A question in JavaChamp about using "instanceof" that I didn't understand

[Amd Dem]

Hi all,

I took the express exam on the JavaChamp site and encountered this question, the question is about the output of the code:

interface Chewable {}
interface Eatable extends Chewable{}
class Food implements Chewable { }
class Meat extends Food {}
class Gum implements Chewable{}
 
public class Tester {
 
    public static void main(String[] args) {
         
       Food food = new Food();
        Meat meat = new Meat();
        Gum gum   = new Gum();
         
        System.out.print(food instanceof Eatable);
        System.out.print(meat instanceof Food);
        System.out.print(gum instanceof Food);
        System.out.print(gum instanceof Chewable);

    }
}

The solution is:

compilation error because of "gum instanceof Food"

The operands of instanceof must be in the same inheritance tree.

falsetruetrue

The solution is not clear for me.

My question is why it caused a compilation error and didn't print "false" instead?

I mean like the first print "food instanceof Eatable" it will print "false".

Why this worked and the third print didn't and they all implement Chewable? What is the difference?

[Matthew Brown]

Hi Amd Dem, welcome to JavaRanch!

In the case of 'food instanceof Eatable' - it is possible that this line could return true. You can have a Food variable referencing something Eatable, as in the following example:

class Toffee extends Food implements Eatable {
}

...

Food food = new Toffee();

But 'gum instanceof Food' cannot possibly be true. Gum and Food are both classes which are unrelated to each other, so there is no way we can assign anything to a Gum reference that is also a Food object. So the compiler takes the view that, since it cannot possibly be true, to try and make the comparison must be an error.

This is closely related to casting. Try the following:

Eatable e = (Eatable)food; // will compile, but throw a ClassCastException
Food f = (Food)gum;        // will not compile because the cast cannot possibly work

[Amd Dem]

It is now clear. Thanks!