JTextArea not updating in gui

Question

I have a Java program that reads a text file of names and sorts them into groups. The number per group is entered via a JTextArea, the "submit" JButton is clicked, then the names are assigned randomly.

Finally the groups are placed in a JTextArea. Everything works fine up until the names are supposed to appear in the JTextArea. They don't show up. This is supposed to be done in the actionPerformed() method.

The only way I can get them to show up is by including a String literal in the JTextArea's append method. Then I have to click "submit" a 2nd time and the names do appear but the String literal ("Groups\n") appears twice, once before and once after.

Why do I need the literal and why doesn't the text appear immediately.

Here's my code:

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

public class GUIForGroups extends JFrame implements ActionListener
{
    JLabel numOfG = new JLabel("How many per group?  ");
    JTextField num = new JTextField(3);
    JButton sub = new JButton("Make Groups!");
    JPanel top = new JPanel();

    JLabel botTitle = new JLabel("The Groups are:");
    private ArrayList<String> roster;
    private ArrayList<String> team;
    //     ArrayList<JLabel> numbers = new ArrayList<JLabel>();
    //     ArrayList<JTextField> teams = new ArrayList<JTextField>();
    JPanel bottom = new JPanel();
    JTextArea area = new JTextArea(400,20);
    JScrollPane scrollPane = new JScrollPane(area); 
    ArrayList<String> groups;
    int n;
    public GUIForGroups()
    {
        setSize(800,400);
        setTitle("Group Picker!");
        Container c = getContentPane();
        c.setLayout( new BoxLayout(c, BoxLayout.Y_AXIS ) );
        top.add(numOfG);
        top.add(num);
        top.add(sub);
        sub.addActionListener(this);
        c.add(top);
        c.add(bottom);
        try {
            roster = getRoster();
        }
        catch (IOException ioe) {
            JOptionPane.showMessageDialog(null,"Error reading in data, exiting");
            System.exit(0);
        }
        setVisible(true);
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);

    }

    public void actionPerformed(ActionEvent evt) 
    {
        String userIn ;
        userIn    = num.getText()  ;
        n  = Integer.parseInt( userIn ) ;
        team = sortInGroups(n);
        String s = setPrint();
        //         /bottom.add(botTitle);

        area.append("Groups\n"+setPrint());  // PROBLEM HERE, NOT WORKING
       //          System.out.print(s);  // was for testing 
        bottom.add(area);
        add(bottom);

        repaint();
    }

    public void paintComponent(Graphics g)
    {
        area.append("The groups are: \n"+setPrint());
        add(bottom);
    }

    public ArrayList<String> getRoster() throws IOException
    {
        roster = new ArrayList<String>();
        Scanner fr = new Scanner(new File("csroster.txt"));
        while (fr.hasNext())
        {
            String name = fr.next();
            roster.add(name);
        }
        return roster;
    }

    public ArrayList sortInGroups(int n)
    {
        int i = 0, j=0;
        String next="";
        ArrayList<String> groups = new ArrayList<String>();
        while (roster.size() > 0 )
        {
            while (i<n && roster.size()>0)
            {
                int num = (int)(Math.random()*roster.size());
                next +=  roster.remove(num) + "   ";
                i++;
            }
            i=0; 
            groups.add(next);
            next = "";
        }
        return groups;
    }

    public String setPrint()
    {
        int teamNum = 1;
        String groups="";
        for (String t: team)
        {
            groups += "Team # "+teamNum+++" is: "+t+"\n";

        }
        return groups;
    }


    public static void main (String[] args) throws IOException
    {
        GUIForGroups gui = new GUIForGroups();
    }
}

Answer 1

You must call revalidate() instead of repaint().

public void actionPerformed(ActionEvent evt) 
{
    ...

    revalidate();
}

or don't add the area to bottom in the actionPerformed method - do it in the constructor. Then no need for revalidate or repaint

public GUIForGroups()
{
    ...
    c.add(bottom);
    bottom.add(area);  // missing a JScrollPane here
    try {
        ...
}


public void actionPerformed(ActionEvent evt) 
{
    String userIn ;
    userIn    = num.getText()  ;
    n  = Integer.parseInt( userIn ) ;
    team = sortInGroups(n);
    String s = setPrint();

    area.append("Groups\n"+setPrint());
}

Answer 2

Replace repaint(); with revalidate() in your action performed. Also you can remove paintComponent method.