Casting a class to an unrelated interface

Question

Obviously, this results in a compilation error because Chair is not related to Cat:

class Chair {}
class Cat {}

class Test {
   public static void main(String[] args) {
       Chair chair = new Char(); Cat cat = new Cat();
       chair = (Chair)cat; //compile error
   }
}

Why is it then that I only get an exception at run time when I cast a Cat reference to the unrelated interface Furniture, while the compiler can obviously tell that Cat does not implement Furniture?

interface Furniture {}

class Test {
   public static void main(String[] args) {
       Furniture f; Cat cat = new Cat();
       f = (Furniture)cat; //runtime error
   }
}

Answer 1

The reason this compiles

interface Furniture {}

class Test {
   public static void main(String[] args) {
       Furniture f; Cat cat = new Cat();
       f = (Furniture)cat; //runtime error
   }
}

is that you may very well have

public class CatFurniture extends Cat implements Furniture {}

If you create a CatFurniture instance, you can assign it to Cat cat and that instance can be casted to Furniture . In other words, it's possible that some Cat subtype does implement the Furniture interface.

In your first example

class Test {
    public static void main(String[] args) {
        Chair chair = new Char(); Cat cat = new Cat();
        chair = (Chair)cat; //compile error
    }
}

it's impossible that some Cat subtype extends Chair unless Cat itself extends from Chair .