Casting a class to an unrelated interface

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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 Chair();
        Cat cat = new Cat();
        chair = (Chair)cat; //compile error
    }
}

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

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