Parece que no es posible arreglar la falta de Graphics2D en el código, allí debo establecer el tamaño de reparación para las animaciones, de lo contrario, algunos comienzan fuera del Rectángulo 490 x 490 congelado o temblando en la pantalla

my SSCCE

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import java.util.ArrayList;
import java.util.Random;
import java.util.Vector;
import java.util.regex.Pattern;
import javax.swing.*;
import javax.swing.table.*;

public class OneTwoThree implements Runnable {

    private String testS;
    private JFrame frame = new JFrame("frameTitle");
    private JScrollPane tblS = new JScrollPane();
    private JTable tbl;
    private Vector<String> rOrH;
    private long t1 = 0L;
    private long t2 = 0L;
    private AlphaChars aChars = new AlphaChars();
    private GradientViewPortOriginal tableViewPort;
    private JViewport viewport = new JViewport();
    private Random random = new Random();
    private Vector<Vector<Integer>> vec = new Vector<Vector<Integer>>();
    private boolean bolo = false;

    public OneTwoThree() {
        t1 = System.currentTimeMillis();
        int regLenght = 3500;
        int chars = 0;
        testS = aChars.getNext(regLenght);
        rOrH = new Vector<String>();
        Vector<Vector<String>> rowD = new Vector<Vector<String>>();
        for (int e = 0; e < regLenght;) {
            chars++;
            if (chars > 50) {
                chars = 1;
                rowD.add(rOrH);
                rOrH = new Vector<String>();
            }
            String str = (testS.substring(e, (e + 1))).toString();
            if (str != null) {
                rOrH.add(str);
            } else {
                rOrH.add("");
            }
            e++;
        }
        rOrH = new Vector<String>();
        for (int i = 0; i < 50; i++) {
            rOrH.add(String.valueOf(i + 1));
        }
        tbl = new JTable(rowD, rOrH);
        TableColumnModel tcm = tbl.getColumnModel();
        for (int i = 0; i < (tcm.getColumnCount()); i++) {
            tcm.getColumn(i).setPreferredWidth(4);
        }
        tbl.setGridColor(tbl.getBackground());
        tbl.setRowHeight(10);
        tbl.setTableHeader(null);
        tbl.setPreferredScrollableViewportSize(tbl.getPreferredSize());
        tbl.setRowSelectionAllowed(false);
        tbl.setColumnSelectionAllowed(false);
        tableViewPort = new GradientViewPortOriginal(tbl);
        viewport = tableViewPort.getViewport();
        RepaintManager.setCurrentManager(new RepaintManager() {

            @Override
            public void addDirtyRegion(JComponent c, int x, int y, int w, int h) {
                Container con = c.getParent();
                while (con instanceof JComponent) {
                    if (!con.isVisible()) {
                        return;
                    }
                    if (con instanceof GradientViewPortOriginal) {
                        c = (JComponent) con;
                        x = 0;
                        y = 0;
                        w = con.getWidth();
                        h = con.getHeight();
                    }
                    con = con.getParent();
                }
                super.addDirtyRegion(c, x, y, w, h);
            }
        });
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(tableViewPort, BorderLayout.CENTER);
        frame.setLocation(20, 20);
        frame.pack();
        frame.setVisible(true);
        addColumnRenderes();
        for (int i = 0; i < 35; i++) {
            Star star = new Star(new Point(random.nextInt(490), random.nextInt(490)));
            star.setColor(new Color(100 + random.nextInt(155), 100 + random.nextInt(155), 100 + random.nextInt(155)));
            star.setxIncr(-3 + random.nextInt(7));
            star.setyIncr(-3 + random.nextInt(7));
            tableViewPort.add(star);
        }
    }

    private void addColumnRenderes() {
        for (int i = 0; i < tbl.getColumnCount(); i++) {
            RowColorRenderer rowRenderer = new RowColorRenderer(i);
            TableColumn column = tbl.getColumnModel().getColumn(i);
            column.setCellRenderer(rowRenderer);
        }
        addTableData();
    }

