Question

I would like to have a java class (MyClass) that holds an id, a name and a value, where the value can be a primitive type (string, int, long) or a list or a map. The class will be persisted by using JPA 2.0

My first thought were about a class with the following structure:

 ,-----------.
 | MyClass   |
 |...........|
 | id        |
 | name      |
 | index     |
 | key       |
 | value     |
 '------------

that resolve by getter and setter if a primitive or a list or a map ist stored – depending on the {index, key, value} combination.

An alternative would be an inheritance hierarchy:

                     .................
                     | AbstractClass |
                 ,.- ................''--..__
            _,-''            |               `--.._
         ,-'                 |            ................
......................       |            |MapValueClass |
| PrimitiveValueClass|       |            ................
......................       |
                    ..................
                    | ListValueClass |
                    ..................

Keeping in mind, that I use JPA: is there a “standard way” or a “best practice” for this problem? Some way, that I can use myClass and access the value by a getter and decide with instanceof Map/List/Integer what sort of value is hold?

Answer 1

Well i found a solution that works fine for me: a base class and value holding derived classes:

Base class:

package jpa.inheritance_demo;

import java.util.List;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;

@Entity
public class Resource {

    @Id
    private long oid;

    @OneToMany
    @JoinColumn(name="OID")
    private List<Property> props;

}

Scalar property class:

package jpa.inheritance_demo;

import javax.persistence.Entity;

@Entity
public class ScalarProperty extends Property {
    private String value;

    public String getValue() {
        return value;
    }

    public void setValue(String value) {
        this.value = value;
    }
}

List property class:

package jpa.inheritance_demo;

import java.util.List;

import javax.persistence.ElementCollection;
import javax.persistence.Entity;

@Entity
public class ListProperty extends Property {

    @ElementCollection
    List<String> value;

    public List<String> getValues() {
        return value;
    }

}

… and don’t forget to add the classes to your persistence.xml