Including JCheckBox in JFrame as and when a client connects

Question

I am trying to write a server application in java and creating a vector v1 to store all the sockets that hit the server.

Next i made an arraylist where i stored all my Client sockets as objects and then created a JCheckBox array where I loop it to add checkbox to the JPanel controlpanel which was initiated in the class contructor but something seems to be going wrong and i can't see any checkbox.

import java.util.*;
import java.net.*;
import javax.swing.*;
import java.awt.event.*;
import java.io.*;
import java.awt.*;

class server extends JFrame implements ActionListener {

    JTextArea t1;
    PrintWriter pw;
    JButton b1, b2;
    static Vector v1 = new Vector();
    static ArrayList checks = new ArrayList();
    static JPanel controlpanel;

    server() {
        setLayout(new FlowLayout());
        t1 = new JTextArea();
        add(t1);
        b1 = new JButton("Send to All");
        b2 = new JButton("Send");
    //b1.addActionListener(this);
        //b2.addActionListener(this);
        add(b2, BorderLayout.SOUTH);

        add(b1, BorderLayout.NORTH);
        b1.addActionListener(this);
        b2.addActionListener(this);
        controlpanel = new JPanel();
        controlpanel.setVisible(true);
        add(controlpanel);
    }

    public void actionPerformed(ActionEvent ae) {

        String m = t1.getText();
        if (ae.getSource() == b1) {
            try {
                Iterator t2 = v1.iterator();
                while (t2.hasNext()) {
                    Socket s = (Socket) t2.next();
                    pw = new PrintWriter(s.getOutputStream(), true);
                    pw.println(m);
                    pw.flush();
                    System.out.println("send" + m);
                }
            } catch (Exception e) {
            }
        }
        if (ae.getSource() == b2) {
            try {
                Iterator itr = checks.iterator();
                while (itr.hasNext()) {
                    Socket schecked = (Socket) itr.next();
                    pw = new PrintWriter(schecked.getOutputStream(), true);
                    pw.println(m);
                    pw.flush();
                    System.out.println("sent " + m);

                }
            } catch (Exception e) {
            }
        }
        t1.setText("");
    }

    public static void main(String z[]) {

        try {
            new server().setVisible(true);
            ServerSocket s = new ServerSocket(2000);
            while (true) {
                System.out.println("waiting...");
                Socket c = s.accept();
                //          System.out.print(c);
                v1.add(c);
                for (int i = 0; i < v1.size(); i++) {
                    checks.add(v1.get(i));
                    //              checks[i].setText()=v1[i];
                }
                JCheckBox checkbox[] = new JCheckBox[checks.size()];

                for (int i = 0; i < checks.size(); i++) {
                    //  Object o=checks.get(i);
                    checkbox[i] = new JCheckBox();
                    //              checkbox[i].setText()=checks.get(i).name;
                    controlpanel.add(checkbox[i]);
                    checkbox[i].setSelected(true);
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Answer 1

As you are adding UI elements after the JFrame has been made visible, you need to call revalidate() on the panel. Also you should never add directly from a thread which is not the Event Dispatch Thread. Finally, it appears every time a new client connects, you're going to re-add all the checkbox you already have added, plus one (i.e. when the first client connects you will have 1 checkbox, when the second connects you'll have 1 + 2 = 3, when the third connects you'll have 1 + 2 + 3 = 6, etc.).

I strongly suggests you mentally run through your code, instruction by instruction, to see what happens. Alternatively, if you're using a decent IDE, you can step through the code in the debugger and inspect the content of controlPanel. If you're not using an IDE... you should.

I don't quite understand what you need check[] and v1[] for but assuming that you do need them, instead of maintaining three arrays you can define a class containing a variable for a check, one for a v1, and one for a checkbox, and maintain a single array of instances of such objects.