Calculated properties

Perhaps the most simple business logic you can add to your application is a calculated property. The properties you have used until now are persistent, i.e., each property is stored in a column in a table in the database. A calculated property is a property that does not store its value in the database but it's calculated any time the property is accessed. See the difference between a persistent and a calculated property.

// Persistent property
@Getter @Setter // Has getter and setter
int quantity; // Has a field, so it's persistent
 
// Calculated property
public int getAmount() { // It has no field and no setter, only a getter
    return quantity * price; // with a calculation
}

Calculated properties are automatically recognized by OpenXava. You can use them in views, tabular lists or any other part of your code.
We are going to use calculated properties to add the money element to our invoicing application. Because, we have details, products, quantities. But what about amounts?

Simple calculated property

The first step will be to add an amount property to the Detail. We want the detail amount to be recalculated and shown to the user when the user chooses a product and type in the quantity:

Adding this feature to your current code is practically adding a calculated property to Detail. Just add the next code to the Detail class:

@Money
@Depends("product.number, quantity")  // When the user changes product or quantity
public BigDecimal getAmount() { // this property is recalculated and redisplayed
    if (product == null || product.getPrice() == null) return BigDecimal.ZERO;
    return new BigDecimal(quantity).multiply(product.getPrice()); 
}

Simply put the calculation in getAmount() and use @Depends to indicate to OpenXava that the amount property depends on product.number and quantity, thus each time the user changes any of these values the property will be recalculated. Note as we in this case we use product instead of getProduct() and quantity instead of getQuantity() because from inside the class you can access directly to its fields.
Now you have to add this new property to the details collection of CommercialDocument:

@ElementCollection
@ListProperties("product.number, product.description, quantity, amount") // amount added
Collection<Detail> details;

Nothing else is required. The mere addition of the getter and modifying the list properties is enough. Try the Invoice and Order modules to see the amount property in action.

Summary

In this lesson you have learned the application of calculated properties, and their differences from persistent properties, when establishing business logic. In future lessons we will see how to work with collections and totals.

Any problem with this lesson? Ask in the forum Everything fine? Go to Lesson 11