    private void addTableData() {
        tbl.setValueAt("8", 4, 25);tbl.setValueAt("4", 5, 24);tbl.setValueAt("1", 5, 25);tbl.setValueAt("4", 5, 26);
        tbl.setValueAt("4", 6, 23);tbl.setValueAt("4", 6, 25);tbl.setValueAt("1", 6, 26);tbl.setValueAt("4", 6, 27);
        tbl.setValueAt("1", 6, 28);tbl.setValueAt("1", 7, 21);tbl.setValueAt("4", 7, 22);tbl.setValueAt("1", 7, 23);
        tbl.setValueAt("4", 7, 24);tbl.setValueAt("1", 7, 27);tbl.setValueAt("8", 7, 28);tbl.setValueAt("4", 8, 23);
        tbl.setValueAt("1", 8, 24);tbl.setValueAt("1", 8, 25);tbl.setValueAt("8", 8, 26);tbl.setValueAt("8", 8, 27);
        tbl.setValueAt("7", 9, 24);tbl.setValueAt("1", 9, 26);tbl.setValueAt("4", 9, 25);tbl.setValueAt("4", 10, 24);
        tbl.setValueAt("1", 10, 25);tbl.setValueAt("8", 10, 26);tbl.setValueAt("4", 11, 23);tbl.setValueAt("4", 11, 27);
        tbl.setValueAt("1", 12, 21);tbl.setValueAt("4", 12, 22);tbl.setValueAt("8", 12, 28);tbl.setValueAt("4", 13, 21);
        tbl.setValueAt("8", 13, 29);tbl.setValueAt("1", 14, 19);tbl.setValueAt("4", 14, 20);tbl.setValueAt("8", 14, 30);
        tbl.setValueAt("4", 15, 19);tbl.setValueAt("4", 15, 20);tbl.setValueAt("4", 15, 21);tbl.setValueAt("1", 15, 22);
        tbl.setValueAt("4", 15, 23);tbl.setValueAt("1", 15, 31);tbl.setValueAt("1", 15, 32);tbl.setValueAt("4", 16, 22);
        tbl.setValueAt("4", 17, 21);tbl.setValueAt("4", 18, 20);tbl.setValueAt("4", 19, 20);tbl.setValueAt("4", 20, 19);
        tbl.setValueAt("4", 21, 17);tbl.setValueAt("4", 21, 18);tbl.setValueAt("4", 22, 16);tbl.setValueAt("4", 22, 17);
        tbl.setValueAt("4", 22, 18);tbl.setValueAt("4", 22, 19);tbl.setValueAt("4", 22, 20);tbl.setValueAt("4", 24, 18);
        tbl.setValueAt("4", 25, 17);tbl.setValueAt("4", 25, 17);tbl.setValueAt("4", 26, 16);tbl.setValueAt("4", 27, 15);
        tbl.setValueAt("4", 27, 16);tbl.setValueAt("4", 27, 18);tbl.setValueAt("4", 28, 17);tbl.setValueAt("4", 29, 16);
        tbl.setValueAt("4", 30, 15);tbl.setValueAt("4", 31, 14);tbl.setValueAt("4", 32, 13);tbl.setValueAt("4", 33, 12);
        tbl.setValueAt("4", 33, 13);tbl.setValueAt("4", 33, 14);tbl.setValueAt("4", 33, 18);tbl.setValueAt("4", 34, 17);
        tbl.setValueAt("4", 35, 16);tbl.setValueAt("4", 36, 15);tbl.setValueAt("4", 37, 14);tbl.setValueAt("4", 38, 13);
        tbl.setValueAt("4", 39, 12);tbl.setValueAt("4", 40, 11);tbl.setValueAt("4", 41, 10);tbl.setValueAt("4", 42, 9);
        tbl.setValueAt("4", 42, 10);tbl.setValueAt("4", 42, 11);tbl.setValueAt("4", 38, 18);tbl.setValueAt("4", 39, 18);
        tbl.setValueAt("4", 40, 18);tbl.setValueAt("4", 41, 17);tbl.setValueAt("4", 42, 16);tbl.setValueAt("4", 43, 15);
        tbl.setValueAt("4", 44, 14);tbl.setValueAt("4", 46, 13);tbl.setValueAt("4", 47, 12);tbl.setValueAt("4", 48, 11);
        tbl.setValueAt("4", 49, 10);tbl.setValueAt("4", 50, 9);tbl.setValueAt("4", 51, 8);tbl.setValueAt("4", 52, 7);
