require type so it fail.

What this means is something may compile fine but may not necesarily run correctly.