我在迭代循环以获取值时得到nullpointer错误。我有一个像下面的xml文件，我想得到driverid，vehicleid，act的值。但是java正在抛出无效指针，我是一个初学者，并不粗鲁。提前谢谢。
package xpath;
import org.w3c.dom.*;
import javax.xml.parsers.*;
import javax.xml.xpath.*;
public class Xpath {
/**
* @param args the command line arguments
*/
public static void main(String[] args) throws Exception {
DocumentBuilderFactory Factory = DocumentBuilderFactory.newInstance();
DocumentBuilder builder = Factory.newDocumentBuilder();
Document doc = builder.parse("C:/Users/HP/Desktop/solution.xml");
//creating an XPathFactory:
XPathFactory factory = XPathFactory.newInstance();
//using this factory to create an XPath object:
XPath xpath = factory.newXPath();
// XPath Query for showing all nodes value
XPathExpression expr = xpath.compile("problem/solutions/solution[cost='505.9208295302417']/routes/route");
Object result = expr.evaluate(doc, XPathConstants.NODESET);
NodeList nodes = (NodeList) result;
for (int i = 0; i < nodes.getLength(); i++) {
System.out.println(nodes.getLength());
;
Element el = (Element) nodes.item(i);
if (el.getNodeType() == Node.ELEMENT_NODE) {
System.out.println("driverid:" + el.getElementsByTagName("driverId").item(i).getTextContent());
System.out.println("vehicleId:" + el.getElementsByTagName("vehicleId").item(i).getTextContent());
System.out.println("Citizen:" + el.getElementsByTagName("act").item(i).getTextContent());
System.out.println("Element currently in: " + el.getNodeName());
}
}
}
}
问题似乎发生在第41行，即
System.out.println("driverid:" + el.getElementsByTagName("driverId").item(i).getTextContent());
（#EDITED）实际上上面提供的xml不是原始的我认为发布一个较小的xml样本会有所帮助，但是没有，抱歉不方便。所以这是完整的xml文件
<?xml version="1.0" encoding="UTF-8"?>
<problem xmlns="http://www.w3schools.com"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.w3schools.com vrp_xml_schema.xsd">
<problemType>
<fleetSize>FINITE</fleetSize>
</problemType>
<vehicles>
<vehicle>
<id>1_1</id>
<typeId>type_1</typeId>
<startLocation>
<id>[x=-20.2675][y=57.4797]</id>
<coord x="-20.2675" y="57.4797"/>
</startLocation>
<endLocation>
<id>[x=-20.2675][y=57.4797]</id>
<coord x="-20.2675" y="57.4797"/>
</endLocation>
<timeSchedule>
<start>0.0</start>
<end>1.7976931348623157E308</end>
</timeSchedule>
<returnToDepot>true</returnToDepot>
</vehicle>
<vehicle>
<id>1_2</id>
<typeId>type_1</typeId>
<startLocation>
<id>[x=-20.2675][y=57.4797]</id>
<coord x="-20.2675" y="57.4797"/>
</startLocation>
<endLocation>
<id>[x=-20.2675][y=57.4797]</id>
<coord x="-20.2675" y="57.4797"/>
</endLocation>
<timeSchedule>
<start>0.0</start>
<end>1.7976931348623157E308</end>
</timeSchedule>
<returnToDepot>true</returnToDepot>
</vehicle>
<vehicle>
<id>2_1</id>
<typeId>type_2</typeId>
<startLocation>
<id>[x=-20.2675][y=57.4797]</id>
<coord x="-20.2675" y="57.4797"/>
</startLocation>
<endLocation>
<id>[x=-20.2675][y=57.4797]</id>
<coord x="-20.2675" y="57.4797"/>
</endLocation>
<timeSchedule>
<start>0.0</start>
<end>1.7976931348623157E308</end>
</timeSchedule>
<returnToDepot>true</returnToDepot>
</vehicle>
<vehicle>
<id>3_1</id>
<typeId>type_3</typeId>
<startLocation>
<id>[x=-20.2675][y=57.4797]</id>
<coord x="-20.2675" y="57.4797"/>
</startLocation>
<endLocation>
<id>[x=-20.2675][y=57.4797]</id>
<coord x="-20.2675" y="57.4797"/>
</endLocation>
<timeSchedule>
<start>0.0</start>
<end>1.7976931348623157E308</end>
</timeSchedule>
<returnToDepot>true</returnToDepot>
</vehicle>
</vehicles>
<vehicleTypes>
<type>
<id>type_1</id>
<capacity-dimensions>
<dimension index="0">120</dimension>
</capacity-dimensions>
<costs>
<fixed>0.0</fixed>
<distance>1.0</distance>
<time>0.0</time>
<service>0.0</service>
<wait>0.0</wait>
</costs>
</type>
<type>
<id>type_2</id>
<capacity-dimensions>
<dimension index="0">160</dimension>
</capacity-dimensions>
<costs>
<fixed>0.0</fixed>
<distance>1.1</distance>
<time>0.0</time>
<service>0.0</service>
<wait>0.0</wait>
</costs>
</type>
<type>
<id>type_3</id>
<capacity-dimensions>
<dimension index="0">300</dimension>
</capacity-dimensions>
<costs>
<fixed>0.0</fixed>
<distance>1.3</distance>
<time>0.0</time>
<service>0.0</service>
<wait>0.0</wait>
</costs>
</type>
</vehicleTypes>
<services>
<service id="5 " type="service">
<location>