        tbl.setValueAt("4", 53, 6);tbl.setValueAt("4", 54, 5);tbl.setValueAt("4", 55, 4);tbl.setValueAt("4", 56, 6);
        tbl.setValueAt("4", 56, 8);tbl.setValueAt("4", 56, 9);tbl.setValueAt("4", 56, 10);tbl.setValueAt("4", 56, 12);
        tbl.setValueAt("4", 56, 13);tbl.setValueAt("4", 58, 10);tbl.setValueAt("4", 59, 11);tbl.setValueAt("4", 59, 12);
        tbl.setValueAt("4", 60, 13);tbl.setValueAt("4", 61, 15);tbl.setValueAt("4", 61, 16);tbl.setValueAt("4", 59, 17);
        tbl.setValueAt("4", 58, 18);tbl.setValueAt("4", 57, 19);tbl.setValueAt("4", 62, 18);tbl.setValueAt("4", 63, 19);
        tbl.setValueAt("4", 63, 20);tbl.setValueAt("3", 62, 23);tbl.setValueAt("3", 63, 23);tbl.setValueAt("3", 64, 23);
        tbl.setValueAt("3", 65, 23);tbl.setValueAt("3", 66, 23);tbl.setValueAt("3", 67, 23);tbl.setValueAt("3", 62, 28);
        tbl.setValueAt("3", 63, 28);tbl.setValueAt("3", 64, 28);tbl.setValueAt("3", 65, 28);tbl.setValueAt("3", 66, 28);
        tbl.setValueAt("3", 67, 28);tbl.setValueAt("8", 14, 26);tbl.setValueAt("8", 14, 27);tbl.setValueAt("8", 16, 27);
        tbl.setValueAt("8", 16, 28);tbl.setValueAt("8", 17, 29);tbl.setValueAt("8", 18, 30);tbl.setValueAt("8", 19, 31);
        tbl.setValueAt("8", 20, 32);tbl.setValueAt("8", 21, 31);tbl.setValueAt("8", 22, 30);tbl.setValueAt("8", 22, 29);
        tbl.setValueAt("3", 20, 27);tbl.setValueAt("3", 21, 26);tbl.setValueAt("3", 22, 25);tbl.setValueAt("3", 23, 24);
        tbl.setValueAt("3", 24, 24);tbl.setValueAt("3", 24, 25);tbl.setValueAt("3", 24, 26);tbl.setValueAt("8", 22, 32);
        tbl.setValueAt("8", 23, 33);tbl.setValueAt("8", 24, 34);tbl.setValueAt("8", 25, 35);tbl.setValueAt("8", 26, 36);
        tbl.setValueAt("8", 27, 37);tbl.setValueAt("8", 29, 34);tbl.setValueAt("8", 29, 35);tbl.setValueAt("8", 28, 36);
        tbl.setValueAt("8", 30, 32);tbl.setValueAt("8", 31, 33);tbl.setValueAt("8", 32, 34);tbl.setValueAt("8", 33, 35);
        tbl.setValueAt("8", 34, 36);tbl.setValueAt("8", 34, 37);tbl.setValueAt("3", 27, 30);tbl.setValueAt("3", 28, 29);
        tbl.setValueAt("3", 29, 28);tbl.setValueAt("3", 30, 28);tbl.setValueAt("3", 31, 29);tbl.setValueAt("3", 32, 30);
        tbl.setValueAt("3", 31, 24);tbl.setValueAt("3", 32, 23);tbl.setValueAt("3", 33, 22);tbl.setValueAt("3", 34, 22);
        tbl.setValueAt("3", 34, 23);tbl.setValueAt("3", 34, 24);tbl.setValueAt("3", 37, 20);tbl.setValueAt("3", 38, 19);
        tbl.setValueAt("3", 39, 18);tbl.setValueAt("3", 40, 18);tbl.setValueAt("3", 41, 19);tbl.setValueAt("3", 42, 20);
        tbl.setValueAt("3", 36, 34);tbl.setValueAt("3", 37, 33);tbl.setValueAt("3", 38, 32);tbl.setValueAt("3", 39, 32);
        tbl.setValueAt("3", 39, 33);tbl.setValueAt("3", 39, 34);tbl.setValueAt("3", 40, 24);tbl.setValueAt("3", 41, 23);
