StudentGabi

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

public class Apl extends JFrame {
    protected JButton ad;
    protected JButton avgBtn;
    protected JButton sortBtn;
    protected JTextField tf[] = new JTextField[3];
    protected List lst;

    protected JPanel contr, plst, footer;
    protected JLabel avgLbl;
    protected AdSt adSt;
    protected Student prs[] = new Student[0];

    Apl(int x, int y, int ln, int ht) {
        this.setLayout(new BorderLayout());
        this.setBounds(x, y, ln, ht);
        this.setVisible(true);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setTitle("Student's directory - version 1");

        contr = new JPanel(new FlowLayout());
        plst = new JPanel(new FlowLayout());
        footer = new JPanel(new FlowLayout());

        ad = new JButton("add");
        contr.add(ad);

        avgBtn = new JButton("Average");
        footer.add(avgBtn);
        avgBtn.addActionListener(new AvgEvent());

        avgLbl = new JLabel("....");
        footer.add(avgLbl);

        sortBtn = new JButton("Sort");
        plst.add(sortBtn);
        sortBtn.addActionListener(new SortEvent());

        tf[0] = new JTextField("name?", 10);
        tf[1] = new JTextField("note?", 3);
        tf[2] = new JTextField("ECN?", 10);
        contr.add(tf[0]);
        contr.add(tf[1]);
        contr.add(tf[2]);

        ad.addActionListener(adSt = new AdSt());

        lst = new List(20);
        plst.add(lst);

        add("North", contr);
        add("Center", plst);
        add("South", footer);

        revalidate();
    }

    class SortEvent implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            for (int i = 0; i < prs.length; i++)
            {
                for (int j = i; j < prs.length; j++)
                {
                    if (prs[i].compareTo(prs[j]) > 0)
                    {
                        Student temp = prs[i];
                        prs[i] = prs[j];
                        prs[j] = temp;
                    }
                }
            }
            lst.removeAll();
            for (int i = 0; i < prs.length; i++)
            {
                lst.add("" + prs[i]);
            }
            revalidate();
        }
    }

    class AdSt implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            Student s;
            int nt;
            String n = tf[0].getText();
            String egn = tf[2].getText();
            try {
                nt = Integer.parseInt(tf[1].getText());
            } catch (NumberFormatException ex) {
                tf[0].setText("name?");
                tf[1].setText("note?");
                tf[2].setText("ECN?");
                return;
            }
            if (!egnCheck(egn))
            {
                tf[0].setText("name?");
                tf[1].setText("note?");
                tf[2].setText("ECN?");
                return;
            }
            Student help[] = new Student[prs.length + 1];
            System.arraycopy(prs, 0, help, 0, prs.length);
            help[help.length - 1] = s = new Student(n, nt, egn);
            prs = help;
            tf[0].setText("name?");
            tf[1].setText("note?");
            tf[2].setText("ECN?");
            lst.add("" + s);
            revalidate();
        }
    }

    class AvgEvent implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            int sum = 0;
            for (int i = 0; i < prs.length; i++) {
                sum += prs[i].note;
            }
            double average = (float)sum / prs.length;
            avgLbl.setText(String.format("Average %.2f", average));
            revalidate();
        }
    }

    public boolean egnCheck(String egn)
    {
        if (egn.length() != 10)
        {
            return false;
        }
        for (int i = 0; i < egn.length(); i++)
        {
            if (!Character.isDigit(egn.charAt(i)))
            {
                return false;
            }
        }
        int[] digits = new int[10];
        for (int i = 0; i < egn.length(); i++)
        {
            digits[i] = Character.getNumericValue(egn.charAt(i));
        }
        int month = digits[2]*10 + digits[3];
        if (month > 12)
        {
            return false;
        }
        int day = digits[4]*10 + digits[5];
        if (month > 31)
        {
            return false;
        }
        int control = 0;
        for (int i = 1; i <= 9; i++)
        {
            control += digits[i-1] * (Math.pow(2, i) % 11);
        }
        control = control % 11;
        if (control == 10)
        {
            control = 0;
        }
        if (control != digits[9])
        {
            return false;
        }
        return true;
    }

    public static void main(String[] arg) {
        Apl apl = new Apl(20, 20, 400, 300);
    }

}