Sending JPA data from server to client? - database

Question

I'm using Eclipse juno IDE.
I Have a client-server application. In the server side I have an Entity (Travels)
and I Have another class that handle the JPA queries. I'm recieving the data from the database
but when I'm trying to send it as a vector to the client i'm getting an exception in the
client side , that says "Cant cast pack.db.Travels to java.util.vector"
Here is my Code:
Entity:
package pack.db;
import java.io.Serializable;
import java.sql.Date;
import java.sql.Time;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
#Entity
public class Travels implements Serializable {
private static final long serialVersionUID = 1L;
#Id
#GeneratedValue(strategy=GenerationType.SEQUENCE)
#Column(name="id")
private int id;
#Column(name="taxi_number")
private String taxiNumber;
#Column(name="travel_date")
private Date travelDate;
#Column(name="travel_time")
private Time travelTime;
#Column(name="cost")
private Double travelCost;
public Travels() {
super();
}
public void setNumber(String number)
{
this.taxiNumber = number;
}
public void setDate(Date date)
{
this.travelDate = date;
}
public void setTime(Time time)
{
this.travelTime = time;
}
public void setCost(Double cost)
{
this.travelCost = cost;
}
}
QueryClass
public Vector retrieveAllTravelsData(Date[] travelDate, Time[] travelTime) {
List<Object[]> allTravels = (List<Object[]>)em.createQuery("SELECT t FROM Travels t WHERE t.travelDate between ?1 and ?2 and " +
"t.travelTime between ?3 and ?4")
.setParameter(1, travelDate[0])
.setParameter(2, travelDate[1])
.setParameter(3, travelTime[0])
.setParameter(4, travelTime[1]).getResultList();
return (Vector) allTravels;
}
So what I want to do is to send "allTravels" as a vector to the client side becasuse
I need to populate a JTABLE in the client-side. so I tried to cast the return data
from the query to OBJECT[] (because the constructor of the JTABLE need Object[][] for the rows) and send it.. but i'm still get the exception in client side that says
"Cannot cast pack.db.Travel to java.util.Vector".. i don't think that i need to add
the travel Entity in the client side.. so how can i send the data to the client?
To be more specific.. I have this code with JDBC implemntation
public Vector retrieveAllTravelsData(Date[] travelDate, Time[] travelTime) {
Vector rows_data = new Vector();
String sql = "SELECT * FROM taxis.travels " + " WHERE travel_date BETWEEN ? AND ? AND travel_time BETWEEN ? AND ?";
try {
statement = (PreparedStatement) connection.prepareStatement(sql);
statement.setDate(1, travelDate[0]);
statement.setDate(2, travelDate[1]);
statement.setTime(3, travelTime[0]);
statement.setTime(4, travelTime[1]);
rs = statement.executeQuery();
ResultSetMetaData meta = rs.getMetaData();
int cols_count = meta.getColumnCount();
while (rs.next()) {
Vector record = new Vector();
for (int i = 0; i < cols_count; i++) {
record.add(rs.getString(i+1));
}
rows_data.addElement(record);
}
} catch (SQLException e) {
while (e != null) {
e.printStackTrace();
e = e.getNextException();
}
}
return rows_data;
Here i can get each column data from each column save it as a record and then put it in the Vector. so how that can be implemented by JPA? is it possible?

Answer 1

Casting an object to another class doesn't magically change the type of the object. It only allows referencing it as a more concrete class. So casting a List to Vector only works if the list is indeed a Vector.
getResultList() returns a List. That's what the javadoc says. The concrete class returned depends on the JPA provider, but I'm pretty sure none of them returns a Vector, since Vector is a class that should not be used anymore, since Java 1.2.
Moreover, this particular query doesn't return Object[], but instances of Travels (which should be named Travel, BTW).
So the method should be:
public List<Travel> retrieveAllTravelsData(Date[] travelDate, Time[] travelTime) {
List<Travel> allTravels = (List<Travel>) em.createQuery("SELECT t FROM Travel t WHERE t.travelDate between ?1 and ?2 and " +
"t.travelTime between ?3 and ?4")
.setParameter(1, travelDate[0])
.setParameter(2, travelDate[1])
.setParameter(3, travelTime[0])
.setParameter(4, travelTime[1]).getResultList();
return allTravels;
}
The server shouldn't care that the client-side needs a Vector to satisfy an old Swing class. If you really need a Vector at client-side, then create one from the returned list:
Vector<Travel> travelsAsVector = new Vector(travelsAsList);

Answer 2

Ok i solve it like that:
public Vector retrieveAllTravelsData(Date[] travelDate, Time[] travelTime) {
javax.persistence.Query q = (javax.persistence.Query)em.createNativeQuery("SELECT *
from Travels WHERE travel_date between ?1 and ?2")
.setParameter(1, travelDate[0])
.setParameter(2, travelDate[1])
.setParameter(3, travelTime[0])
.setParameter(4, travelTime[1]);
List<Object[]> result = (List<Object[]>)q.getResultList();
Vector rows = new Vector();
for (int i = 0 ; i < result.size(); i++)
{
Vector rec = new Vector();
for (int j = 0; j < columnCount; j++)
{
rec.add(result.get(i)[j].toString()); // returns the specific column value and add it to the vector
}
rows.addElement(rec);
}
return rows;
}