@Override
    public void keyReleased(KeyEvent e) {

        switch(e.getKeyCode()) {
            case KeyEvent.VK_0:
            case KeyEvent.VK_NUMPAD0:
                this.button0.setIsDown(false);
                break;
            case KeyEvent.VK_1:
            case KeyEvent.VK_NUMPAD1:
                this.button1.setIsDown(false);
                break;
            case KeyEvent.VK_2:
            case KeyEvent.VK_NUMPAD2:
                this.button2.setIsDown(false);
                break;
            case KeyEvent.VK_3:
            case KeyEvent.VK_NUMPAD3:
                this.button3.setIsDown(false);
                break;
            case KeyEvent.VK_4:
            case KeyEvent.VK_NUMPAD4:
                this.button4.setIsDown(false);
                break;
            case KeyEvent.VK_5:
            case KeyEvent.VK_NUMPAD5:
                this.button5.setIsDown(false);
                break;
            case KeyEvent.VK_6:
            case KeyEvent.VK_NUMPAD6:
                this.button6.setIsDown(false);
                break;
            case KeyEvent.VK_7:
            case KeyEvent.VK_NUMPAD7:
                this.button7.setIsDown(false);
                break;
            case KeyEvent.VK_8:
            case KeyEvent.VK_NUMPAD8:
                this.button8.setIsDown(false);
                break;
            case KeyEvent.VK_9:
            case KeyEvent.VK_NUMPAD9:
                this.button9.setIsDown(false);
                break;
            case KeyEvent.VK_ESCAPE:
                this.clearButton.setIsDown(false);
                break;
            case KeyEvent.VK_ENTER:
                this.display.clear();
                this.enterButton.addActionListener(new ActionListener() {
                    @Override
                    public void actionPerformed(ActionEvent e) {
                        for(ActionListener a : actions) {
                            a.actionPerformed(e);
                        }
                    }
                });
                this.enterButton.setIsDown(false);
                break;
            default:
                break;
        }
    }