import javax.swing.JOptionPane;

public class InputValidation
{
    public static void main(String [ ] args)
    {
        String message;

        message = JOptionPane.showInputDialog("Please Enter a, b or c:");

        while(!message.equals("a") && !message.equals("b") && !message.equals("c"))
        {
            message = JOptionPane.showInputDialog("Please Enter a, b or c:");
        }
    }
}