Prevalence: Transparent, Fault-Tolerant Object Persistence
Pages: 1, 2, 3, 4, 5

Adding the Business Objects

The prevalent system is now ready to have some business objects associated with it. The first business class to add is Item. Right-click on the PrevalentOrderSystem class in the Eclipse Package Explorer and select Prevayler -> Create Business Object. Note that you will only see the Prevayler option on the context menu if you select the correct object in the Package Explorer: you need to select the class, not the source file (see Figure 5).

Figure 5. Adding a business object

The Create Business Object dialogue opens. This allows you to specify the name of a BO class, and to specify whether the prevalent system will contain a single object or a list of objects. Enter Item in the BO Class Name box and select List, as you want the system to be able to contain many items (see Figure 6). Enter items in the "field name" box.

Figure 6. Specifying details of the business object

Click Finish and a new class, Item, is added to the project. The PrevalentSystem class is altered so that it has a field of the type ArrayList, and methods to add a new Item, retrieve an Item by id, and remove an Item. The addItem method is shown in the listing below. This method uses the nextItemID field to ensure that a unique id is added to the Item before storing in the ArrayList. You will need to modify the auto-generated code slightly, as shown in the listing below--this version passes a reference to an Item into the method and then sets its id. The listing also shows a method, getItems, which returns the whole list--this is not generated by Preclipse, so you need to add it yourself.

public class PrevalentOrderSystem implements Serializable {
    private List items = new ArrayList();
    private long nextItemID = 1;

        public PrevalentOrderSystem() {
          super();
        }

    public Item addItem(Item newItem) {
        newItem.setID(nextItemID++);
        this.items.add(newItem);
        return newItem;
    }    

    public List getItems(){
        return items;
    }

    // other methods omitted for brevity...
}

The Item class, at this stage, has only an id field, which will be used to identify particular objects stored in the prevalent system. You need to add additional fields and methods to match the specification in Figure 1. The completed Item class is as follows (with getters/setters omitted for brevity). Note that the id field is not set in the constructor--in this example, ids are only set when an object is made persistent by a transaction.

public class Item implements Serializable {

    private long id;
    private int currentStock;
    private double price;
    private long itemCode;

    
    public Item() {
        this.setCurrentStock(100);
    }
    
    public double getPostage() {
        return 0.0;
    }

    public void reStock(int quantity) {
        setCurrentStock(getCurrentStock() + quantity);
    }

    public void order(int quantity) 
            throws InsufficientStockException{
        if (getCurrentStock() >= quantity) {
            setCurrentStock(getCurrentStock() - quantity);
        } else {
            throw new InsufficientStockException
                (currentStock);
        }
    }

    public String toString() {
        return (id + " - " + this.getClass() + 
            ":" + currentStock + " in stock");
    }

    // getters/setters omitted...
}

The Item class throws a custom exception, InsufficientStockException, which can be added to the project as an ordinary Java class, not using any Prevayler-specific options.

public class InsufficientStockException extends Exception {
    private int currentStock;
    
    public InsufficientStockException(int currentStock){
        this.currentStock = currentStock;
    }
    
    public String toString(){
        return (super.toString() + ": current stock level is " 
                + currentStock);
    }

    // getters/setters omitted...
}

The subclasses of Item can now be added to the project as ordinary Java classes--they become persistent BO classes because they are subclasses of an existing BO class. An instance of BookItem, for example, will inherit its id field from the superclass.

public class BookItem extends Item {
    private String title;
    private String authors;
    private String publisher;

    public BookItem() {
    }

    public BookItem(double price, String title, String 
            authors, String publisher) {
        this.setPrice(price);
        this.setTitle(title);
        this.setAuthors(authors);
        this.setPublisher(publisher);
    }

    public double getPostage() {
        return (getPrice() * 0.10);
    }

    public String toString(){
        return(super.toString() + ":" + this.title);
    }

    // getters/setters omitted...
}

public class SoftwareItem extends Item {
    private String title;
        private String version;
    
        public SoftwareItem() {
    }
    
        public SoftwareItem(double price, String title, String version){
          this.setPrice(price);
        this.setTitle(title);
        this.setVersion(version);
        }
    
        public double getPostage(){
        return 2.50;
        }

    public String toString(){
        return(super.toString() + ":" + this.title);
    }

    // getters/setters omitted...
}

public class MultipackItem extends Item {
    private Item item;
    private int multiple;
    private double discount;
    
    public MultipackItem(){}

    public MultipackItem(Item item, int multiple, double 
         discount) {
        this.setItem(item);
        this.setMultiple(multiple);
        this.setDiscount(discount);
    }

    public double getPrice() {
        double packPrice = 
            (getItem().getPrice() * 
            getMultiple() * (1 - getDiscount()));
        return packPrice;
    }

    public double getPostage() {
        double packPostage = (getItem().getPostage() * 
            getMultiple() * 0.5);
        return packPostage;
    }
    
    public String toString(){
        return(super.toString() + "(" + 
            this.item.toString() + ")");
    }

    // getters/setters omitted...
}

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