ui/src/main/java/ucar/util/prefs/ui/Field.java
protected boolean validate( StringBuffer buff) {
    if (!_validate(buff)) return false;
    Object editValue = getEditValue();
    if (editValue == null)
      return false;

    for (FieldValidator v : validators) {
      if (!v.validate(this, editValue, buff)) return false;
    }

    if (acceptIfDifferent( editValue)) {
      setEditValue( validValue);
      sendEvent();
    }
    return true;
  }
protected boolean validate( StringBuffer buff) {
    if (!_validate(buff)) return false;
    Object editValue = getEditValue();
    if (editValue == null)
      return false;

    for (FieldValidator v : validators) {
      if (!v.validate(this, editValue, buff)) return false;
    }

    if (acceptIfDifferent( editValue)) {
      setEditValue( validValue);
      sendEvent();
    }
    return true;
  }
See if edit value is valid, put error message in buff.