Java: Overriding static variable of parent class?

Question

I have the following class which I'm using as the base of all the models in my project:

public abstract class BaseModel
{
    static String table;
    static String idField = "id";       

    public static boolean exists(long id) throws Exception
    {
        Db db = Util.getDb();
        Query q = db.query();
        q.select( idField ).whereLong(idField, id).limit(1).get(table);

        return q.hasResults();
    }

    //snip..
}

I'm then trying to extend from it, in the following way:

public class User extends BaseModel
{
    static String table = "user";
    //snip
}

However, if I try to do the following:

if ( User.exists( 4 ) )
   //do something

Then, rather than the query: "SELECT id FROM user WHERE id = ?", it is producing the query: "SELECT id from null WHERE id = ?". So, the overriding of the table field in the User class doesn't seem to be having any effect.

How do I overcome this? If I added a setTable() method to BaseModel, and called setTable() in the constructor of User, then will the new value of table be available to all methods of the User class as well?

Solution 1

You cannot override static methods or fields of any type in Java.

public class User extends BaseModel
{
    static String table = "user";
    //snip
}

This creates a new field User#table that just happens to have the same name as BaseModel#table. Most IDEs will warn you about that.

If you change the value of the field in BaseModel, it will apply to all other model classes as well.

One way is to have the base methods generic

protected static boolean exists(String table, long id) throws Exception
{
    Db db = Util.getDb();
    Query q = db.query();
    q.select( idField ).whereLong(idField, id).limit(1).get(table);

    return q.hasResults();
}

and use it in the subclass

public static boolean exists(long id)
{
    return exists("user", id);
}

If you want to use the field approach, you have to create a BaseDAO class and have a UserDAO (one for each model class) that sets the field accordingly. Then you create singleton instances of all the daos.

Solution 2

Because Java doesn't allow you to override static members, you basically need to resort to the slightly more verbose but overall nicer singleton pattern, wherein you're still conceptually writing "static" code, but you're technically using (global/singleton/"static") instances, so you're not restricted by the limitations of static.

(note that you also need to use methods because fields don't participate in polymorphism, and thus cannot be overridden)

public abstract class BaseTable {
    public abstract String table();
    public String idField() { return "id"; }

    public boolean exists(long id) {
        // don't build queries this way in real life though!
        System.out.println("SELECT count(*) FROM " + table() + " WHERE " + idField() + " = " + id);
        return true;
    }
}

public class UserTable extends BaseTable {
    public static final User INSTANCE = new UserTable();
    private UseTabler() {}

    @Override public String table() { return "user"; }
}

public class PostTable extends BaseTable {
    public static final Post INSTANCE = new PostTable();
    private PostTable() {}

    @Override public String table() { return "post"; }
}

public static void main(String[] args) {
    UserTable.INSTANCE.exists(123);
    PostTable.INSTANCE.exists(456);
}

Outputs:

SELECT count(*) FROM user WHERE id = 123
SELECT count(*) FROM post WHERE id = 456

Solution 3

In order to do what you are looking to do, don't make table static in the BaseModel. Then in the other classes that inherit from BaseModel, you can set table in the default constructor to whatever you wish.

static {
    table = "user";
}

Comments

[Rohit Jain]

Why would an IDE warn about data hiding?

[Cephalopod]

@Click: I extended my answer.

[gregory boero.teyssier]

exists() is only one example ,I have another 10-20 such helper methods. I don't want to overload all of them, I'd rather just change the table name property so they will all work.

[Cephalopod]

@Rohit: because it causes unexpected behavior, as in OP's case.

[gregory boero.teyssier]

Then I can't call User.exists() as a static method.

[Rohit Jain]

@Arian It's totally an expected behaviour. It's just that OP didn't expected that. OP is talking about overriding fields, which is never done. Not even for instance field.

[StormeHawke]

@ClickUpvote this could be an indication that you're misusing the static keyword. I'd recommend taking a look at your design and reworking it to get rid of the need for a static exists() method

[ChuckCottrill]

stackoverflow.com/questions/685300/…

[gregory boero.teyssier]

@StormeHawke What's wrong with my design? Will it be clearer if I made an instance of my user model just to check if the given user id is valid?

[Cephalopod]

"OP didn't expected that" so it was unexpected ;) ... also, who downvoted my answer? At least give some reason, coward!

[StormeHawke]

@ClickUpvote Not saying there is something wrong with your design, only that there could be something you need to look at if you're running into something like this where the design of the Java language doesn't allow you to do what you're wanting to do

[ChuckCottrill]

stackoverflow.com/questions/1448092/…

[gregory boero.teyssier]

Is there any way to check, from within BaseModel.exists(), what the instance type of the class is? E.g using getClass().getName() or instance of? So if I could do if .. instance of User table = "user" .

[Brian Dishaw]

@ClickUpvote you could if you made the User's default constructor static as well. I didn't include that in my sample (shame on me) because I thought you would see that is the only way it would work with your current implementation.

[gregory boero.teyssier]

@BrianDishaw If I made that static.. then could I also instantiate multiple new User objects with their own properties? Or will I be able to have only one User object at a time?

[Brian Dishaw]

@ClickUpvote If you made a constructor static, then you would be able to interact with your other static methods in your base class. Nothing would prevent you from instantiating other User objects or having additional constructors that were non static with additional properties.

[Erik Kaplun]

here's how it would look like in Scala with some naive reflection added, without the singleton boilerplate: pastebin.com/3iujwygZ