如何更正迭代产品数组列表时的错误？

Question

this is the method that list the products by keyword:::"produitsParMC" after selecting these products from database,note that i have created a class for product,my problem in that when i try to fetch these products by keyword an exception saying "Exception in thread "main" java.lang.NullPointerException at dao.App.main(App.java:21)"

public class ProduitDaoImpl implements IProduitDao {



    public Produit save(Produit p) {

        Connection connection=SingletonConnection.getConnection();
        try {
            PreparedStatement ps=connection.prepareStatement("INSERT INTO PRODUITS(DESIGNATION,PRIX,QUANTITE) VALUES (?,?,?)");
            ps.setString(1, p.getDesignation());
            ps.setDouble(2, p.getPrix());
            ps.setInt(3, p.getQuantite());
            ps.executeUpdate();
            PreparedStatement ps2=connection.prepareStatement("SELECT MAX(ID) as MAXID FROM PRODUITS");
            ResultSet rs=ps2.executeQuery();
            if(rs.next()){
                //if because we have one value to view,,next: in order to positionate in the first enregistrement
            p.setId(rs.getLong("MAXID"));
            }

            ps.close();
            //connection.close();//si on utilise un singletoon on ne doit pas fermer la connectio car elle est crée une seule fois 
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }       return null;
    }

    public List<Produit> produitsParMC(String mc) {
        List<Produit> produits=new ArrayList<Produit>();
        Connection connection=SingletonConnection.getConnection();
        try {




            PreparedStatement ps=connection.prepareStatement("SELECT * FROM PRODUITS WHERE DESIGNATION LIKE ?");
            ps.setString(1, mc);
            ResultSet rs=ps.executeQuery();
            while(rs.next()){

                Produit p=new Produit();
                p.setId(rs.getLong("ID"));
                /*System.out.println(p.getId()+"cccc");*/
                p.setDesignation(rs.getString("DESIGNATION"));
                p.setPrix(rs.getDouble("Prix"));
                p.setQuantite(rs.getInt("QUANTITE"));
                produits.add(p);
            }

        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        return null;
    }

    public Produit getProduit(Long id) {
        // TODO Auto-generated method stub
        return null;
    }

    public Produit update(Produit p) {
        // TODO Auto-generated method stub
        return null;
    }

    public void deleteProduit(Long id) {
        // TODO Auto-generated method stub

    }

}

here is my main class

package dao;

import java.util.ArrayList;
import java.util.List;


public class App {
    public static void main(String[] args) {
        ProduitDaoImpl dao=new ProduitDaoImpl();


        Produit p4=new Produit("mac 000", 222, 111);
        dao.save(p4);
        System.out.println(p4.toString());
        List<Produit> pro=new ArrayList<Produit>();
        pro= dao.produitsParMC("asus 6500");


        System.out.println("my list is :");

        for(Produit p :pro){

            System.out.println(p.toString());
        }

    }

}

finally here is the output containing the error

Produit [id=195, designation=mac 000, prix=222.0, quantite=111]
my list is :
Exception in thread "main" java.lang.NullPointerException
    at dao.App.main(App.java:21)

Answer 1

The output of produitsParMC is null since in your function you return null; you should return the ArrayList

Answer 2

At this pro= dao.produitsParMC("asus 6500"); line, pro is null , so your for each loop in this case looks like this:

for(Produit p : null) {
    System.out.println(p.toString());
}

That produces a NullPointerException . Instead of returning null at the end of your ProduitDaoImpl#produitsParMC method, return the produits ArrayList which you create at the beginning of that method.