        tbl.setValueAt("3", 42, 22);tbl.setValueAt("3", 43, 22);tbl.setValueAt("3", 43, 23);tbl.setValueAt("3", 43, 24);
        tbl.setValueAt("3", 42, 30);tbl.setValueAt("3", 43, 29);tbl.setValueAt("3", 44, 28);tbl.setValueAt("3", 45, 28);
        tbl.setValueAt("3", 46, 29);tbl.setValueAt("3", 47, 30);tbl.setValueAt("3", 50, 21);tbl.setValueAt("3", 51, 20);
        tbl.setValueAt("3", 52, 19);tbl.setValueAt("3", 53, 19);tbl.setValueAt("3", 53, 20);tbl.setValueAt("3", 53, 21);
        tbl.setValueAt("3", 50, 31);tbl.setValueAt("3", 51, 30);tbl.setValueAt("3", 52, 29);tbl.setValueAt("3", 53, 29);
        tbl.setValueAt("3", 53, 30);tbl.setValueAt("3", 53, 31);tbl.setValueAt("3", 47, 15);tbl.setValueAt("3", 48, 14);
        tbl.setValueAt("3", 49, 13);tbl.setValueAt("3", 50, 13);tbl.setValueAt("3", 51, 14);tbl.setValueAt("3", 52, 15);
        tbl.setValueAt("3", 51, 25);tbl.setValueAt("3", 52, 24);tbl.setValueAt("3", 53, 23);tbl.setValueAt("3", 54, 23);
        tbl.setValueAt("3", 55, 24);tbl.setValueAt("3", 46, 25);tbl.setValueAt("3", 50, 38);tbl.setValueAt("3", 51, 37);
        tbl.setValueAt("3", 52, 36);tbl.setValueAt("3", 53, 36);tbl.setValueAt("3", 54, 37);tbl.setValueAt("3", 55, 38);
        tbl.setValueAt("3", 58, 28);tbl.setValueAt("3", 59, 27);tbl.setValueAt("3", 60, 26);tbl.setValueAt("3", 58, 26);
        tbl.setValueAt("3", 59, 27);tbl.setValueAt("3", 60, 28);tbl.setValueAt("8", 37, 36);tbl.setValueAt("8", 38, 37);
        tbl.setValueAt("8", 38, 38);tbl.setValueAt("8", 40, 39);tbl.setValueAt("8", 41, 40);tbl.setValueAt("8", 42, 41);
        tbl.setValueAt("8", 43, 41);tbl.setValueAt("8", 44, 38);tbl.setValueAt("8", 44, 39);tbl.setValueAt("8", 44, 40);
        tbl.setValueAt("8", 43, 35);tbl.setValueAt("8", 44, 36);tbl.setValueAt("8", 45, 37);tbl.setValueAt("8", 46, 38);
        tbl.setValueAt("8", 47, 39);tbl.setValueAt("8", 48, 40);tbl.setValueAt("8", 49, 41);tbl.setValueAt("8", 50, 42);
        tbl.setValueAt("8", 51, 43);tbl.setValueAt("8", 52, 44);tbl.setValueAt("8", 53, 45);tbl.setValueAt("8", 54, 43);
        tbl.setValueAt("8", 55, 42);tbl.setValueAt("8", 56, 41);tbl.setValueAt("8", 59, 31);tbl.setValueAt("8", 60, 32);
        tbl.setValueAt("8", 61, 33);tbl.setValueAt("8", 62, 34);tbl.setValueAt("8", 58, 36);tbl.setValueAt("8", 59, 37);
        tbl.setValueAt("8", 60, 38);tbl.setValueAt("8", 61, 39);tbl.setValueAt("8", 62, 40);tbl.setValueAt("8", 62, 41);
        tbl.setValueAt("8", 60, 35);tbl.setValueAt("8", 61, 36);tbl.setValueAt("8", 62, 37);tbl.setValueAt("8", 58, 45);
        tbl.setValueAt("8", 59, 42);tbl.setValueAt("8", 59, 43);tbl.setValueAt("8", 59, 44);tbl.setValueAt("8", 62, 40);
        frame.setTitle(" - - >   Merry Christmast   < - - ");
        t2 = System.currentTimeMillis();
        System.out.println("miliSec:" + (t2 - t1)); //aver. 32 miliSec.
        new Thread(this).start();
    }

