Thread: what s wrong with my code

hi every bady
my english is bad sorry
so, i have a problem in implement method named viewAllFactor(User us,ResultSet rs)
when i call this method using <T> like objet dont work because T obj is null
i ask if i can try something to have a good result
i have 2 classes BaseDao<T> (DAO Pattern) and Class User extends from BaseDao
this is the code
thiks

package appweb.core;
public abstract  class BaseDao<T> {
public ArrayList<T> viewall() {
			ResultSet rset = null;
			ArrayList<T> listAll = new ArrayList<T>();
			String sql = "SELECT* FROM "+ this.tableName;
			System.out.println(sql);
			try {		
 
				PreparedStatement ps =  this.connect.prepareStatement(sql);
				rset = ps.executeQuery(sql);
				while (rset.next()) {
					T obj ; [B][COLOR="#A52A2A"](mast initialis obj with null bath when i do that it s wrong in the call method viewAllFactor(obj,rset) [/COLOR][/B]
					this.viewAllFactor(obj,rset);
					listAll.add(obj);
				}
				rset.close();
				ps.close();
 
 
			} catch (SQLException e) {
				e.printStackTrace();
			}
					return listAll;
		}

public class UserDao extends BaseDao<User> {
.....
public void viewAllFactor(User u,ResultSet rs) {
 
	try{
 
			u.setId(rs.getInt("id"));                                [COLOR="#800000"]  error hier[/COLOR]
			u.setName(rs.getString("name"));
			u.setLogin(rs.getString("login"));
			u.setPass(rs.getString("pw"));
			u.setIdpr(rs.getInt("idProfil"));
			}
		catch (SQLException e) {e.printStackTrace();} 
 
}

sorry for my english

Originally Posted by essayd100

so, i have a problem in implement method named viewAllFactor(User us,ResultSet rs)
when i call this method using <T> like objet dont work because T obj is null
i ask if i can try something to have a good result
i have 2 classes BaseDao<T> (DAO Pattern) and Class User extends from BaseDao

From the posted code, I understand that you are trying to do a "generic" select-all logic. Generic in the sense that, being parameterized (mainly by the tableName), it should ideally work for any table/type.

It's a good idea in general, but there is a particular issue you need to resolve, and in the right way. At some point you must instantiate an object of a concrete class. User is the type you need at this moment, but you could have other "value" classes, eg. Product, Person, etc.

Where is the instantiation of the User?

You can't do new T(); in viewall() because T is just only a "placeholder". You need to delegate the instantiation of User (or any other type) to some code that is not known by viewall().

When there are these type of problems, you need to analyze the problem trying to "see" what changes and what doesn't change.
Your viewall() executes an "algorithm" (in general): creating the SQL string, preparing the statement, execute the statement, iterating on the result set, etc... This doesn't change. It's the same for any table/type to query. What changes is the concrete class to use and the get of all the values, because this depends on the table/type.

It seems you have understood this concept, because I see an abstract class BaseDao and a concrete class UserDao. So you are on the right way! You just need to refine the concept.

In BaseDao you should have an abstract method:

protected abstract T viewAllFactor(ResultSet rs);

In any concrete subclass, eg. in UserDao you can have the concrete method:

protected User viewAllFactor(ResultSet rs) { ..... }

And in the body you just need to: 1) instantiate the User, 2) use getXyz on ResultSet to get values to assign to User, 3) return the User object.
In viewall you receive the object and add it to the list.