Operacje na plikach:

File file = new File(nazwa_pliku);

try {
    BufferedReader reader = new BufferedReader(new FileReader(file));
    BufferedWriter writer = new BufferedWriter(new FileWriter(file));

    String line = null;
    while ((line = reader.readLine()) != null) {
        writer.write(line);
        writer.newLine();
    }

    reader.close();
    writer.close();
} catch (IOException e) {
    System.err.println(e);
    System.exit(1);
}

Implementacje dla JPanel
Runnable, KeyListener, MouseListener, MouseMotionListener, MouseWheelListener

Implmentacja w JFrame
this.addKeyListener(JPanel);
this.addMouseListener(JPanel);
this.addMouseWheelListener(JPanel);
this.addMouseMotionListener(JPanel);

Wątki
Thread nazwaWatka = new Thread(this);
nazwaWatka.start();

JFrame
super("Kolokwium II");
this.setSize(400, 600);
this.setDefaultCloseOperation(EXIT_ON_CLOSE);
this.setLocationRelativeTo(null);
this.setResizable(false);

AppPanel framePanel = new AppPanel();
this.setContentPane(framePanel);

this.setVisible(true);


public class ApiFrame extends JFrame {
    private JButton loadFile;
    private JButton saveFile;
    private JLabel labelInfo;
    private String textInfo;
    private ArrayList <Double> arrayOfNumbers = new ArrayList <Double>();

    public static void main(String[] args) {
        ApiFrame apiFrame = new ApiFrame();
    }
    public ApiFrame() {
        super("Kolokwium");

        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
        this.setLocationRelativeTo(null);
        this.setResizable(false);

        final JPanel apiPanel = new JPanel();
        this.setContentPane(apiPanel);
        apiPanel.setLayout(new BorderLayout());

        loadFile = new JButton("Wczytaj dane");
        saveFile = new JButton("Zapisz dane");

        textInfo = "Nie wczytano danych";

        labelInfo = new JLabel(textInfo,JLabel.CENTER);

        apiPanel.add(labelInfo,BorderLayout.PAGE_START);
        apiPanel.add(loadFile,BorderLayout.WEST);
        apiPanel.add(saveFile,BorderLayout.EAST);

        loadFile.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent arg0) {
                // TODO Auto-generated method stub
                File file = new File("numbers.dat");
                try {
                    BufferedReader reader = new BufferedReader(new FileReader(file));

                    String line = null;
                    while ((line = reader.readLine()) != null) {
                        line = line.replace(",", ".");
                        arrayOfNumbers.add(Double.parseDouble(line));
                        System.out.println(line);
                    }

                    reader.close();

                    labelInfo.setText("Wczytano dane");

                }
                catch (IOException e) {
                    System.err.println(e);
                    System.exit(1);
                }
            }
        });

        saveFile.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent arg0) {
                // TODO Auto-generated method stub

                File file = new File("max.dat");

                try {
                    BufferedWriter writer = new BufferedWriter(new FileWriter(file));
                    String line = maxNumber().toString();

                    writer.write(line);
                    writer.newLine();
                    System.out.println(line);
                    writer.close();
                }
                catch (IOException e) {
                    System.err.println(e);
                    System.exit(1);
                }
                labelInfo.setText("Zapisano dane");
            }
        });

        this.pack();
        this.setVisible(true);
    }

    public Double maxNumber() {
        double p = arrayOfNumbers.get(0);

        for (int i = 1; i < arrayOfNumbers.size(); i++) {
            if (arrayOfNumbers.get(i) > p) p = arrayOfNumbers.get(i);
        }

        return (double) p;
    }
}


public AppPanel() {
        this.setLayout(new BorderLayout());
        //Top Panel
        JPanel topPanel = new JPanel();
        topPanel.setPreferredSize(new Dimension(this.getWidth(), 50));

        textField = new JTextField(20);
        textField.setText("0");
        textLabel = new JLabel("Długość boku: ");
        textLabel.setLabelFor(textField);

        topPanel.add(textLabel);
        topPanel.add(textField);

        this.add(topPanel,BorderLayout.PAGE_START);

        //Bottom Panel
        JPanel bottomPanel = new JPanel();
        bottomPanel.setPreferredSize(new Dimension(this.getWidth(), 50));

        refreshButton = new JButton("Wczytaj obiekty");
        bottomPanel.add(refreshButton);

        this.add(bottomPanel,BorderLayout.PAGE_END);

        refreshButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {
                repaint();
            }
        });
    }

    public void paint(Graphics g) {
        super.paint(g);

        if (getLength() > 0) {
            paintRectangle(getLength(), getCountFromFile(), g);
            g.dispose();
        }
    }

    public void paintRectangle(int length, int count, Graphics g) {
        int i, t;

        Dimension d = getSize();

        for (i = 0; i < count; i++) {
             Graphics2D rectangle = (Graphics2D)g;
             rectangle.setColor(Color.BLUE);
             rectangle.drawRect(Math.round(d.width/2-length/2)+i*10, Math.round(d.height/2-length/2)+i*10, length, length);
        }
    }

    public void refreshPaint() {
        this.paintPanel.repaint();
    }

    public int getLength() {
        String length = this.textField.getText();
        return Integer.parseInt(length);
    }

    public int getCountFromFile() {
        String s = null;
        try {
            File file = new File("count.txt");

            FileReader inputFil = new FileReader(file);
            BufferedReader in = new BufferedReader(inputFil);
            s = in.readLine();
            in.close();
             inputFil.close();
        }
        catch(Exception e) {

        }

        if (s != null) {
            return Integer.parseInt(s);
        }
        else {
            return 0;
        }
    }
}