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Button submit = new Button("submit") {
    public void onSubmit() {
        // handle form submission
    }
}
form.add(submit);

TextField foo = new TextField("foo") {
    public boolean isRequired() {
        Form form = (Form) findParent(Form.class);
        return form.getRootForm().findSubmittingButton() == submitButtonbutton;
    }
}
form.add(foo);