Transforming List<Map<String,String>> into a XML grouping by a value of the map

MapToXMLTransformer.java

public class MapToXMLTransformer {
    /*
     SKU_ID, DATE_EFF, DATE_INAC, VAT_CODE_ATT, PCNT_TO_APLY
    
    Origin = List<HashMap<String,String>>
    [ [skuid=10, date_eff=JAN, date_inac=FEB, vat_code_att=S, pcnt_to_aply=80],
      [skuid=10, date_eff=JAN, date_inac=FEB, vat_code_att=Z, pcnt_to_aply=20],
      [skuid=10, date_eff=MAR, date_inac=APR, vat_code_att=S, pcnt_to_aply=100]
    ]
    
    Destination = String
    <SkuTax skuId="10">
      <SkuTaxPeriod date_eff="JAN" date_inac="FEB">
        <Tax vatCode="S" pcnt="80"/>
        <Tax vatCode="Z" pcnt="20"/>
      </SkuTaxPeriod>
      <SkuTaxPeriod date_eff="MAR" date_inac="APR">
        <Tax vatCode="S" pcnt="100"/>
      </SkuTaxPeriod>
    </SkuTax>

     */

    private String getSkuNode(Map<String, String> taxElement) {
        return "<SkuTax skuId=\"".concat(taxElement.get("SKU_ID").concat("\">"));
    }

    private String xmlGroupNode(Map<String, String> group) {
        return "<SkuTaxPeriod date_eff=\"".concat(group.get("DATE_EFF")).concat("\" date_inac=\"").concat(group.get("DATE_INAC")).concat("\">");
    }

    private String xmlTaxNode(Map<String,String> mapa) {
        return "<Tax vatCode=\"" + mapa.get("VAT_CODE_ATT") + "\" pcnt=\"" + mapa.get("PCNT_TO_APLY") + "\"/>";
    }

    public String transform(List<Map<String,String>> message_body) {
        StringBuilder result = new StringBuilder();
        message_body.stream().
                            collect(Collectors.groupingBy(group -> xmlGroupNode(group))).
                            entrySet().stream().forEach(period -> {
                                    result.append(period.getKey());
                                    period.getValue().forEach(taxElement -> {
                                        if (!result.toString().contains("<SkuTax")) result.append(getSkuNode(taxElement));
                                        result.append(xmlTaxNode(taxElement));
                                            });
                                    result.append("</SkuTaxPeriod>");
                                    });
        return result.append("</SkuTax>").toString();
    }