Showing validation messages

In this recipe, we will make an input text field for a PIN code with the following rules:

• It can contain only numbers. When a user enters letters, then a Just numbers allowed error is shown.

• When there are less than four numbers, a Too few numbers error is shown.

• When there are more than four numbers, a Too many numbers error is shown.

• The content of the field is valid only when it contains exactly four numbers.

These rules will be part of validation and the error message will appear together with the field. So when we move over the field, the error pops up, as shown in the following screenshot:

How to do it...

Carry out the following steps:

1. First, we create a TextField with an Enter PIN Code label. Then, we need to set setImmediate to true, so the requests from UI are sent immediately to the server.

   public class MyVaadinUI extends UI {
   
     @Override
     protected void init(VaadinRequest request) {
       final VerticalLayout layout = new VerticalLayout();
       layout.setMargin...