If only one object field is required in a method, what should be passed as a parameter - an object or a field value?

Lets say there is a method that searches for book authors by book id. What should be passed as a parameter to such method - only book.id (int) or whole book object?

Or another example. In java I need to do some work with current url of the page. What should be passed to such method - only request.getRequestURL() or whole request?

I kind of see benefits from each method but can't come up with good rule wh开发者_JS百科en to use what.

Thanks.

I am not sure if there is a "rule" to what is best, but I most often pass just the paramaters I need into the method. So in your first example I would only pass in the book.id and in your second example I would only pass in the request.getRequestURL().

I try to avoid passing in more than I need.

I'm going to be a dissenter and argue for passing the entire Book object.

Reason 1: Type checking. If you just pass an integer ID, there's no way to know, looking at code, if you've got the correct "kind" of integer ID. Maybe you've been passing around an integer variable that you think is the Book ID, but it's actually the Author ID. The compiler is not going to help you catch this mistake, and the results are going to be buggy in unexpected ways.

Reason 2: Future proofing. Some have made the argument that if you just pass the ID, you give yourself the option to change the structure of the Book object later, without breaking the doSomethingWithBook(int ID) method. And that's true. On the other hand, if you pass the entire Book object, you give yourself the option to change the internals of doSomethingWithBook(Book book) (maybe it will want to search based on some other field in the future) without breaking any of the (possibly numerous) places you've called doSomethingWithBook. I'd argue that the latter helps you more.

In the case of the Request, I would give a different answer, since I would consider a Request object to be tightly linked to a certain type of interface (web) and therefore would want to limit the use of that object. One question I like to ask myself: if I wanted to switch this web application to be, say, a command-line application, how many classes would have to change? If I'm passing around the Request, that's going to "infect" more classes with web-specific logic.

Weaker connectivity is preferred unless there are specific reasons. When pass book id only to search method you are free to change Book interface without worrying that it might affect other functions. At some moment in future you may discover that you need to do exactly the same job with some URL outside request handler, so avoiding unneeded dependency on request is good. But note, that if you frequently call do_smth(request.getRequestURL()) it may become quite annoying.

This is related to the Law of Demeter, which basically states that objects and methods should only receive exactly what they need, rather than going through another object to get what they actually need. If you need to use multiple fields from a Book in your method, it might be better to just take a book. But in general, you'll have less coupling in a system if you only depend on exactly what you need.

In both your examples, just using the ID or URL would probably be preferable. Particularly in the case of the URL, where (if you want to test the method) it's easy to create a URL to test with but harder (and completely unnecessary) to create a request to pass to the method which will then only use the URL anyway. The method also becomes more generally applicable to other situations than one in which you have a request object.

I would give each method only as much as necessary (so for the second question: just give it request.getRequestURL()).

For the first one I would think about defining both methods (but prefer the id-one, as you can easily get the ID if you have a Book, but not the other way around).

findAuthorsForBookId(int bookId)
findAuthorsForBook(Book b)

Call book.authors().

(Note: this is a dissenting view regarding the accepted answer.)

Well, there is an implicit rule set in context of domain modeling. If the receiver is performing tasks independent of the domain model then you pass the field. Otherwise, you should pass the object and the model specific action is made explicit by the act of the receiver accessing the id property of the 'Book' object. Most importantly, if accessing the property ever evolves beyond simply returning the reference of a field (e.g. certain actions in the property accessor) then clearly you do NOT want to chase all instances in your code where you dereferenced the property before passing it into various methods.

Further considerations are the consequences (if any) of accessing the field before the call cite, or, inside the receiver.

There's no rule actually, you should be straightforward with the info you need, in that case the book.id. If you consider extending / sharing your search in the future, the you can have an overloaded method to accept a book object so that you can search by other attributes of the book object.

Think about maintaining the code in the long run. Any method you expose is a method you'll have to support for your users going forward. If bookId is all that's needed for the forseeable future, then I'd go with just passing in that: that way, anyone who has a bookId can use your method, and it becomes more powerful.

But if there's a good chance that you may need to refactor the lookup to use some other attributes of Book, then pass in Book.

If you're writing a DAO of sorts, you should consider having a BookSelector which can be built up like: new BookSelector().byId(id).bySomethingElse(somethingElse) and pass this selector instead of having a proliferation of findByXYZ methods.

I agree with the previous posters. I wanted to add that if you find yourself needing multiple properties of the object (id, title, author) then I'd suggest passing the object (or an interface to the object). Short parameter lists are generally preferable.

Lets say there is a method that searches for book authors by book id. What should be passed as a parameter to such method - only book.id (int) or whole book object?

I am making the assumption that "book authors" is an attribute of a book. Therefore, I imagine something like the following class:

class Book {
    private int id;
    private List<Author> authors;
    // ... maybe some other book information
    public int getID() {
        return this.id
    }
    public void setID(int value) {
        this.id = value
    }
    public List<Author> getAuthors() {
        return this.authors.clone();
    }
    // ...
}

Given an instantiated Book object (aBook), to determine the list of authors, I would expect that I can call aBook.getAuthors(), which requires no parameters.

I would discourage the creation of partially instantiated domain objects. In other words, given a bookid, and looking for a list of authors, I would want the client code to look more like this:

Book aBook = library.findBook(bookid);
List<Author> authors = aBook.getAuthors();

and less like this:

Book bookQuery = new Book().setID(bookid); // partially instantiated Book
Book aBook = library.findBook(bookQuery);
List<Author> aBook = book.getAuthors();

The first version reduces the number of throwaway objects that are created by the client code. (In this case, bookQuery, which isn't a real book.)

It also makes the code easier to read--and therefore to maintain. This is because bookQuery is not doing what the maintenance programmer would expect. For example, I'd expect two Books with the same ID to have the same tite, authors, ISBN, etc. These assertions would fail for bookQuery and aBook.

Thirdly, it minimizes the chance that you will someday pass an invalid (partially instantiated) Book object to a method that is expecting a real Book. This is a bug where the failure (in the method) may happen far away from the cause (the partial instantiation).