"The compiler does not allow a cast from interface reference to..." OCPJSE11

[Nicholas Barbosa]

Look this afirmation, "The compiler does not allow a cast from interface reference to a object reference if the object type does not implement the interface"

Look this code.

interface InOne {

}

class ConcreteInOne implements InOne {

}

public class Concrete2 {

	public static void main(String[] args) {
		InOne one = new ConcreteInOne();

		Concrete2 con2 = (Concrete2) one;
	}
}

How this code compile, following this rule in OCP JSE 11 book??Of course, will throw a CastClassException.

[Nicholas Barbosa]

Nicholas Barbosa wrote:Look this afirmation, "The compiler does not allow a cast from interface reference to a object reference if the object type does not implement the interface"

Look this code.

interface InOne {

}

class ConcreteInOne implements InOne {

}

public class Concrete2 {

	public static void main(String[] args) {
		InOne one = new ConcreteInOne();

		Concrete2 con2 = (Concrete2) one;
	}
}

How this code compile, following this rule in OCP JSE 11 book??Of course, will throw a CastClassException.

I think, this follow the same rule as instanceof operator for interfaces.The compiller allows because the Concrete2 class its not final,so there is a possibility that yours child classes(of Concrete2) ,implement the InOne interface,making compatible.

interface InOne {

}

class ConcreteInOne implements InOne {

}

public class Concrete2 {

	public static void main(String[] args) {
		InOne one = new Child();

		Concrete2 con2 = (Concrete2) one;
		
	}
}

class Child extends Concrete2 implements InOne {
}

This code compiles, however If I marked Concrete2 as final, never will compile.Because no one can extends Concrete2.

[Campbell Ritchie]

Please give us fuller details of the book, including authors. Is it Boyarsky and Selikoff?

The full details are in the Java® Language Specification (=JLS). The JLS and the language are written on the assumption that programmers are competent and when they wrote code the know what they are doing. If you write a narrowing reference conversion, it is conceivable that your no 2 class has a subtype implementing the InOne interface, so it “believes” you,even though you are “lying” to it with that cast.

[Nicholas Barbosa]

Campbell Ritchie wrote:Please give us fuller details of the book, including authors. Is it Boyarsky and Selikoff?

The full details are in the Java® Language Specification (=JLS). The JLS and the language are written on the assumption that programmers are competent and when they wrote code the know what they are doing. If you write a narrowing reference conversion, it is conceivable that your no 2 class has a subtype implementing the InOne interface, so it “believes” you,even though you are “lying” to it with that cast.

Yes, its OCP JSE11 book, from selikoff.

You said, what i meant in the previous comment. Java believes that a child class has a subtype implementing the InOne interface, thats why if I mark  the parent class as final, wont compile.

Look the example again.(works if concrete2 its not final)

interface InOne {
 
}
 
class ConcreteInOne implements InOne {
 
}
 
public class Concrete2 {
 
    public static void main(String[] args) {
        InOne one = new Child();
 
        Concrete2 con2 = (Concrete2) one;
         
    }
}
 
class Child extends Concrete2 implements InOne {
}

[Campbell Ritchie]

That sounds correct, but the common practice of saying, “child,” or “parent,” is misleading. The best terminology I have heard is what they say in C#: “base,” and, “derived,” classes.

[Jeanne Boyarsky]

You are correct. it should say "cannot implement" rather than "does not"