如何从Java中将子元素从XML提取到字符串？

import java.io.StringReader;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.xml.sax.InputSource;

public class XmlExtractor {
    public static void main(String[] args) {
        String xmlString = "<root><child1>Hello</child1<child2>World</child2></root>";
        String extractedString = extractChildElementsToString(xmlString);
        System.out.println(extractedString);
    }

    public static String extractChildElementsToString(String xmlString) {
        try {
            DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
            DocumentBuilder dBuilder = dbFactory.newDocumentBuilder();
            InputSource is = new InputSource(new StringReader(xmlString));
            Document doc = dBuilder.parse(is);

            NodeList childNodes = doc.getDocumentElement().getChildNodes();
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i< childNodes.getLength(); i++) {
                Node childNode = childNodes.item(i);
                if (childNode.getNodeType() == Node.ELEMENT_NODE) {
                    Element childElement = (Element) childNode;
                    sb.append(childElement.getTextContent());
                }
            }
            return sb.toString();
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }
}