compiling creates ...$.class for every button

Question

I have a problem with compiling. If I compile my code, it creates a GUI.class and GUI$x.class, wehere x stands for the amount of buttons I put in ( that means, if i have two buttons it creates a GUI$1.class and a GUI$2.class). The shown GUI is correct and works fine. Why does it create those $.class files and how can i correct it? Here is a code snippet:

public class GUI implements ActionListener,ListSelectionListener,FocusListener {
    static DefaultListModel<String> inputHistory;
    static DefaultListModel<String> resultHistory;
    static JTextField input;
    JList<String> listInput;
    JList<String> listResult;
    static JLabel status;
    JButton enter;
    JButton delete;
    JButton copy;
    JButton paste;
    static JButton numpad;
    boolean keyboardActivated = true;

    public GUI() {

        JFrame frame = new JFrame("calc");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setResizable(false);
        frame.setSize(705, 400);
        frame.getContentPane().setLayout(null);

        input = new JTextField();
        input.setBounds(10,10,560,50);
        input.setActionCommand("input");
        input.addActionListener(this);

        enter = new JButton("=");
        enter.setBounds(580,10,100,50);
        enter.addActionListener(this);  

        delete = new JButton("C");                                                  
        delete.setBounds(580,90,100,50);
        delete.addActionListener(new ActionListener(){  
            public void actionPerformed(ActionEvent e){  
                input.setText("");
            }
        });

        copy = new JButton("copy");
        copy.setBounds(580,150,100,50);
        copy.addActionListener(new ActionListener(){  
            public void actionPerformed(ActionEvent e){  
                Toolkit.getDefaultToolkit().getSystemClipboard().setContents (              
                           new StringSelection(input.getText()), null
                 );
            }
        });

        paste = new JButton("paste");
        paste.setBounds(580,210,100,50);
        paste.addActionListener(new ActionListener(){  
            public void actionPerformed(ActionEvent e){                                     
                Clipboard clipboard = Toolkit.getDefaultToolkit().getSystemClipboard();
                DataFlavor flavor = DataFlavor.stringFlavor;
                if (clipboard.isDataFlavorAvailable(flavor)) {
                  try {
                    String text = (String) clipboard.getData(flavor);
                    input.setText(text);
                  } catch (UnsupportedFlavorException f) {
                    System.out.println(f);
                  } catch (IOException f) {
                    System.out.println(f);
                  }
                }
            }
        });

        JLabel label1 = new JLabel("input history");
        label1.setBounds(10, 70, 200, 20);
        inputHistory = new DefaultListModel<>();
        listInput = new JList<>(inputHistory);
        listInput.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
        listInput.addListSelectionListener(this);
        listInput.addFocusListener(this);
        JScrollPane sp = new JScrollPane();
        sp.setViewportView(listInput);
        sp.setBounds(10,90,280,250);

        JLabel label2 = new JLabel("result history");
        label2.setBounds(290,70,280,20);
        resultHistory = new DefaultListModel<>();
        listResult = new JList<>(resultHistory);
        listResult.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
        listResult.addListSelectionListener(this);
        listResult.addFocusListener(this);
        JScrollPane sp1 = new JScrollPane();
        sp1.setViewportView(listResult);
        sp1.setBounds(290,90,280,250);

        status = new JLabel("");
        status.setForeground(Color.red);
        status.setBounds(10,340,650,20);

        numpad = new JButton("Numpad");
        numpad.setBounds(580,290,100,50);
        numpad.addActionListener(new ActionListener(){  
            public void actionPerformed(ActionEvent e){  
                new Numpad();                                                   
                numpad.setEnabled(false);                                           
            }
        });

        frame.getContentPane().add(input);
        frame.getContentPane().add(enter);
        frame.getContentPane().add(label1);
        frame.getContentPane().add(label2);
        frame.getContentPane().add(sp);
        frame.getContentPane().add(sp1);
        frame.getContentPane().add(status);
        frame.getContentPane().add(numpad);
        frame.getContentPane().add(delete);
        frame.getContentPane().add(copy);
        frame.getContentPane().add(paste);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        new GUI();
    }

    public void actionPerformed(ActionEvent e) {                                    
        String result = null;

        try {
            result = Postfix.eval(input.getText());
        } catch (Exception ex) {
            status.setText(ex.getMessage());
        }

        if (result != null) {                                                       
            inputHistory.insertElementAt(input.getText(), 0);                       
            resultHistory.insertElementAt(result , 0);                          
            input.setText(result);
            input.setCaretPosition(0);
            status.setText("");
        }
    }
}