I am confused about the Category.java class @ 3.2.4. Why does it Have a Category instance variable itself? I understand it having the Set for for sub-categories, but isn't "this" sufficient for a Category to know about itself?

Scott

The instance variable has the name "parentCategory". This is a reference to another instance of Category, the "parent". This is a hierarchy of categories.

Christian --

Do you ever sleep? I get better customer support from you than I do with IBM! Okay bra, let me ask my question another way. Consider the class below:


class Category {

private Long id;
private Set children;

}

This code allows me to treat the 1-n self-association without the Category instance variable.

Peace,
Scott

Bad idea. The parentCategory reference is a direct pointer-representation of the foreign key column in the database schema. It's good to have that. I repeat this several times in the book: Map all foreign keys to many-to-one associations. Then make these associations bidirectional (with a collection) if you _have_ to. Not just because you feel like it.

What about the scenario where I am not saving the self-association? Why would the self reference need to exist then?

There is no "self" reference pointer. The "parentCategory" is a pointer that represents the PARENT_CATEGORY_ID foreign key column. This is a reference to a _different_ category, the parent of the current one.

Yes, the childCategories of this mysterious parent is a mapping of this very same foreign key. But it's not the same representation in Java.

Do you understand that the categories form a tree, a data structure that has some recursive properties?

Thanks Christian! As soon as you you said "tree," I got it.

Is there a way to retrieve the most current eBook periodically? I'd rather not mess with the errata alongside my PDF.

Thanks again,
Scott

There are no regular updates to the ebook, maybe every half year when a new print run has to be done. I don't expect any major changes, minor errata only.