Question on Object cast (Java OCA 8 Programmer I Study Guide, Sybex)

[Raghavendra Desoju]

interface CanClimb{
	int maxHeight();
}
interface HasClaws{
	boolean isSharp();
}

public class Koala implements CanClimb, HasClaws {


     
	public boolean isSharp() {
		return true;
	}
	public int maxHeight() {
		return 15;
	}

	public static void main(String []args){
	
		Object koala = new Koala();                          //   A
		CanClimb canClimb = (CanClimb) koala;                //   B 
		HasClaws hasClaws = (HasClaws) canClimb;             //   C
		System.out.print(canClimb.maxHeight());
		System.out.print(hasClaws.isSharp());
	}

}

Hi,

I expected above code would throw compile time exceptions. However it compiled and printed "15true".
Here are my questions:-
1. Does Interface in java inherit from java.lang.Object?
2. If not, why does compiler sense an issue when Object "koala" (please see comment A) cast'ed to an interface reference variable next line (comment B) since they do not share same inheritance hierarchy?
3. Why does compiler not throw an error when reference object "canClimb" (line with comment B) cast'ed to another interface reference object HasClaws (line with comment C) since they do not share same inheritance hierarchy?

Please clarify.

Thanks,
Raghu

[Roel De Nijs]

Raghavendra Desoju wrote:1. Does Interface in java inherit from java.lang.Object?

No! Only classes extend from java.lang.Object (directly or indirectly).

Raghavendra Desoju wrote:2. If not, why does compiler sense an issue when Object "koala" (please see comment A) cast'ed to an interface reference variable next line (comment B) since they do not share same inheritance hierarchy?

The compiler knows that reference variable koala is of type Object. Because class Object is not final, you can cast a reference variable to any interface without a compiler error (you might get a runtime ClassCastException if the interface is incompatible). If class Object would have been final, you'll get a compiler error on lineB. A more detailed explanation about casting and interfaces can be found here.

Raghavendra Desoju wrote:3. Why does compiler not throw an error when reference object "canClimb" (line with comment B) cast'ed to another interface reference object HasClaws (line with comment C) since they do not share same inheritance hierarchy?

The compiler knows that reference variable canClimb is of type CanClimb. Because interface CanClimb is an interface (and therefore not final), you can cast a reference variable to any interface without a compiler error (you might get a runtime ClassCastException if the interface is incompatible). You can also cast to a any non-final class (you might get a runtime ClassCastException if the class is incompatible). You can even cast to a final class, but only if this class implements CanClimb; otherwise you'll get a compiler error.
Let's provide some code snippets for all these different cases

Koala hasClaws1 = (Koala) canClimb; // ok

String hasClaws2 = (String) canClimb; // compiler error

interface Moveable {}
Moveable hasClaws3 = (Moveable) canClimb; // runtime exception

final class Animal {}
Animal hasClaws4 = (Animal) canClimb; // compiler error

class Tiger {}
Tiger hasClaws5 = (Tiger) canClimb; // runtime exception

final class Ant implements CanClimb {
  public int maxHeight() { return 5; }
}
Ant hasClaws6 = (Ant) canClimb; // runtime exception

Hope it helps!
Kind regards,
Roel

[edit] fixed a mistake (it's getting late ): you don't get a compiler error when casting canClimb to final class Ant (because it implements the CanClimb interface), but you'll get a runtime ClassCastException because Koala (the actual object canClimb is refering to) IS-NOT-A Ant.

[Raghavendra Desoju]

Thank you Roel !!

[Roel De Nijs]

I just noticed I made a mistake in one of the code snippets. I edited my post and fixed it!