RichModel1

RichModel1.java

public class Order {

   private BigDecimal total;
   private List<OrderItem> items = new ArrayList<OrderItem>();

   /**
     * The total is defined as the sum of all {@link OrderItem#getTotal()}.
     *
     * @return the total of this {@link Order}.
     */
   public BigDecimal getTotal() {
      if (total == null) {
         /*
          * we have to calculate the total and remember the result
          */
          BigDecimal orderItemTotal = BigDecimal.ZERO;
          List<OrderItem> items = getItems();

          for (OrderItem orderItem : items) {
             BigDecimal itemTotal = orderItem.getTotal();
            /*
             * add the total of an OrderItem to our total.
             */
             orderItemTotal = orderItemTotal.add(itemTotal);
          }
          this.total = orderItemTotal;
      }
   return total;
   }

   /**
     * Adds the {@link OrderItem} to this {@link Order}.
     *
     * @param orderItem
     *            the {@link OrderItem} to add. Must not be null.
     */
    public void addItem(OrderItem orderItem) {
       if (orderItem == null) {
          throw new IllegalArgumentException("orderItem must not be null");
       }
       if (this.items.add(orderItem)) {
           /*
            * the list of order items changes so we reset the total field to
            * let getTotal re-calculate the total.
            */ 
            this.total = null;
       }
    }

    /**
      *
      * @return the {@link OrderItem} that belong to this {@link Order}. Clients
      *         may not modify the returned {@link List}. Use
      *         {@link #addItem(OrderItem)} instead.
      */
    public List<OrderItem> getItems() {
         /*
          * we wrap our items to prevent clients from manipulating our internal
          * state.
          */
         return Collections.unmodifiableList(items);
    }

}

import java.math.BigDecimal;

public class OrderItem {
    private BigDecimal price;
    private int quantity;
    private String name = "no name";

    public OrderItem(BigDecimal price, int quantity, String name) {
       if (price == null) {
          throw new IllegalArgumentException("price must not be null");
       }
       if (name == null) {
          throw new IllegalArgumentException("name must not be null");
       }
       if (price.compareTo(BigDecimal.ZERO) < 0) {
          throw new IllegalArgumentException(
                "price must be a positive big decimal");
       }
       if (quantity < 1) {
          throw new IllegalArgumentException("quantity must be 1 or greater");
       }
       this.price = price;
       this.quantity = quantity;
       this.name = name;
    }

    public BigDecimal getPrice() {
       return price;
    }

    public int getQuantity() {
       return quantity;
    }

    public String getName() {
       return name;
    }

   /**
     * The total is defined as the {@link #getPrice()} multiplied with the
     * {@link #getAmount()}.
     *
     * @return
     */
    public BigDecimal getTotal() {
       int quantity = getQuantity();
       BigDecimal price = getPrice();
       BigDecimal total = price.multiply(new BigDecimal(quantity));
       return total;
    }
}