Create class instance from class name

Question

I have something like this:

@Entity
public class CallCardAttribute implements Serializable, IEntity {
private static final long serialVersionUID = 1L;

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
@Column(nullable = false)
private String name;
@Column(nullable = false)
private String type;;
@Column(nullable = false)
@ManyToOne(optional = false)
private Validator validator;

public CallCardAttribute() {}
...

This class represents a single attribute of a CallCard. Since there can be many of them and the number can be different for any single one, I am storing the Attributes as a Map<Attribute, String>. To keep it in just one table, all the values are converted to Strings, regardless of the Java type in the application itself.

But when loading them from the database, I need to cast them to the right type. So I store it as the type parameter in the Attribute class as a Class name string.

I've figured out that I could use reflection to get an instance of the Class specified by the string and than fill it with the value.

Like in this snippet:

Integer i = 17;
String iVal = i.toString();
String iType = i.getClass().getName();

Object reVal = Class.forName(iType).newInstance();

But I need to cast reVal to the correct type, which can be any of String/Calendar/Integer/Double...

Can this be done? And if so, how?

In your snippet, "reVal" is a java.lang.Class type. How would you cast "reVal" from a java.lang.Class to a java.lang.String if the types are incompatible? It seems like an invalid question to me, can you clarify this? — piepera
– piepera, Commented Jan 18, 2012 at 18:13
Reflection should be used judiciously, its uses has a bad impact on performance — Apurv
– Apurv, Commented Jan 18, 2012 at 18:20
Edited and hopefully fixed. As for the one table thing - it's modelled this way and I can't change that. Aside from that, if the value was stored as different types, how would it be mapped by the JPA? — Alexandar Živkovič
– Alexandar Živkovič, Commented Jan 18, 2012 at 18:21
It would be mapped according to the mapping provided by you in your Entity (if you use OneToMany) or in your Embeddable (if you use ElementCollection). — Jon Martin Solaas
– Jon Martin Solaas, Commented Jan 18, 2012 at 19:58

Highland Mark · Accepted Answer · 2012-01-18 18:09:31Z

0

use instanceof to determine which to cast to: if (reVal instanceof String) result = (String) eval; ... You'll need a separate variable of each type to cast to

answered Jan 18, 2012 at 18:09

Highland Mark

1,0101 gold badge7 silver badges12 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Jon Martin Solaas · Accepted Answer · 2012-01-18 18:19:19Z

0

If you don't want to use @OneToMany on the attribute collection you can use a JPA 2.0 ElementCollection.

http://en.wikibooks.org/wiki/Java_Persistence/ElementCollection

You'll still end up with attribs in a separate table.

answered Jan 18, 2012 at 18:19

Jon Martin Solaas

7671 gold badge10 silver badges19 bronze badges

3 Comments

Alexandar Živkovič Over a year ago

So, can I map something like Map<Attribute, T> using JPA? Where T is any of String/Calendar/numbers?

Jon Martin Solaas Over a year ago

yes, see for instance hwellmann.blogspot.com/2010/07/jpa-20-mapping-map.html or google "jpa map elementcollection onetomany" ... but it won't work for you if you can only use one table. Edit: And T can't be just any Object, it must be of the type you have annotated with Embeddable or Entity.

Jon Martin Solaas Over a year ago

Maybe you could serialize/deserialize to a blob and cast using instanceof after deserialization. But it'd also require a change of the schema (and it'd be pretty nasty in general). stackoverflow.com/questions/2121860/…

Collectives™ on Stack Overflow

Create class instance from class name

2 Answers 2

Comments

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related