public void actionPerformed(ActionEvent event) {
            double a = Double.parseDouble( txtA.getText());
            double b = Double.parseDouble( txtB.getText());
            double c = 0;
            String op = (String) ops.getSelectedItem();
            if (op.equalsIgnoreCase("add"))
                c = a + b;
            else if (op.equalsIgnoreCase("subtract"))
                    c = a - b;
            else if (op.equalsIgnoreCase("multiply"))
                    c = a * b;
            else if (op.equalsIgnoreCase("divide"))
                    c = a / b;
            txtC.setText("" + c);
        }