    @Override
    public void run() {
        while (true) {
            try {
                Thread.sleep(1000);
            } catch (Exception e) {
                e.printStackTrace();
            }
            if (!bolo) {
                for (int i = 0; i < 30; i++) {
                    Vector<Integer> vecLocal = new Vector<Integer>();
                    final int r1 = random.nextInt(tbl.getRowCount() - 1);
                    final int c1 = random.nextInt(tbl.getColumnCount() - 1);
                    vecLocal.add(r1);
                    vecLocal.add(c1);
                    vec.add(vecLocal);
                    addSnowFlag();
                    try {
                        Thread.sleep(250);
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
                bolo = true;
            } else {
                Vector<Integer> vecLocal = new Vector<Integer>();
                final int r = random.nextInt(tbl.getRowCount() - 1);
                final int c = random.nextInt(tbl.getColumnCount() - 1);
                vecLocal.add(r);
                vecLocal.add(c);
                vec.add(vecLocal);
                addSnowFlag();
                try {
                    Thread.sleep(500);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }
    }

    private void addSnowFlag() {
        if (vec.size() > -1) {
            Vector<Integer> vecLocal = vec.elementAt(vec.size() - 1);
            final int r = vecLocal.elementAt(0);
            final int c = vecLocal.elementAt(1);
            String str = String.valueOf(tbl.getValueAt(r, c));
            if (!str.isEmpty()) {
                if (!Pattern.compile("\\d").matcher(str).find()) {
                    if (!Pattern.compile("[123456789]").matcher(str).find()) {
                        Runnable doRun = new Runnable() {

