private class btnCalculateListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            double num1, num2, num3;
            try {
                num1 = Double.parseDouble(txtNumber1.getText());
            } catch (Exception ex) {
                JOptionPane.showMessageDialog(GUIWindow.this, "Please enter an integer or a decimal in the first number box", programTitle, JOptionPane.ERROR_MESSAGE);
                return;
            }
            try {
                num2 = Double.parseDouble(txtNumber2.getText());
            } catch (Exception ex) {
                JOptionPane.showMessageDialog(GUIWindow.this, "Please enter an integer or a decimal in the second number box", programTitle, JOptionPane.ERROR_MESSAGE);
                return;
            }
            try {
                num3 = Double.parseDouble(txtNumber3.getText());
            } catch (Exception ex) {
                JOptionPane.showMessageDialog(GUIWindow.this, "Please enter an integer or a decimal in the third number box", programTitle, JOptionPane.ERROR_MESSAGE);
                return;
            }
            double average = (num1+num2+num3)/3.0;
            lblResult.setText("" + average);
        }
    }