About Question enthuware.ocajp.i.v7.2.1002 :

[admin]

I1 and I2 don't necessarily need to have any is-a relationship. You can have a class that extends C3 and implements I1 as well, right? The compiler can see this possibility and accepts the code.
In case of your example, the type of a is U1 and you are trying to assign it to U2. But there is no way the variable a could ever point to an object of a class that satisfies is-a relationship to U1. The compiler figures this out and rejects the cast.

If you had something like this:
I1 a = null;
U2 b = (U2) a;
This is ok even if U2 does not implement I1 because the compiler knows that it is possible to write a class that extends U2 and implements I1. Therefore, it is possible for a to point to an object of a class that is-a U2. This proves that there is no need for I1 and U2 to have any is-a relationship between themselves.

HTH,
Paul.

[eddie3]

Oh, that makes perfect sense now. Thank you!

[flex567]

The explanation says:

This is valid because at run time i1 actually refers to an object that implements I2.

but there are no objects, only references?

[admin]

You can cast null to any type. So even if there is no object, the JVM has no problem with the assignment.