Problem with casting

[Federico Er]

Hi there,

i'm training for the SCJP exam and i just can't get something out of my mind, two pieces of code from the second chapter's Self Test

class Dog {
	public void bark() { System.out.print("woof "); }
}

class Hound extends Dog {
	public void bark() { System.out.print("howl "); }
	public void sniff() { System.out.print("sniff "); }
}

public class DogShow {
	public static void main(String[] args) {
		new DogShow().go();
	}
	void go() {
		new Hound(). bark();
		((Dog) new Hound()).bark();
		((Dog) new Hound()).sniff();
	}
}

And the other one is

public class Redwood extends Tree {
	public static void main(String[] args) {
		new Redwood().go();
	}
	void go() {
		go2(new Tree(), new Redwood());
		go2((Redwood) new Tree(), new Redwood());
		}
	void go2(Tree t1, Redwood r1) {
		Redwood r2 = (Redwood) r1;
		Tree t2 = (Tree) r1;
	}
}

class Tree { }

It must be really silly, but i don't understand why the first one causes a compile error in line 15 while the second one an exception at runtime.

Thanks in advance

[Tom Reilly]

What is the compiler error? What is the runtime error?

[Federico Er]

Thanks for answering!

In the DogShow example is a compiler error in line 17, sorry, not 15:

DogShow.java:17: cannot find symbol
symbol  : method sniff()
location: class Dog
		((Dog) new Hound()).sniff();
		                   ^
1 error

But in the Redwoodone it compiles just fine and i get an Exception at Runtime

Exception in thread "main" java.lang.ClassCastException: Tree cannot be cast to Redwood
	at Redwood.go(Redwood.java:7)
	at Redwood.main(Redwood.java:3)

I don't understand why in DogShow i'm stoped by the compiler instead of recivieng later a ClassCastException

[Tom Reilly]

1. You are casting Hound to a Dog. Dog does not have a sniff() method. The compiler at this point only knows about Dog so it complains.
2. A Redwood IS A Tree. But a Tree IS NOT A Redwood. You are telling the compiler that a Tree is a Redwood so it believes you. The JVM knows you are lying.

[Federico Er]

Oh i see,

i get the compiler error when javac notices that there is no sniff() method on Dog. On the other hand, i get a ClassCastException when the casting itself is wrong...

Thanks alot, really, for the fast answer!