Java：覆盖父类的静态变量？

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?

Answer 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.

Answer 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

Answer 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";
}