How to validate TextBox fields?

I'm having problems in getting started with GWT Validation. I just want to validate some form fields on a page.
I started like this, but don't know how to go on:

class Person {
  @Size(min = 4, message = "Name too short.")
  private String name;

  Person(name) {
     this.name = name;
  }
}

class MyForm {
    @UiField
    TextBox name;

    @UiHandler
    void onClick("saveButton") {
        new Person(name);
        //...
    }
}

What do I have to do in order to validate the field now, and show the error message from the annotation on the page?

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