                            @Override
                            public void run() {
                                tbl.setValueAt("9", r, c);
                            }
                        };
                        SwingUtilities.invokeLater(doRun);
                        if (bolo) {
                            Vector<Integer> vecLocal1 = vec.elementAt(0);
                            final int r1 = vecLocal1.elementAt(0);
                            final int c1 = vecLocal1.elementAt(1);
                            Runnable doRun1 = new Runnable() {

                                @Override
                                public void run() {
                                    tbl.setValueAt(aChars.getNext(1), r1, c1);
                                    vec.removeElementAt(0);
                                }
                            };
                            SwingUtilities.invokeLater(doRun1);
                        }
                    }
                }
            }
        }
    }

    private class GradientViewPortOriginal extends JScrollPane {

        private static final long serialVersionUID = 1L;
        private final int h = 80;
        private BufferedImage img = null;
        private BufferedImage shadow = new BufferedImage(1, h, BufferedImage.TYPE_INT_ARGB);
        private JViewport viewPort;
        private ArrayList<Star> stars = new ArrayList<Star>();

        public GradientViewPortOriginal(JComponent com) {
            super(com);
            viewPort = this.getViewport();
            viewPort.setScrollMode(JViewport.BLIT_SCROLL_MODE);
            viewPort.setScrollMode(JViewport.BACKINGSTORE_SCROLL_MODE);
            viewPort.setScrollMode(JViewport.SIMPLE_SCROLL_MODE);
            setPreferredSize(this.getPreferredSize());
            paintBackGround(new Color(250, 250, 150));
        }

        public void add(Star star) {
            stars.add(star);
        }
        //
        private Timer timer = new Timer(33, new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                for (Star star : stars) {
                    star.move();
                }
                repaint();
            }
        });

        public void startAnimation() {
            if (!timer.isRunning()) {
                timer.start();
            }
        }

        @Override
        public void addNotify() {
            super.addNotify();
            timer.start();
        }

        private void paintBackGround(Color g) {
            Graphics2D g2 = shadow.createGraphics();
            g2.setPaint(g);
            g2.fillRect(0, 0, 1, h);
            g2.setComposite(AlphaComposite.DstIn);
            g2.setPaint(new GradientPaint(0, 0, new Color(0, 0, 0, 0f), 0, h,
                    new Color(0.8f, 0.8f, 0.8f, 0.5f)));
            g2.fillRect(0, 0, 1, h);
            g2.dispose();
        }

        @Override
        public void paint(Graphics g) {
            if (img == null || img.getWidth() != getWidth() || img.getHeight() != getHeight()) {
                img = new BufferedImage(getWidth(), getHeight(), BufferedImage.TYPE_INT_ARGB);
            }
            Graphics2D g2 = img.createGraphics();
            super.paint(g2);
            ((Graphics2D) g).setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
            Rectangle bounds = getViewport().getVisibleRect();
            g2.scale(bounds.getWidth(), -1);
            int y = (getColumnHeader() == null) ? 0 : getColumnHeader().getHeight();
            g2.drawImage(shadow, bounds.x, -bounds.y - y - h, null);
            g2.scale(1, -1);
            g2.drawImage(shadow, bounds.x, bounds.y + bounds.height - h + y, null);
            g2.dispose();
            g.drawImage(img, 0, 0, null);
            for (Star star : stars) {
                g.setColor(star.getColor());
                g.fillPolygon(star);
            }
        }
    }

    private class RowColorRenderer extends DefaultTableCellRenderer {

        private static final long serialVersionUID = 1L;
        private int colNo = 0;

        RowColorRenderer(int col) {
            colNo = col;
        }

        @Override
        public Component getTableCellRendererComponent(JTable table, Object value,
                boolean isSelected, boolean hasFocus, int row, int column) {
            Component comp = super.getTableCellRendererComponent(table, value,
                    isSelected, hasFocus, row, column);
            if (!isSelected) {
                if (table.getValueAt(row, colNo) != null) {
                    String str = table.getValueAt(row, colNo).toString();
                    if (!str.isEmpty()) {
                        if (Pattern.compile("\\d").matcher(str).find()) {
                            if (((Pattern.compile("[9]").matcher(str).find()))) {
                                setBackground(new Color(250, 250, 150));
                                setForeground(new Color(250, 250, 150));
                                setFont(new Font("Serif", Font.BOLD, 9));
                            } else if (((Pattern.compile("[1]").matcher(str).find()))) {
                                setBackground(tbl.getBackground());
                                setForeground(Color.orange);
                                setFont(new Font("Serif", Font.BOLD, 8));
                            } else {
                                setBackground(tbl.getBackground());
                                setForeground(Color.gray);
                                setFont(new Font("Serif", Font.BOLD, 9));
                            }
                        } else {
                            setBackground(tbl.getBackground());
                            setForeground(Color.lightGray);
                            setFont(new Font("Serif", Font.PLAIN, 6));
                        }
                    }
                }
            }
            setHorizontalAlignment(CENTER);
            return this;
        }
    }

