XPathExpression无法对数字求和

Question

我解析XML并使用XPath想要评估汇总。

我试试：

long resultSum = 0L;
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
factory.setNamespaceAware(false);
DocumentBuilder builder = factory.newDocumentBuilder();
Document doc = builder.parse(getOutputFileName());
XPathFactory xPathFactory = XPathFactory.newInstance();
XPath xPath = xPathFactory.newXPath();
String xPathExpression = "sum(//entries//entry/@field)";
XPathExpression expr = xPath.compile(xPathExpression);
resultSum = (Long) expr.evaluate(doc, XPathConstants.NUMBER);

但是，我有错误：

Exception in thread "main" java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.Long

在这一行：

resultSum = (Long) expr.evaluate(doc, XPathConstants.NUMBER);

为什么会这样？

Answer 1

XPathConstants.NUMBER评估为Double，为了将结果分配给Long，您需要执行以下操作：

resultSum = ((Double) expr.evaluate(doc, XPathConstants.NUMBER)).longValue();