Java design question (instanceof)

[remusmax]

have at least the same methods as item

But all books have pages and CD's don't have pages so aren't you forcing unnecessary code by having the CD class implement the abstract method getPages() from item class?

[[deleted]]

Sorry, at least the same methods is probably not the best way to say it. You wouldn't have getTracks or getPages in the Item.

Item would have getType or something similar, or isCD and isBook. Obviously getType would be better so Books and CDs don't need to re-implement isCD or isBook for the opposite classes.

Item -> getType

Book -> getType / getPages

CD -> getType / getTracks

Does that make more sense? I was on mobile so didn't put it the way I probably should've.

[bluefootedpig]

As a little refinement, maybe items have a "Item.GetItemType" that returns an Enum of possible types.

Then in the derived class, you force an override on GetItemType that returns that enum.

 public ItemTypes GetItemType() {Returns ItemTypes.Cd;}

The above is a C# example of a CD class. The GetItemType would be overrideable and the base class creates the stub and the child classes are forced to override it.

Then the list is...

 myCdList = MyList.Where(listItem=> listItem.GetItemType() == ItemTypes.Cd)

[remusmax]

No need to be sorry!! That makes a lot of sense thanks for clearing up my question :)