    private class AlphaChars {

        public static final int MIN_LENGTH = 2000;
        private java.util.Random rand = new java.util.Random();
        private char[] AlphaChars = {
            'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q',
            'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',};
        /*'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q',
        'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
        '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', '+', '-', '*', '/', '<', '>', '&',
        '#', '@', '{', '}', '?', ':', '_', '"', '!', ')', '('};*/

        public String getNext() {
            StringBuilder strbuf = new StringBuilder();
            for (int i = 0; i < MIN_LENGTH; i++) {
                strbuf.append(getAlphaChars()[getRand().nextInt(getAlphaChars().length)]);
            }
            return strbuf.toString();
        }

        public String getNext(int reqLenght) {
            StringBuilder strbuf = new StringBuilder();
            for (int i = 0; i < reqLenght; i++) {
                strbuf.append(getAlphaChars()[getRand().nextInt(getAlphaChars().length)]);
            }
            return strbuf.toString();
        }

        public java.util.Random getRand() {
            return rand;
        }

        public void setRand(java.util.Random aRand) {
            rand = aRand;
        }

        public char[] getAlphaChars() {
            return AlphaChars;
        }

        public void setAlphaChars(char[] aAlphaChars) {
            AlphaChars = aAlphaChars;
        }
    }

    private class Star extends Polygon {

        private static final long serialVersionUID = 1L;
        private Point location = null;
        private Color color = Color.YELLOW;
        private int xIncr, yIncr;
        static final int WIDTH = 500, HEIGHT = 500;

        Star(Point location) {
            int x = location.x;
            int y = location.y;
            this.location = location;
            this.addPoint(x, y + 8);
            this.addPoint(x + 8, y + 8);
            this.addPoint(x + 11, y);
            this.addPoint(x + 14, y + 8);
            this.addPoint(x + 22, y + 8);
            this.addPoint(x + 17, y + 12);
            this.addPoint(x + 21, y + 20);
            this.addPoint(x + 11, y + 14);
            this.addPoint(x + 3, y + 20);
            this.addPoint(x + 6, y + 12);
        }

        public void setColor(Color color) {
            this.color = color;
        }

        public void move() {
            if (location.x < 0 || location.x > WIDTH) {
                xIncr = -xIncr;
            }
            if (location.y < 0 || location.y > WIDTH) {
                yIncr = -yIncr;
            }
            translate(xIncr, yIncr);
            location.setLocation(location.x + xIncr, location.y + yIncr);
        }

        public void setxIncr(int xIncr) {
            this.xIncr = xIncr;
        }

        public void setyIncr(int yIncr) {
            this.yIncr = yIncr;
        }

        public Color getColor() {
            return color;
        }
    }

    public static void main(String args[]) {
        Runnable doRun = new Runnable() {

            @Override
            public void run() {
                OneTwoThree hnvit = new OneTwoThree();
            }
        };
        SwingUtilities.invokeLater(doRun);
    }
}

Deseo a Merry Christmast para todos los contribuyentes de Java aquí

EDITAR

Por favorchange private class Star extends Polygon { métodomove() a --->

public void move() {
    if (location.x < 0 || location.x > frame.getContentPane().getWidth() - 20) {
        xIncr = -xIncr;
    }
    if (location.y < 0 || location.y > frame.getContentPane().getHeight() - 20) {
        yIncr = -yIncr;
    }
    translate(xIncr, yIncr);
    location.setLocation(location.x + xIncr, location.y + yIncr);
}