src/main/java/org/efaps/ui/wicket/models/field/factories/DateUIFactory.java
@Override
    public Comparable<?> getCompareValue(final AbstractUIField _uiField)
        throws EFapsException
    {
        Comparable<?> ret = (Comparable<?>) _uiField.getValue().getReadOnlyValue(_uiField.getParent().getMode());
        if (ret instanceof String && StringUtils.isEmpty((String) ret)) {
            ret = null;
        }
        return ret;
    }
@Override
    public Comparable<?> getCompareValue(final AbstractUIField _uiField)
        throws EFapsException
    {
        Comparable<?> ret = (Comparable<?>) _uiField.getValue().getReadOnlyValue(_uiField.getParent().getMode());
        if (ret instanceof String && StringUtils.isEmpty((String) ret)) {
            ret = null;
        }
        return ret;
    }
{@inheritDoc}