我写了一个简单的XMLPhoneBook程序，它从用户（从输入控制台）获取输入，如姓名，姓氏和电话号码，并将其保存到名为＆＃34; PhoneBook.xml ＆的XML文件中＃34 ;.该程序还会检查现有的xml文件，并在文件已存在时更新该文件，或者创建新文件。我编写的第一个版本没有错误处理类的代码是成功的。
当我尝试使用错误处理类更新我的代码时，我开始从javax.xml.transform获取NullPointerException。我已经发布了下面两个版本的代码供您理解。
没有错误处理的主类XMLPhoneBook。
/**
*
* @author Sanjayan Ravi
*/
/**
*
* Class XMLPhoneBook is the Main Class that controls getContactInfo function and storeContactInfo.
*/
public class XMLPhoneBook
{
GetContactInformation gci;
StoreContactInformation sci;
XMLPhoneBook()
{
gci = new GetContactInformation();
}
/**
*
* getContactInfo function triggers the functions getFirstName, getLastName, getPhoneNumber of the GetContactInformation class.
*/
void getContactInfo(){
gci.getFirstName();
gci.getLastName();
gci.getPhoneNumber();
}
/**
*
* The variables FName, LName, PhoneNumber of the class GetContactInformation contains the user entered data, these values are passed to the class StoreContactInformation for creating/updating an XML file with user data.
*/
void storeContactInfo(){
sci = new StoreContactInformation(gci.FName,gci.LName,gci.PhoneNumber);
}
public static void main(String[] args)
{
XMLPhoneBook pb = new XMLPhoneBook();
pb.getContactInfo();
pb.storeContactInfo();
}
}
没有错误处理的类GetContactInformation。
/**
*
* Class GetContactInformation is responsible for getting inputs from the user such as First Name, Last Name and Phone Number.
* The variables FName(String), LName(String), PhoneNumber(long) are used for containing the user data during run time.
* The Variable reader(Scanner) is used for getting inputs from the user via the console.
*/
import java.util.Scanner;
public class GetContactInformation {
String FName;
String LName;
long PhoneNumber;
Scanner reader;
GetContactInformation(){
FName=null;
LName=null;
PhoneNumber=0;
}
/**
*
* The function getFirstName prompts the user for this first name and gets the inputs from the console.
* Then the value for the first name is passed to the function setFirstName.
*/
void getFirstName(){
System.out.println("Enter Your FirstName = ");
reader = new Scanner(System.in);
setFirstName(reader.nextLine());
}
/**
*
* The function getLastName prompts the user for this last name and gets the inputs from the console.
* Then the value for the last name is passed to the function setLastName.
*/
void getLastName(){
System.out.println("Enter Your LastName = ");
reader = new Scanner(System.in);
setLastName(reader.nextLine());
}
/**
*
* The function getPhoneNumber prompts the user for this phone number and gets the inputs from the console.
* Then the value for the phone number is passed to the function setPhoneNumber.
*/
void getPhoneNumber(){
try{
System.out.println("Enter Your PhoneNumber = ");
reader = new Scanner(System.in);
setPhoneNumber(Long.parseUnsignedLong(reader.nextLine()));
}catch(java.lang.NumberFormatException e){
}
}
/**
*
* The function setFirstName assigns the value it receives to the variable FName.
*/
String setFirstName(String FN){
return FName=FN;
}
/**
*
* The function setLastName assigns the value it receives to the variable LName.
*/
String setLastName(String LN){
return LName=LN;
}
/**
*
* The function setPhoneNumber assigns the value it receives to the variable PhoneNumber.
*/
long setPhoneNumber(long PN){
return PhoneNumber=PN;
}
}
没有错误处理的最后一个类StoreContactInformation。
/**
*
* The StoreContactInformation is responsible for storing the user data such as first name, last name and phone number into an XML file called PhoneBook.xml.
* The StoreContactInformation first checks if the XML file already exist then if the file exist it simply updates the file or else creates a new XML file with the provided user data.
*
*/
import java.io.File;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerException;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;
import javax.xml.transform.OutputKeys;
import org.w3c.dom.Attr;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.xml.sax.SAXException;
import java.io.IOException;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
public class StoreContactInformation {
File f;
String filePathString ;
DocumentBuilderFactory docFactory ;
DocumentBuilder docBuilder ;
Document doc;
TransformerFactory transformerFactory;
Transformer transformer;
DOMSource source;
StreamResult result;
String FirstName;
String LastName;
long PhoneNumber;
StoreContactInformation(String fn, String ln, long pn){
filePathString="PhoneBook.xml"; //File Path is the current working directory.
f = new File(filePathString);
FirstName=fn;
LastName=ln;
PhoneNumber=pn;
docFactory = DocumentBuilderFactory.newInstance();
try {
docBuilder = docFactory.newDocumentBuilder();
}
catch (ParserConfigurationException pce)
{
pce.printStackTrace();
}
transformerFactory = TransformerFactory.newInstance();
try
{
transformer = transformerFactory.newTransformer();
}
catch (TransformerException tfe)
{
tfe.printStackTrace();
}
// check if the XML file exist already !
if(f.exists() && !f.isDirectory()) {
//if exist
UpdateXMLFile();
WriteToXMLFile();
}else{
//if do not exist
CreateXMLFile();
WriteToXMLFile();
}
}
/**
*
* The function CreateXMLFile creates a new XML file if one doesn't exist already.
