Question

I have an assignment and i got a library including an interface class. [InfoItem]

I implement this class [Item].

Now i am required to write a method watchProgram(InfoItem item) [other class, importing InfoItem], which (as shown) requires an
InfoItem.

The passed parameter item has a variable ‘Recorded’ [boolean] which i want to edit using a method changeRecorded() that i defined in the implementation of InfoItem.

I cannot edit the interface and i get an error message that the method is not found [cannot find symbol]..

Any hints, suggestions, solutions?
Thanks!!

-Samuel-

Answer 1

In the method watchProgram, all Java knows is that the argument is an InfoItem. That argument may or may not be an Item, and thus it may or may not have that method changeRecorded. Since Java can’t guarantee that the object has that method, it can’t insert the code to call the method when the class is compiled. As far as the Java compiler is concerned, the method changeRecorded doesn’t exist in the argument item.

Usually when you run into a situation like this, it’s a sign that you shouldn’t really be calling that changeRecorded method in the first place. Think very carefully about why you think you need it and how you could change your own code to work without using it. For instance, if someone were to call your watchProgram method with some other implementation of InfoItem that doesn’t have a changeRecorded method, what should watchProgram do?

If, after some careful thought, you decide that it really is necessary for you to call the changeRecorded method when the passed-in argument is an instance of Item, you can use a cast to do so:

watchProgram(InfoItem item) {
    // do stuff
    if (item instanceof Item) { // thanks to ruslan for if statement
        Item castedItem = (Item)item;
        castedItem.changeRecorded();
    }
    // do stuff
}

But as I said, this sort of thing should be used sparingly. The whole point of object-oriented programming (specifically, polymorphism) is to make it so you don’t have to do this.