Question

having a look at the netbeans affable bean tutorial. Why do we need the use of synchronized methods here?

public synchronized int getNumberOfItems() {

        numberOfItems = 0;

        for (ShoppingCartItem scItem : items) {

            numberOfItems += scItem.getQuantity();
        }

        return numberOfItems;
    }


    public synchronized double getSubtotal() {

        double amount = 0;

        for (ShoppingCartItem scItem : items) {

            Product product = (Product) scItem.getProduct();
            amount += (scItem.getQuantity() * product.getPrice().doubleValue());
        }

        return amount;
    }

Answer 1

Looks like every method related to the ShoppingCartItems management is synchronized. Certainly to prevent a concurrent access in the items List (List<ShoppingCartItem> items;).

Without the synchronized, you could have 1+ Thread accessing a ‘read’ method such as getSubtotal () while the items List is being updated through public synchronized void addItem(Product product) by another Thread.

The source can be found here