* The first set of data stored in the XML file is given the unique id="1".
* The root element of the XML file is "ContactInformation" which contains child nodes "Contact" with there unique ids.
* The element "contact further contains child nodes "FirstName","LastName" and "PhoneNumber".
*/
void CreateXMLFile(){
doc = docBuilder.newDocument();
Element rootElement = doc.createElement("ContactInformation");
doc.appendChild(rootElement);
Element contact = doc.createElement("Contact");
rootElement.appendChild(contact);
Attr attr = doc.createAttribute("id");
attr.setValue("1");
contact.setAttributeNode(attr);
Element firstname = doc.createElement("FirstName");
firstname.appendChild(doc.createTextNode(FirstName));
contact.appendChild(firstname);
Element lastname = doc.createElement("LastName");
lastname.appendChild(doc.createTextNode(LastName));
contact.appendChild(lastname);
Element PN = doc.createElement("PhoneNumber");
PN.appendChild(doc.createTextNode(java.lang.Long.toString(PhoneNumber)));
contact.appendChild(PN);
}
/**
*
* The function WriteToXMLFile is responsible for writing the data into output XML file "PhoneBook.xml".
* WriteToXMLFile also formats the data to be written into pretty print way with INDENT spaces and line breaks etc.
*/
void WriteToXMLFile(){
try
{
transformer.setOutputProperty(OutputKeys.INDENT, "yes");
transformer.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "2");
source = new DOMSource(doc);
result = new StreamResult(new File(filePathString));
transformer.transform(source, result);
System.out.println("Contact Saved !");
}
catch (TransformerException tfe)
{
tfe.printStackTrace();
}
}
/**
*
* The function UpdateXMLFile updates the existing XML file with the new user data.
* The new set of user data gets a unique id for the element "Contact" which is calculated and generated by UpdateXMLFile.
*
*/
void UpdateXMLFile(){
try
{
doc = docBuilder.parse(filePathString);
}catch(SAXException sae){
}catch (IOException ioe) {
ioe.printStackTrace();
}
int AttrValue=1;
Node ContactInfo = doc.getFirstChild();
NodeList NumberChildNodes = ContactInfo.getChildNodes();
for(int i=0;i<NumberChildNodes.getLength();i++)
{
Node n = NumberChildNodes.item(i);
if ("Contact".equals(n.getNodeName()))
{
AttrValue+=1;
}
}
Element contact = doc.createElement("Contact");
ContactInfo.appendChild(contact);
Attr attr = doc.createAttribute("id");
attr.setValue(Integer.toString(AttrValue));
contact.setAttributeNode(attr);
Element firstname = doc.createElement("FirstName");
firstname.appendChild(doc.createTextNode(FirstName));
contact.appendChild(firstname);
Element lastname = doc.createElement("LastName");
lastname.appendChild(doc.createTextNode(LastName));
contact.appendChild(lastname);
Element PN = doc.createElement("PhoneNumber");
PN.appendChild(doc.createTextNode(java.lang.Long.toString(PhoneNumber)));
contact.appendChild(PN);
}
}
以上代码是我的第一个版本的XMLPhoneBook，它运行良好：
C:\Users\xxxxxx\Documents\Java\xml>java XMLPhoneBook
Enter Your FirstName =
XXXXXX
Enter Your LastName =
YYYYYY
Enter Your PhoneNumber =
789634569863
Contact Saved !
当我尝试使用错误处理类更新上述代码时，问题就出现了，如下所示。
获取的主类XMLPhoneBook是例外：
/**
*
* @author Sanjayan Ravi
*/
package xmlphonebook;
/**
*
* Class XMLPhoneBook is the Main Class that controls getContactInfo function and storeContactInfo.
*/
public class XMLPhoneBook
{
GetContactInformation gci;
StoreContactInformation sci;
XMLPhoneBook()
{
gci = new GetContactInformation();
}
/**
*
* getContactInfo function triggers the functions getFirstName, getLastName, getPhoneNumber of the GetContactInformation class.
*/
void getContactInfo()throws Exception {
gci.getFirstName();
gci.getLastName();
gci.getPhoneNumber();
}
/**
*
* The variables FName, LName, PhoneNumber of the class GetContactInformation contains the user entered data, these values are passed to the class StoreContactInformation for creating/updating an XML file with user data.
*/
void storeContactInfo()throws Exception{
sci = new StoreContactInformation(gci.FName,gci.LName,gci.PhoneNumber);
}
public static void main(String[] args)
{
XMLPhoneBook pb = new XMLPhoneBook();
try{
pb.getContactInfo();
pb.storeContactInfo();
}catch(Exception e){
ErrorHandler er = new ErrorHandler(e);
}
}
}
类GetContactInformation：
package xmlphonebook;
/**
*
* Class GetContactInformation is responsible for getting inputs from the user such as First Name, Last Name and Phone Number.
