Accessing an object using a reference variable of an unrelated Interface

[Andy James]

Why is casting an object of type Person to an unrelated Interface Printable allowed, as follows:

interface Printable {}
class Person {}
class Test {
	Person person = new Person();
	Printable pr = (Printable)person;
}

cheers
Andy

[Henry Wong]

Andy James wrote:Why is casting an object of type Person to an unrelated Interface Printable allowed, as follows:

interface Printable {}
class Person {}
class Test {
	Person person = new Person();
	Printable pr = (Printable)person;
}

cheers
Andy

What should the compiler do in this case?

interface Printable {}
class Person {}
class PrintablePerson extends Person implements Printable {}
class Test {
	Person person = new PrintablePerson();
	Printable pr = (Printable)person;
}

Basically, it is not possible, at the time of the cast, to determine whether the Person reference is referring to a Person object that implements Printable (or not) -- hence, it is allowed at compile time (but will fail at runtime). On the other hand, try making the Person class final, and see what happens.

Henry