error in lista.add(obj) and list<obj> java

Question

I'm trying resolve a problem with List in Java but I can't. I have the following code:

Productos producto = ... //come from a database, here there isn't errors

List<Productos> lista = new ArrayList<Productos>();

lista.set(0,producto);    

And I can't introduce 'producto' in my list and I don't know why. I had tried it using "lista.add(producto)" too but it doesn't work. What can I do?

Edit1: Producto is not null, it's initialized and it has some attributes with correct values. I used lista.add(0,producto) too but it didn't work :( .The error is:

Grave: java.lang.NumberFormatException: null
at java.lang.Integer.parseInt(Integer.java:454)
at java.lang.Integer.parseInt(Integer.java:527)
at clientesCarrito.ClienteCarritoCompraServlet.doPost(ClienteCarritoCompraServlet.java:68)

Yeah, it looks like an error to have done a "bad casting" to Int but I don't think..this is my part of my code:

EntityManager em = entityManagerFactory.createEntityManager();
.
.
int id = Integer.parseInt(request.getParameter("IdProd"));
Productos producto = em.find(Productos.class,id);
HttpSession session = request.getSession(false);

if(session.getAttribute("carProductos")==null){
    List<Productos> lista = new ArrayList<Productos>();
    lista.add(producto);
    session.setAttribute("carProductos", lista);
}else{
    List<Productos> lista = (List<Productos>) session.getAttribute("carProductos");
    lista.add(producto);
    session.setAttribute("carProductos", lista);
}

I have done debug in Eclipse and "IdProd" is 5 and "id" is 5 too. Now it looks like "lista" catch "producto" and introduces itself but when it's at bottom suddenly show that error and crash. After of this code, I don't use again "lista" or "producto". Somebody can help me?

Answer 1

Following is the declaration for java.util.ArrayList.add() method public void add(int index, E element)

so use

lista.add(0, producto)

Answer 2

The set method in a List raises an error when setting an element in a position not between 0 and size() - 1. On an empty List you can never set an element. From: https://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#set%28int,%20E%29

Throws: IndexOutOfBoundsException - if the index is out of range (index < 0 || index >= size())

What I guess you want to do is to add an element to the list; since the List is empty the only valid position is zero:

lista.add(producto);

The add method adds the element to the end of the list. In the case of an empty list, it adds the element as the only element.