Question

I work on Java/HTML project. I’ve set a hashmap as a session attribute. I request the hashmap from session and put key/value in it
map.put("some string", "1")
. When this action is performed the second time, I print the hashmap content and the only value, that was ‘1’ after the last operation on the hashmap, becomes ‘-1’.

Hashmap is the best data structure, in my opinion, for this project. Can anyone help?

public class Cart {
    private HashMap<String, asd> list;

    public Cart(){
        list = new HashMap<String, asd>();
    }

    public HashMap<String, asd> getMap(){
        return list;
    }

    /*
     * Parameter: code
     *      -1: increase quantity by 1
     *      0: delete product from the product list
     *      else: set the product quantity to the passed value
     */
    public void alterProduct(int code, String product){
        if(list.containsKey(product)) {
            if(code == -1) plusOne(product);
            if(code == 0) remove(product);
            else setValue(product, code);
        }else {
            asd asd = new asd();
            asd.a = 1;
            list.put(product, asd);
        }
    }

    private void plusOne(String product){
        asd asd = list.get(product);
        asd.a = asd.a + 1;
        list.put(product, asd);
    }

    private void remove(String product){
        list.remove(product);
    }

    private void setValue(String product, int code){
        asd asd = new asd();
        asd.a = code;
        list.put(product, asd);
    }
}

class asd{
    int a;

    public String toString(){
        return ""+a;
    }
}

JSP code where I set Cart object as session attribute:

<%
    Cart myCart = new Cart();
    session.setAttribute("cart",myCart);
%>

Servlet code:

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        Cart cart = (Cart) request.getSession().getAttribute("cart");
        cart.alterProduct(-1, (String) request.getSession().getAttribute("name"));
        doGet(request, response);
    }

After I call alterProduct the second time for the same (String) request.getSession().getAttribute("name") the hashmap value for the same key is ‘-1’.

Answer 1

What is type/value of product? How it is connected to the “cart”?

I guess what’s happen that you mess up data types. Another option is you have bug in the Cart.toString() method. I suggest you change the code with “plain” data type and recheck it. If it fails, use your Cart class without messy conversion and debug the code.

You have bug here:

  public void alterProduct(int code, String product){
    if(list.containsKey(product)) {
      if(code == -1) plusOne(product);
      if(code == 0) remove(product);
      else setValue(product, code);
  }


  private void setValue(String product, int code){
    asd asd = new asd();
    asd.a = code;
    list.put(product, asd);
  }

Consider what happen when you call art.alterProduct(-1, "something") second time.
list.containsKey(product) is true (you use the same product”), code is -1. So

if(code == -1) plusOne(product); is executed as expected.

But then you have something weired

if(code == 0) remove(product);
else setValue(product, code);

code ==0 is evaluated to false, so else instruction is called. You are calling setValue(product, -1)

As you can see above setValue will assign -1 to the asd.a that is observed by you.