How to represent a FOREIGN KEY in the Data Model class

How should I represent a FOREIGN KEY field in the database on the java data model class? Is it represented as an Array of object? Because I can't populate the entire Entity class with just one SELECT Query from the database, I'll then have to iterate through all the row of the FOREIGN KEY reffered table.

TABLE CLIENT
-- clientId (PRIMARY KEY)
-- clientName
-- clientPhoneNumber
-- clientBirthDate

TABLE ORDER
-- orderId (PRIMARY KEY)
-- orderDate
-- orderNumber
--clientId  (FOREIGN KEY)

Should the Data Object of Client looks like that:

public class ClientData{
   private int clientId;
   private String clientName;
   private String clientPhoneNumber;
   private String clientBirthDate;
   ...
}

or this

public class ClientData{
   private int clientId;
   private String clientName;
   private String clientPhoneNumber;
   private String clientBirthDate;
   private ArrayList<OrderData> orderList;
   ...
}