Adding object to ArrayList giving a NullPointerException in Java

Question

I have an array of integers which I'm trying to loop through, then get a product object by ID based on the array. I am then trying to add the created product object to another array but this is giving me a null pointer exception error.

Code for adding object to array:

ArrayList<Integer> cartIds = new ArrayList<Integer>();

void viewCart(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException{

        ProductDAO productDAO = new ProductDAO();
        ArrayList<Product> cartArrayList = null;

        for(int i = 0; i< cartIds.size(); i++){
            Product product = productDAO.getProductByIdCart(cartIds.get(i));

            Product cartProduct = new Product(product.getId(), product.getMake(), product.getModel(), product.getEngine(), product.getPower(), product.getSpeed(), 
                    product.getCategory(), product.getYear(), product.getPrice());

            cartArrayList.add(cartProduct);
        }

        request.getSession(true).setAttribute(IConstants.SESSION_KEY_CART, cartArrayList);
        System.out.println(cartArrayList);
        RequestDispatcher rd = request.getRequestDispatcher("/shop-main.jsp");
        rd.forward(request, response);

    }

My DAO:

public Product getProductByIdCart(int id) {

    DBManager dmbgr = new DBManager();
    Connection con = dmbgr.getConnection();
    int carId = 0;
    String make = null;
    String model = null;
    String engine = null;
    String power = null;
    String speed = null;
    String category = null;
    String year = null;
    String price = null;
    String location = null;
    Product carData = new Product();

    String query = "SELECT * FROM PRODUCTDATA WHERE PRODUCT_ID = "+ id +"";
    try {
        PreparedStatement stmt = con.prepareStatement(query);
        ResultSet rs = stmt.executeQuery();
        while (rs.next()) {
            carId = (rs.getInt(1));
            make = (rs.getString(2));
            model = (rs.getString(3));
            engine = (rs.getString(4));
            power = (rs.getString(5));
            speed = (rs.getString(6));
            category = (rs.getString(7));
            year = (rs.getString(8));
            price = (rs.getString(9));
            location = (rs.getString(10));
            Product tempCar = new Product();
            tempCar.setId(carId);
            tempCar.setMake(make);
            tempCar.setModel(model);
            tempCar.setEngine(engine);
            tempCar.setPower(power);
            tempCar.setSpeed(speed);
            tempCar.setCategory(category);
            tempCar.setYear(year);
            tempCar.setPrice(price);
            tempCar.setImg_location(location);
            carData = tempCar;
            System.out.println(tempCar);
        }

    } catch (SQLException e) {
        e.printStackTrace();
    }


    return carData;

}

And my product constructor:

public Product(int id, String make, String model, String engine, String power, String speed, String category, String year, String price){
    this.id = id;
    this.make = make;
    this.model = model;
    this.engine = engine;
    this.power = power;
    this.speed = speed;
    this.category = category;
    this.year = year;
    this.price = price;
}

I think you are talking about this array 'cartArrayList', just initialize it. — Akshay Batra, Jan 12 '19 at 12:26

score 2 · Answer 1 · answered Jan 12 '19 at 12:25

2

You should initialize cartArrayList: ArrayList<Product> cartArrayList = new ArrayList<>();

answered Jan 12 '19 at 12:25

Yura Miroshnichenko

91
1
2

This worked, thanks for your help. – aa2397 Jan 12 '19 at 12:30

score 0 · Answer 2 · answered Jan 12 '19 at 12:25

0

I think you have initialized cartArrayList to null and added to it. This results in null pointer exception. Replace that line with:

ArrayList<Product> cartArrayList = new ArrayList<Product>();

answered Jan 12 '19 at 12:25

sk_462

587
1
7
16

score 0 · Answer 3 · answered Jan 12 '19 at 12:28

0

Initialize your cartArrayList

ArrayList<Product> cartArrayList = new ArrayList<Product>();

answered Jan 12 '19 at 12:28

sreeroop

64
1
9

score 0 · Answer 4 · answered Jan 12 '19 at 12:30

You need initialize the collection(ArrayList in your case) before using it. like this:

ArrayList<Product> cartArrayList = new ArrayList<Product>();

If you use java 7+, Type infer feature allow you ignore the type when initialize.

ArrayList<Product> cartArrayList = new ArrayList<>();

Also, in java, we prefer use interface type instead specific implement. Here like :

List<Product> cartArrayList = new ArrayList<>();

Adding object to ArrayList giving a NullPointerException in Java

4 Answers4