XPathExpression不能總和

Question

我解析XML並使用XPath想評估summ。

我嘗試：

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，I'have錯誤：

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

在這一行：

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

這究竟是爲什麼？

Answer 1

XPathConstants.NUMBER計算結果爲Double，以它的結果分配給一個Long你需要做這樣的事情：

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