How to implement Seed and Next when extending UserVersionType

I'm trying to implement a String based UserVersionType. I have found examples enough to understand how to use the UserType methods to an extent. I can't find anything that shows me exactly what to do with next() and seed().

So I have something like this

public class StringVersionType implements UserType, UserVersionType {
    ...

    public int compare(Object o1, Object o2) {
        String a = (String) o1;
        String b = (String) o2;

        return a.compareTo(b);
    }

    public Object next(Object arg0, SharedSessionContractImplementor arg1)           
    {

           return "DUMMY SEED";  // + LocalTime.now().toString();
    }

    public Object seed(SharedSessionContractImplementor session){
        return "DUMMY SEED"; // LocalTime.now().toString();
    }

}

I've tried adding simple code to return a string that is always the same and code that might change the version number. I always get an error on update. Looking at the hibernate console output when I add almost anything to these UserVersionType methods hibernate stops doing a select and then an update but always goes straight to a new insert query and so fails on a primary key still exists.

Obviously I'm misunderstanding what seed and next should do but I can't find any useful documentation ?

Can anyone tell me more about how to use them ?

Seed:

Generate an initial version. Parameters: session - The session from which this request originates. May be null; currently this only happens during startup when trying to determine the "unsaved value" of entities. Returns: an instance of the type

@Override
  public Object 
 seed(SharedSessionContractImplementor session) 
  {
  return ( (UserVersionType) userType ).seed( 
  session );
 }

For properties mapped as either version or timestamp, the insert statement gives you two options. You can either specify the property in the properties_list, in which case its value is taken from the corresponding select expressions, or omit it from the properties_list, in which case the seed value defined by the org.hibernate.type.VersionType is used

next:

Increment the version. Parameters: session - The session from which this request originates. current - the current version Returns: an instance of the type

public Object next(Object current, 
SessionImplementor session) {
return ( (UserVersionType) userType ).next(  current, session );
}

From the docs:

"UPDATE statements, by default, do not effect the version or the timestamp attribute values for the affected entities. However, you can force Hibernate to set the version or timestamp attribute values through the use of a versioned update. This is achieved by adding the VERSIONED keyword after the UPDATE keyword. Note, however, that this is a Hibernate specific feature and will not work in a portable manner. Custom version types, org.hibernate.usertype.UserVersionType, are not allowed in conjunction with a update versioned statement."

Other Docs:

Dedicated version number The version number mechanism for optimistic locking is provided through a @Version annotation.

The @Version annotation

  @Entity
   public class Flight implements Serializable {
   ...
  @Version
  @Column(name="OPTLOCK")
   public Integer getVersion() { ... }
   }   

Here, the version property is mapped to the OPTLOCK column, and the entity manager uses it to detect conflicting updates, and prevent the loss of updates that would be overwritten by a last-commit-wins strategy.

The version column can be any kind of type, as long as you define and implement the appropriate UserVersionType.

Your application is forbidden from altering the version number set by Hibernate. To artificially increase the version number, see the documentation for properties LockModeType.OPTIMISTIC_FORCE_INCREMENT or LockModeType.PESSIMISTIC_FORCE_INCREMENTcheck in the Hibernate Entity Manager reference documentation.

Database-generated version numbers If the version number is generated by the database, such as a trigger, use the annotation @org.hibernate.annotations.Generated(GenerationTime.ALWAYS).

Declaring a version property in hbm.xml

 <version
    column="version_column"
    name="propertyName"
    type="typename"
    access="field|property|ClassName"
    unsaved-value="null|negative|undefined"
    generated="never|always"
    insert="true|false"
    node="element-name|@attribute- 
    name|element/@attribute|."
 />

This is all I can find from the documentation that can help you understand why and how to use those methods. Give me feed back about the irrelevent parts, due to my misunderstanding to the question, to remove it.