Yousaf Yousaf - 13 days ago 6
Java Question

creating objects in java - stackoverflow error

Suppose I have two classes A and B. If i create an object of class A in class B and also create object of class B in class A, it causes Stack Overflow error. One solution to this problem is that i can create an object of class A inside any function in class B and vice versa but if I do this then object of class A is created every time that particular function in which object of class A is created, is called.

Question is how can i make objects of class A and B inside each other effectively ?

Consider following example.

Room Class

public class Room {

String roomno;
String reserved;
String category;
String airconditioned;
String bedtype;
String rent;

Connection con;
PreparedStatement ps;
ResultSet rs;

AddRoom adr = new AddRoom();
RemoveRoom rr = new RemoveRoom();
UpdateRoom ur = new UpdateRoom();
// AllRooms alr = new AllRooms();

public Room()
{
roomno = "";
reserved = "";
category = "";
airconditioned = "";
bedtype = "";
rent = "";
make_connection();
}

public void make_connection()
{
try{
String driver = "net.ucanaccess.jdbc.UcanaccessDriver";
Class.forName(driver);
String login = "jdbc:ucanaccess://C:\\MsDatabase\\EmployeeDB.accdb";
con = DriverManager.getConnection(login);

}catch(Exception ex){ System.out.println(ex);}
}

public void add_room(AddRoom obj)
{
try{
adr = obj;
if("".equals(adr.get_jtextfield1().getText())||"".equals(adr.get_jtextfield2().getText())||
"".equals(adr.get_jtextfield3().getText())||"".equals(adr.get_jtextfield4().getText())||
"".equals(adr.get_jtextfield5().getText())||"".equals(adr.get_jtextfield6().getText()))
{
JOptionPane.showMessageDialog(null, "None of the fields can be left empty");
}
else
{
roomno = adr.get_jtextfield1().getText();
reserved = adr.get_jtextfield2().getText();
category = adr.get_jtextfield3().getText();
airconditioned = adr.get_jtextfield4().getText();
bedtype = adr.get_jtextfield5().getText();
rent = adr.get_jtextfield6().getText();

String sql = "INSERT INTO RoomInfo(RoomNumber,Reserved,RoomCategory,AirConditioned,BedType,RentPerDay)"
+ "VALUES(?,?,?,?,?,?)";

ps = con.prepareStatement(sql);

ps.setInt(1, new Integer(roomno));
ps.setString(2, reserved);
ps.setString(3, category);
ps.setString(4, airconditioned);
ps.setString(5, bedtype);
ps.setInt(6, new Integer(rent));
ps.executeUpdate();
JOptionPane.showMessageDialog(null, "Room Added Successfully");
}

}catch(Exception ex){
JOptionPane.showMessageDialog(null, "Input in Room Number and "
+ "Rent Per Day should be a number");
}
}

}


AddRoom Class

public class AddRoom extends javax.swing.JFrame {

Room objr = new Room();
public AddRoom() {
initComponents();
}

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
//Room objr = new Room();
objr.add_room(this);
}

private void jButton2ActionPerformed(java.awt.event.ActionEvent evt) {
AdminHome admh = new AdminHome();
admh.setVisible(true);
dispose();
}

/* Create and display the form */
java.awt.EventQueue.invokeLater(new Runnable() {
@Override
public void run() {
new AddRoom().setVisible(true);
}
});
}

public JTextField get_jtextfield1()
{
return jTextField1;
}

public JTextField get_jtextfield2()
{
return jTextField2;
}

public JTextField get_jtextfield3()
{
return jTextField3;
}

public JTextField get_jtextfield4()
{
return jTextField4;
}

public JTextField get_jtextfield5()
{
return jTextField5;
}
public JTextField get_jtextfield6()
{
return jTextField6;
}

// Variables declaration - do not modify
private javax.swing.JButton jButton1;
private javax.swing.JButton jButton2;
private javax.swing.JLabel jLabel1;
private javax.swing.JLabel jLabel2;
private javax.swing.JLabel jLabel3;
private javax.swing.JLabel jLabel4;
private javax.swing.JLabel jLabel5;
private javax.swing.JLabel jLabel6;
private javax.swing.JLabel jLabel7;
private javax.swing.JPanel jPanel1;
private javax.swing.JTextField jTextField1;
private javax.swing.JTextField jTextField2;
private javax.swing.JTextField jTextField3;
private javax.swing.JTextField jTextField4;
private javax.swing.JTextField jTextField5;
private javax.swing.JTextField jTextField6;
// End of variables declaration
}


Note that i have created object of Room class in AddRoom class and also created object of AddRoom class in Room class. Now if i do this then i get stack overflow error but if if i make object of Room class inside any function in AddRoom class then stack overflow error is not displayed and program runs fine.

Answer

You are doing with that Circle references/dependencies and that is not a good idea...

what you need to implement is a callback, so A can call methods from B and B can inform A see image above:

enter image description here


Example:

//Interface
public interface ICallback{
    void onMessage(String msg);
}

//the class A call methods from B
public class A implements ICallback{

    private B b;

    public A(){
        b= new B();
        b.setCallback(this);
        b.printSomething(5);
        b.printSomething(0);
    }
    @Override
    public void onMessage(String msg){
    }
}

//the Class B can communicate to A with the callback
public class B {
    private ICallback cb;
    public B(){
    }
    public void setCallback(ICallback cb){
        this.cb = cb;
    }
    public void printSomething(int i){
        if(i==0){
            cb.onMessage("this is zero");
        }
    }
}
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