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What is the purpose of a primary key class (xxxPK.java) in Hibernate/Java and when is it appropriate to define one?

I am not familiar with Hibernate. I am trying to understand the purpose of defining a primary key class in Hibernate and where it would be appropriate.
I have to add new classes to some old code (early versions of Hibernate). I have a table called LOG which has the following fields among other fields
ID NUMBER
TRANS_ID VARCHAR2(40)
ID is the primary key (populated by oracle sequence). TRANS_ID is unique and populated by the application. Only search needed is by the TRANS_ID. There are no composite keys.
I believe I will need to define an entity class called Log.java and DAO class called LogDAO.java but do I need to define a LogPK.java for the Id field? I would not like to define one if this is not needed in my situation.

JPA Entity not having the column from the table but query based on the non existing column in the entity

I have an entity called Person which represent person_tb
person_tb has 10 fields that includes zip and country. But these columns are not specified in the Person entity. The entity contains only 8 columns out of 10 columns in the actual table.
Can I still run the query against person entity referring to the non-existing fields (but existing in the actual table)? (I am assuming its not possible)
If the above is not possible, can I write native query based on the missing fields in the entity?

Yes - you can use Native SQL Queries for this purpose.

Mapping class to a table without a table name

I have many tables in my DB with exactly the same structure: same columns names and types.
The only difference between these tables is the their names (which I can only know in runtime).
I would like to create a mapping of a class to a table but giving the name of the table only during runtime (no static #Table annotation).
Is it possible?
Is there any other way to achieve my goal?

Directly - no. Because this is not a regular use-case. Normally you should not have dynamcally-generated tables. It is a valid use when you move some records to an archive table (or tables), but otherwise avoid it.
Anyway, you can make that work: make a native query, and map the result to your non-entity object. You will be able to select from any table and transform the result to an object. However, you can't insert or update that way.

Don't think associating or changing the table mapped to an entity dynamically is possible.
You can check #MappedSuperClass, which will allow you to define all fields in a class once and inherit them, so that there is no repetition and entities are empty class with mappings.
http://docs.jboss.org/hibernate/annotations/3.5/reference/en/html_single/#d0e1168
If you know the table name dynamically you can always instantiate the mapped class accordingly.

Hibernate: #UniqueConstraint Across Multiple Tables?

I have a data model in which a number of entities inherit some common attributes from a single superclass entity. I am using InheritanceType.JOINED on the superclass, which causes Hibernate to create a single table for attributes defined in the superclass, with subclass tables containing only columns that are added by the subclass (so to load the attributes for a subclass instance, a join is performed between the two tables). That is all working fine.
What I'd like to do, however, is specify a unique constraint that includes fields in both the subclass and superclass tables. For instance, say that my superclass entity is something like:
Thing: {id, name}
...and then I have some subclass entities like:
Company: {address} //inherits 'id' and 'name' from 'Thing'
Employee: {company} //inherits 'id' and 'name' from 'Thing'
...and I want to configure Hibernate to automatically enforce that a given Company cannot have two Employee's with the same name. The company field is in the Employee table, but the name field is in the Thing table, so is there any way to get Hibernate to enforce this constraint, or do I need to do it programmatically whenever I add a new Employee?

If it's not possible in the Database it won't be possible with Hibernate. You can't create one constraint on multiple tables with SQL so neither in Hibernate.
You could work around this by creating a new Entity holding only the company and employee id and setting a unique constraint on those 2 fields but I would recommend enforcing this programmatically.

You could not use InheritanceType.JOINED, then everything ends up in a huge table, and you could write your constraint. As said before: What you want is just not possible in a relational DB.

JPA to retrieve name-value from child table without using model for child

This is something I'd really like to be able to do - resolve names based on id values without fetching the whole child model.
Here is an example of what I have, a Table say Employee and a Name_Details table
The Employee may look like this
Create Table Employee {
emp_idinteger not null generated by default as identity; -- generated pk
Department varchar(44);
emp_name_id Integer; -- fk to Name_Details table
...other details such as hire_date etc..
}
now emp_id is a foreign key to the name_details table which may look like this:-
Create Table Name_Details {
id Integer;
Name varchar(32);
Address Varchar(127);
Postcode Varchar(10);
--other details..
}
My question is that I'd like to model the first table Employee with a Java class but I am not interested in setting up a one-to-one relationship between the Employee class and the Name_Details table to fetch that whole Name_details table (as its quite large) the only information I want from this second class is just the Name field (found by joining the emp_name_id column with the Name_Details.id column.
So is it possible in JPA to say declare something like a transient variable in my Employee class called say String employeeName and have this retrieved by JPA based on the above relationShip?
This is a simplified example of what I have wgere there are several tables with name-value pairs and the master table has the values. I need this for reading and not updating/deleting etc.
I am not using JPA v1.x with EJB3 (and not hibernate) on WPS 6.1
Thx G

There are a few options.
The first is to just create a Name class that maps to Name_Details but only maps the name and id fields. Employee would then have a OneToOne to Name, and only the name would be read.
A second option is define Name_Details as a #SecondaryTable in Employee and map only the name from it. The JPA spec restricts the secondary table join to have to share the same id, but depending on what JPA provider you are using, using the 1-1 foreign key may be possible (EclipseLink does support this). You could also define a view the does the join and map to the view.
A third solution is to still map all of the Name_Details fields but define them as LAZY. If your JPA provider supports LAZY basics (or fetch groups), then these will not be fetch unless accessed.