Casting a class to an unrelated interface

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
   }
}

Solution 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.