<id>[x=57.49129867553711][y=-20.29949951171875]</id>
<coord x="57.49129867553711" y="-20.29949951171875"/>
</location>
<capacity-dimensions>
<dimension index="0">100</dimension>
</capacity-dimensions>
<duration>10.0</duration>
<timeWindows>
<timeWindow>
<start>0.0</start>
<end>1.7976931348623157E308</end>
</timeWindow>
</timeWindows>
</service>
<service id="2 " type="service">
<location>
<id>[x=57.49300003051758][y=-20.298799514770508]</id>
<coord x="57.49300003051758" y="-20.298799514770508"/>
</location>
<capacity-dimensions>
<dimension index="0">20</dimension>
</capacity-dimensions>
<duration>10.0</duration>
<timeWindows>
<timeWindow>
<start>0.0</start>
<end>1.7976931348623157E308</end>
</timeWindow>
</timeWindows>
</service>
<service id="1 " type="service">
<location>
<id>[x=57.49380111694336][y=-20.29840087890625]</id>
<coord x="57.49380111694336" y="-20.29840087890625"/>
</location>
<capacity-dimensions>
<dimension index="0">80</dimension>
</capacity-dimensions>
<duration>10.0</duration>
<timeWindows>
<timeWindow>
<start>0.0</start>
<end>1.7976931348623157E308</end>
</timeWindow>
</timeWindows>
</service>
<service id="3 " type="service">
<location>
<id>[x=57.49290084838867][y=-20.298200607299805]</id>
<coord x="57.49290084838867" y="-20.298200607299805"/>
</location>
<capacity-dimensions>
<dimension index="0">80</dimension>
</capacity-dimensions>
<duration>10.0</duration>
<timeWindows>
<timeWindow>
<start>0.0</start>
<end>1.7976931348623157E308</end>
</timeWindow>
</timeWindows>
</service>
<service id="4 Jean Nicolas Yung" type="service">
<location>
<id>[x=57.49209976196289][y=-20.298599243164062]</id>
<coord x="57.49209976196289" y="-20.298599243164062"/>
</location>
<capacity-dimensions>
<dimension index="0">100</dimension>
</capacity-dimensions>
<duration>10.0</duration>
<timeWindows>
<timeWindow>
<start>0.0</start>
<end>1.7976931348623157E308</end>
</timeWindow>
</timeWindows>
</service>
</services>
<solutions>
<solution>
<cost>505.9214355631349</cost>
<routes>
<route>
<driverId>noDriver</driverId>
<vehicleId>1_1</vehicleId>
<start>0.0</start>
<act type="service">
<serviceId>5 </serviceId>
<arrTime>109.9819741964403</arrTime>
<endTime>119.9819741964403</endTime>
</act>
<end>229.9639483928806</end>
</route>
<route>
<driverId>noDriver</driverId>
<vehicleId>3_1</vehicleId>
<start>0.0</start>
<act type="service">
<serviceId>2 </serviceId>
<arrTime>109.98268205388193</arrTime>
<endTime>119.98268205388193</endTime>
</act>
<act type="service">
<serviceId>1 </serviceId>
<arrTime>119.98357684436793</arrTime>
<endTime>129.98357684436792</endTime>
</act>
<act type="service">
<serviceId>3 </serviceId>
<arrTime>129.98449911991617</arrTime>
<endTime>139.98449911991617</endTime>
</act>
<act type="service">
<serviceId>4 Jean Nicolas Yung</serviceId>
<arrTime>139.98539391040217</arrTime>
<endTime>149.98539391040217</endTime>
</act>
<end>259.9672978232725</end>
</route>
</routes>
</solution>
<solution>
<cost>505.9208295302417</cost>
<routes>
<route>
<driverId>noDriver</driverId>
<vehicleId>1_1</vehicleId>
<start>0.0</start>
<act type="service">
<serviceId>5 </serviceId>
<arrTime>109.9819741964403</arrTime>
<endTime>119.9819741964403</endTime>
</act>
<end>229.9639483928806</end>
</route>
<route>
<driverId>noDriver</driverId>
<vehicleId>3_1</vehicleId>
<start>0.0</start>
<act type="service">
<serviceId>4 Jean Nicolas Yung</serviceId>
<arrTime>109.98190391287031</arrTime>
<endTime>119.98190391287031</endTime>
</act>
<act type="service">
<serviceId>2 </serviceId>
<arrTime>119.98282618841856</arrTime>
<endTime>129.98282618841856</endTime>
</act>
<act type="service">
<serviceId>1 </serviceId>
<arrTime>129.98372097890456</arrTime>
<endTime>139.98372097890456</endTime>
</act>
<act type="service">
<serviceId>3 </serviceId>
<arrTime>139.9846432544528</arrTime>
<endTime>149.9846432544528</endTime>
</act>
<end>259.9668316441239</end>
</route>
</routes>
</solution>
</solutions>
</problem>
答案 0 :(得分：0)
你的xpath错了。它应该是
//solution[cost/text()='505.9208295302417']/routes/route
它为您提供了应该能够迭代的所有route元素的列表。
你的元素包含一个solutions元素，它根本没有在xml中出现。此外，如果要检查包含的数据，还需要包含text()。
为了进一步参考，最好使用online xpath tester来确保检索所需的数据。