如何通过Java和XPath改变XML编码？

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathConstants;
import javax.xml.xpath.XPathExpression;
import javax.xml.xpath.XPathFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;

public class XMLEncoder {
    public static void main(String[] args) {
        try {
            // 创建Document对象
            DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
            DocumentBuilder builder = factory.newDocumentBuilder();
            Document document = builder.parse("input.xml");

            // 创建XPath对象
            XPathFactory xpathFactory = XPathFactory.newInstance();
            XPath xpath = xpathFactory.newXPath();

            // 编写XPath表达式
            XPathExpression expr = xpath.compile("//book[@id='1']/title");

            // 执行XPath表达式并获取结果
            NodeList nodes = (NodeList) expr.evaluate(document, XPathConstants.NODESET);

            // 遍历节点列表，修改编码
            for (int i = 0; i < nodes.getLength(); i++) {
                Node node = nodes.item(i);
                node.setTextContent("新的编码");
            }

            // 将修改后的Document保存为XML文件
            // ...

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}