What is “loose coupling?” Please provide examples

Consider a simple shopping cart application that uses a CartContents class to keep track of the items in the shopping cart and an Order class for processing a purchase. The Order needs to determine the total value of the contents in the cart, it might do that like so:

Tightly Coupled Example:

public class CartEntry
{
    public float Price;
    public int Quantity;
}

public class CartContents
{
    public CartEntry[] items;
}

public class Order
{
    private CartContents cart;
    private float salesTax;

    public Order(CartContents cart, float salesTax)
    {
        this.cart = cart;
        this.salesTax = salesTax;
    }

    public float OrderTotal()
    {
        float cartTotal = 0;
        for (int i = 0; i < cart.items.Length; i++)
        {
            cartTotal += cart.items[i].Price * cart.items[i].Quantity;
        }
        cartTotal += cartTotal*salesTax;
        return cartTotal;
    }
}

Notice how the OrderTotal method (and thus the Order class) depends on the implementation details of the CartContents and the CartEntry classes. If we were to try to change this logic to allow for discounts, we’d likely have to change all 3 classes. Also, if we change to using a List<CartEntry> collection to keep track of the items we’d have to change the Order class as well.

Now here’s a slightly better way to do the same thing:

Less Coupled Example:

public class CartEntry
{
    public float Price;
    public int Quantity;

    public float GetLineItemTotal()
    {
        return Price * Quantity;
    }
}

public class CartContents
{
    public CartEntry[] items;

    public float GetCartItemsTotal()
    {
        float cartTotal = 0;
        foreach (CartEntry item in items)
        {
            cartTotal += item.GetLineItemTotal();
        }
        return cartTotal;
    }
}

public class Order
{
    private CartContents cart;
    private float salesTax;

    public Order(CartContents cart, float salesTax)
    {
        this.cart = cart;
        this.salesTax = salesTax;
    }

    public float OrderTotal()
    {
        return cart.GetCartItemsTotal() * (1.0f + salesTax);
    }
}

The logic that is specific to the implementation of the cart line item or the cart collection or the order is restricted to just that class. So we could change the implementation of any of these classes without having to change the other classes. We could take this decoupling yet further by improving the design, introducing interfaces, etc, but I think you see the point.