Operator Verification Error

I do some practice expanding the homework project that I recently wrote. This is not for evaluation, but I want to add some errors to my code.

The program requires you to enter a name, select from the drop-down list, select from the list and select the switch. My goal is to fill out an error message if any of the required elements is empty.

The code that I have so far checked for an error is given below, but I'm not sure how to take a separate element without it and fill it in the message field, since the entire error check is in the same "if" expression.

Error checking code:

// Listener to handle the print button.
class ButtonListener implements ActionListener
{
    ButtonListener() {}

    public void actionPerformed(ActionEvent e)
    {
        JFrame frame1 = new JFrame("Show Message Dialog");

        // Checks for required entries
        if (error == 0 || name == "" || flag == 0)
        {
            JOptionPane.showMessageDialog(frame1,
                    "You must complete the form: " +     missing, "ERROR",
                JOptionPane.ERROR_MESSAGE);
        }
        else
        {
            // Get values from fields
            setText();
            System.out.println("Passenger Name: " + name + "\n");
            System.out.println("Age Group: " + ageValue + "\n");
            for (int i = 0; i < value.length; i++)
            {
                System.out.println("Destination: " + value[i]   + "\n");
            }
            System.out.println("Departure Day: " + day  + "\n");
        }
    }
}

Thank!