Question

I’m new to java so apologies if I’ve got totally the wrong end of the stick.

I’m trying to write a generic (in the English sense of the word!) Data Access class.
eg I currently have:

public class DA<T> {
public static Dao getAccountDao() throws NamingException, SQLException {
    Context ctx = new InitialContext();
    DataSource dataSource = (DataSource)ctx.lookup("java:comp/env/jdbc/test");
    ConnectionSource connectionSource = new DataSourceConnectionSource(dataSource, new MysqlDatabaseType());            
    Dao<Account, Integer> accountDao =  DaoManager.createDao(connectionSource, Account.class);
    return accountDao;
}
}

And I can call this with:

Dao<Account, Integer> accountDao = DA.getAccountDao();

But I’ll need a version of this for every Dao/model. So I’m trying to make something that can be called like:

Dao<SomeClass, Integer> someClassDao = DA.getDao(SomeClass);

Is this even possible?

I’ve tried things like:

public class DA {
public static Dao getDao(<T>) throws NamingException, SQLException {
    Context ctx = new InitialContext();
    DataSource dataSource = (DataSource)ctx.lookup("java:comp/env/jdbc/test");
    ConnectionSource connectionSource = new DataSourceConnectionSource(dataSource, new MysqlDatabaseType());            
    Dao<T, Integer> accountDao =  DaoManager.createDao(connectionSource, T.class);
    return accountDao;
}

}

but Netbeans gives the error: illegal start of type

My brain is struggling with generics, is this something they can do?!

EDIT: With help from the posts below I’ve got to:

public class DA<T> {
public static Dao<T, Integer> getDao(T daoType) throws NamingException, SQLException {
    Dao<T, Integer> accountDao =  DaoManager.createDao(T.class);
    return accountDao;
}

}

Which generates two errors:
non-static type variable T cannot be referenced from a static context
and if I remove the static keyword, I get:
cannot select from a type variable
I need to read up on how generics and static work together, but the 2nd looks like a consequence of erasure (http://www.coderanch.com/t/386358/java/java/Converting-type-parameters-class) , so not sure if it’s going to be possible.

Should have mentioned earlier, the Dao stuff is using an ORM library called ORMLite, so createDao etc isn’t my code.

Answer 1

To access what you mean by T.class, you’ll have to pass the class object into the method:

class Account {}

class Dao<TEntity, TId> {}

class DA {
    // your DaoManager.createDao() will also need a similar signature
    public static <TEntity> Dao<TEntity, Integer> getDao(Class<TEntity> daoType) {
        /// yadda blah, create DAO as appropriate
        return new Dao<TEntity, Integer>();
    }
}

public class Test {
    public static void main(String[] args) {
        // Pass the class literal for what you want TEntity to be as a parameter
        Dao<Account, Integer> dao = DA.getDao(Account.class);
    }
}