JPA: How to initialise an entity for a self-referencing table?

Question

I am working on a project that requires a self-referencing table:
mysql> show create table attributes\G
*************************** 1. row ***************************
       Table: attributes
Create Table: CREATE TABLE `attributes` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `parent` int(11) DEFAULT NULL,
  `type` int(11) DEFAULT NULL,
  `name` varchar(128) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `parent` (`parent`),
  KEY `type` (`type`),
  CONSTRAINT `attributes_ibfk_1` FOREIGN KEY (`parent`) REFERENCES `attributes` (`id`),
  CONSTRAINT `attributes_ibfk_2` FOREIGN KEY (`type`) REFERENCES `attributes` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=26 DEFAULT CHARSET=latin1
I used NetBeans to generate an entity class from the table:
@Entity
@Table(name = "attributes")
@XmlRootElement
@NamedQueries({
    @NamedQuery(name = "Attributes.findAll", query = "SELECT a FROM Attributes a"),
    @NamedQuery(name = "Attributes.findById", query = "SELECT a FROM Attributes a WHERE a.id = :id"),
    @NamedQuery(name = "Attributes.findByName", query = "SELECT a FROM Attributes a WHERE a.name = :name")})
public class Attributes implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "id")
    private Integer id;
    @Size(max = 128)
    @Column(name = "name")
    private String name;
    @OneToMany(mappedBy = "parent")
    private Collection<Attributes> attributesCollection;
    @JoinColumn(name = "parent", referencedColumnName = "id")
    @ManyToOne
    private Attributes parent;
    @OneToMany(mappedBy = "type")
    private Collection<Attributes> attributesCollection1;
    @JoinColumn(name = "type", referencedColumnName = "id")
    @ManyToOne
    private Attributes type;
    @OneToMany(cascade = CascadeType.ALL, mappedBy = "attributes")
    private Collection<ItemAttributes> itemAttributesCollection;
    @OneToMany(mappedBy = "ivalue")
    private Collection<ItemAttributes> itemAttributesCollection1;
But how can I write a constructor for this entity? The auto-generated code gets around the issue by doing nothing; the constructor is empty. I can't help thinking that if I set the parent and type references to anything with new Attributes(), then it will recurse out of control. What else can/shall I do? I know how to rewrite it to not use the entity relations, but I'd prefer to make it work.

Answer 1

Cymae wrote:
I don't want to call the hash table creation method because from what i understand about interfaces the idea is that your main method doesnt know the table is actually a hash table...is that right?That's not exactly the idea. The idea to use the interface as the type instead of the implementation, is that your class probably doesn't need to know the full type. This makes it easy to change the implementation of the interface if needed. However, somebody at some point has to create the concrete object, a HashTable.
Basically, an interface describes a behavior, and a class that implements an interface decides how to actually perform this behavior. It is obviously impossible to perform the behavior if you are not told how to perform it.
Table table = new HashTable() is the correct way to do it. This means that if you ever need you Table implementation to change, the only thing you need to change in your whole class is this line. For example you might want Table table = new FileTable().