Anon Exercise Un-Fucked

// Un fucked code from: http://pastebin.com/xJJ4jYdE
import java.awt.*;
import java.awt.event.*;
import java.text.DecimalFormat;
import javax.swing.*;

class HoveredButton extends JButton implements MouseListener{
    private Color defaultForeground;

    public HoveredButton(String label){
        super(label);
        addMouseListener(this);
    }

    public void mouseClicked(MouseEvent evt){}
    public void mousePressed(MouseEvent evt){}
    public void mouseReleased(MouseEvent evt){}
    public void mouseEntered(MouseEvent evt){
        defaultForeground = evt.getComponent().getForeground();
        setForeground(Color.WHITE);
    }

    public void mouseExited(MouseEvent evt){
        setForeground(defaultForeground);
    }
}

public class Calculator extends JFrame implements ActionListener {
    private static final DecimalFormat FORMAT = new DecimalFormat("###,##0.######;-###,##0.######");

    private JTextField display = null;
    private String lastOperation = "";
    private double buffer = 0.0;
    private String current = "0";

    public static void main(String[] args){
        Calculator calc = new Calculator();
        calc.setVisible(true);
    }

    private HoveredButton addButton(JPanel panel, String label, Color color){
        final HoveredButton button = new HoveredButton(label);
        Action action = new AbstractAction(label){
                public void actionPerformed(ActionEvent e){button.doClick();}
            };
        action.putValue(Action.ACCELERATOR_KEY, KeyStroke.getKeyStroke(label));
        button.getActionMap().put("autoAction", action);
        button.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put((KeyStroke)action.getValue(Action.ACCELERATOR_KEY), "autoAction");
        button.addActionListener(this);
        button.setBackground(color);
        panel.add(button);
        return button;
    }

    public Calculator() {
        setTitle("My Calculator v1.0");
        setSize(248, 222);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new BorderLayout());

        display = new JTextField(15);
        display.setText("0");
        display.setHorizontalAlignment(JTextField.RIGHT);
        display.setEditable(false);

        JPanel panel = new JPanel();
        panel.setLayout(new GridLayout(5, 4));
        addButton(panel, "7", Color.LIGHT_GRAY);
        addButton(panel, "8", Color.LIGHT_GRAY);
        addButton(panel, "9", Color.LIGHT_GRAY);
        addButton(panel, "+", Color.CYAN);

        addButton(panel, "4", Color.LIGHT_GRAY);
        addButton(panel, "5", Color.LIGHT_GRAY);
        addButton(panel, "6", Color.LIGHT_GRAY);
        addButton(panel, "-", Color.CYAN);

        addButton(panel, "1", Color.LIGHT_GRAY);
        addButton(panel, "2", Color.LIGHT_GRAY);
        addButton(panel, "3", Color.LIGHT_GRAY);
        addButton(panel, "*", Color.CYAN);

        addButton(panel, ".", Color.LIGHT_GRAY);
        addButton(panel, "0", Color.LIGHT_GRAY);
        addButton(panel, "Sqrt", Color.CYAN);
        addButton(panel, "/", Color.CYAN);

        addButton(panel, "C", Color.MAGENTA);
        addButton(panel, "<-", Color.MAGENTA);
        addButton(panel, "%", Color.CYAN);
        addButton(panel, "=", Color.GREEN);

        JMenuBar menuBar = new JMenuBar();
        JMenu m_file = new JMenu("File");
        JMenuItem i_exit = new JMenuItem("Exit");
        i_exit.addActionListener(this); //Should have a separate actionlistener for the menu, but I can't be bothered.

        m_file.add(i_exit);
        menuBar.add(m_file);

        setJMenuBar(menuBar);
        add(display, BorderLayout.PAGE_START);
        add(panel, BorderLayout.CENTER);

        pack();
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        String cmd = e.getActionCommand();

        if(cmd.equals("<-")){
            if(current.length() > 1){
                current=Double.parseDouble(current.substring(0, current.length()-1))+"";
            }else{current="0";}
        }else if(cmd.equals("C")){
            current = "0";
            buffer = 0.0;
        }else if(cmd.equals("%")){
            buffer = Double.parseDouble(current)/100;
            current = buffer+"";
        }else if(cmd.equals("+")||cmd.equals("-")||cmd.equals("*")||cmd.equals("/")){
            buffer = Double.parseDouble(current);
            current = "0";
            lastOperation = cmd;
        }else if(cmd.equals("Sqrt")){
            buffer = Math.sqrt(Double.parseDouble(current));
            current = buffer+"";
            lastOperation = cmd;
        }else if(cmd.equals("=") && !lastOperation.isEmpty()){
            try{
                buffer = calculate(buffer, Double.parseDouble(current), lastOperation.charAt(0));
                current = buffer+"";
                lastOperation = "";
            }catch(IllegalArgumentException ex){/*Apparently we don't care about this.*/}
        }else if(cmd.equals(".")){
            if(current.indexOf('.')==-1)
                current = current+".";
        }else if(cmd.equals("Exit")){
            System.exit(0);
        }else if(Character.isDigit(cmd.charAt(0))){
            current = current+cmd;
        }

        display.setText(FORMAT.format(Double.parseDouble(current)));
    }

    double calculate(double loperand, double roperand, char operator) {
        switch(operator){
        case '+': return loperand + roperand;
        case '-': return loperand - roperand;
        case '*': return loperand * roperand;
        case '/': return loperand / roperand;
        }
        throw new IllegalArgumentException("calculate: Illegal operator encountered");
    }
}