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It s a good idea to add a convenience method to the Category class that groups these operations, allowing reuse and helping ensure correctness, and in the end guarantee data integrity:
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public void addChildCategory(Category childCategory) { if (childCategory == null) throw new IllegalArgumentException("Null child category!"); if (childCategory.getParentCategory() != null) childCategory.getParentCategory().getChildCategories() .remove(childCategory); childCategory.setParentCategory(this); childCategories.add(childCategory); }
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The addChildCategory() method not only reduces the lines of code when dealing with Category objects, but also enforces the cardinality of the association. Errors that arise from leaving out one of the two required actions are avoided. This kind of grouping of operations should always be provided for associations, if possible. If you compare this with the relational model of foreign keys in a relational database, you can easily see how a network and pointer model complicates a simple operation: instead of a declarative constraint, you need procedural code to guarantee data integrity.
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Implementing the domain model
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Because you want addChildCategory() to be the only externally visible mutator method for the child categories (possibly in addition to a removeChildCategory() method), you can make the setChildCategories() method private or drop it and use direct field access for persistence. The getter method still returns a modifiable collection, so clients can use it to make changes that aren t reflected on the inverse side. You should consider the static methods Collections.unmodifiableCollection(c) and Collections.unmodifiableSet(s), if you prefer to wrap the internal collections before returning them in your getter method. The client then gets an exception if it tries to modify the collection; every modification is forced to go through the relationship-management method. A different kind of relationship exists between the Category and Item classes: a bidirectional many-to-many association, as shown in figure 3.4.
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Figure 3.4 Category and the associated Item class
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In the case of a many-to-many association, both sides are implemented with collection-valued attributes. Let s add the new attributes and methods for accessing the Item relationship to the Category class, as shown in listing 3.2.
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Listing 3.2 Category to Item scaffolding code
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public class Category { ... private Set items = new HashSet(); ... public Set getItems() { return items; } public void setItems(Set items) { this.items = items; } }
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Domain models and metadata
The code for the Item class (the other end of the many-to-many association) is similar to the code for the Category class. You add the collection attribute, the standard accessor methods, and a method that simplifies relationship management, as in listing 3.3.
Listing 3.3 Item to Category scaffolding code
public class Item { private String name; private String description; ... private Set categories = new HashSet(); ... public Set getCategories() { return categories; } private void setCategories(Set categories) { this.categories = categories; } public void addCategory(Category category) { if (category == null) throw new IllegalArgumentException("Null category"); category.getItems().add(this); categories.add(category); } }
The addCategory() method is similar to the addChildCategory() convenience method of the Category class. It s used by a client to manipulate the link between an Item and a Category. For the sake of readability, we won t show convenience methods in future code samples and assume you ll add them according to your own taste. Using convenience methods for association handling isn t the only way to improve a domain model implementation. You can also add logic to your accessor methods.
Adding logic to accessor methods
One of the reasons we like to use JavaBeans-style accessor methods is that they provide encapsulation: The hidden internal implementation of a property can be changed without any changes to the public interface. This lets you abstract the internal data structure of a class the instance variables from the design of the
Implementing the domain model
database, if Hibernate accesses the properties at runtime through accessor methods. It also allows easier and independent refactoring of the public API and the internal representation of a class. For example, if your database stores the name of a user as a single NAME column, but your User class has firstname and lastname properties, you can add the following persistent name property to the class:
public class User { private String firstname; private String lastname; ... public String getName() { return firstname + ' ' + lastname; } public void setName(String name) { StringTokenizer t = new StringTokenizer(name); firstname = t.nextToken(); lastname = t.nextToken(); ) .... }
Later, you ll see that a Hibernate custom type is a better way to handle many of these kinds of situations. However, it helps to have several options. Accessor methods can also perform validation. For instance, in the following example, the setFirstName() method verifies that the name is capitalized:
public class User { private String firstname; ... public String getFirstname() { return firstname; } public void setFirstname(String firstname) throws InvalidNameException { if ( !StringUtil.isCapitalizedName(firstname) ) throw new InvalidNameException(firstname); this.firstname = firstname; ) .... }
Hibernate may use the accessor methods to populate the state of an instance when loading an object from a database, and sometimes you ll prefer that this validation
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