* The variables FName(String), LName(String), PhoneNumber(long) are used for containing the user data during run time.
* The Variable reader(Scanner) is used for getting inputs from the user via the console.
*/
import java.util.Scanner;
public class GetContactInformation {
String FName;
String LName;
long PhoneNumber;
Scanner reader;
GetContactInformation(){
FName=null;
LName=null;
PhoneNumber=0;
}
/**
*
* The function getFirstName prompts the user for this first name and gets the inputs from the console.
* Then the value for the first name is passed to the function setFirstName.
*/
void getFirstName()throws Exception{
System.out.println("Enter Your FirstName = ");
reader = new Scanner(System.in);
setFirstName(reader.nextLine());
}
/**
*
* The function getLastName prompts the user for this last name and gets the inputs from the console.
* Then the value for the last name is passed to the function setLastName.
*/
void getLastName()throws Exception{
System.out.println("Enter Your LastName = ");
reader = new Scanner(System.in);
setLastName(reader.nextLine());
}
/**
*
* The function getPhoneNumber prompts the user for this phone number and gets the inputs from the console.
* Then the value for the phone number is passed to the function setPhoneNumber.
*/
void getPhoneNumber()throws Exception{
System.out.println("Enter Your PhoneNumber = ");
reader = new Scanner(System.in);
setPhoneNumber(Long.parseUnsignedLong(reader.nextLine()));
}
/**
*
* The function setFirstName assigns the value it receives to the variable FName.
*/
String setFirstName(String FN)throws Exception{
return FName=FN;
}
/**
*
* The function setLastName assigns the value it receives to the variable LName.
*/
String setLastName(String LN)throws Exception{
return LName=LN;
}
/**
*
* The function setPhoneNumber assigns the value it receives to the variable PhoneNumber.
*/
long setPhoneNumber(long PN)throws Exception{
return PhoneNumber=PN;
}
}
类StoreContactInformation：
package xmlphonebook;
/**
*
* The StoreContactInformation is responsible for storing the user data such as first name, last name and phone number into an XML file called PhoneBook.xml.
* The StoreContactInformation first checks if the XML file already exist then if the file exist it simply updates the file or else creates a new XML file with the provided user data.
*
*/
import java.io.File;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;
import javax.xml.transform.OutputKeys;
import javax.xml.transform.TransformerException;
import org.w3c.dom.Attr;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import java.util.Scanner;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
public class StoreContactInformation {
File f;
String filePathString ;
DocumentBuilderFactory docFactory ;
DocumentBuilder docBuilder ;
Document doc;
TransformerFactory transformerFactory;
Transformer transformer;
DOMSource source;
StreamResult result;
String FirstName;
String LastName;
long PhoneNumber;
int choice;
Scanner reader;
OutputKeys OutputKeys;
StoreContactInformation(String fn, String ln, long pn) throws Exception {
filePathString="PhoneBook.xml"; //File Path is the current working directory.
f = new File(filePathString);
FirstName=fn;
LastName=ln;
PhoneNumber=pn;
docFactory = DocumentBuilderFactory.newInstance();
docBuilder = docFactory.newDocumentBuilder();
transformerFactory = TransformerFactory.newInstance();
// check if the XML file exist already !
if(f.exists() && !f.isDirectory()) {
//if exist
UpdateXMLFile();
WriteToXMLFile();
}else{
//if do not exist
CreateXMLFile();
WriteToXMLFile();
}
}
/**
*
* The function CreateXMLFile creates a new XML file if one doesn't exist already.
* The first set of data stored in the XML file is given the unique id="1".
* The root element of the XML file is "ContactInformation" which contains child nodes "Contact" with there unique ids.
* The element "contact further contains child nodes "FirstName","LastName" and "PhoneNumber".
*/
void CreateXMLFile(){
doc = docBuilder.newDocument();
Element rootElement = doc.createElement("ContactInformation");
doc.appendChild(rootElement);
Element contact = doc.createElement("Contact");
rootElement.appendChild(contact);
Attr attr = doc.createAttribute("id");
attr.setValue("1");
contact.setAttributeNode(attr);
Element firstname = doc.createElement("FirstName");
firstname.appendChild(doc.createTextNode(FirstName));
contact.appendChild(firstname);
Element lastname = doc.createElement("LastName");
lastname.appendChild(doc.createTextNode(LastName));
contact.appendChild(lastname);
Element PN = doc.createElement("PhoneNumber");
PN.appendChild(doc.createTextNode(java.lang.Long.toString(PhoneNumber)));
contact.appendChild(PN);
}
/**
*
* The function WriteToXMLFile is responsible for writing the data into output XML file "PhoneBook.xml".
* WriteToXMLFile also formats the data to be written into pretty print way with INDENT spaces and line breaks etc.
*/
void WriteToXMLFile() throws Exception{
transformer.setOutputProperty(OutputKeys.INDENT, "yes");
transformer.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "2");
source = new DOMSource(doc);
result = new StreamResult(new File(filePathString));
transformer.transform(source, result);
System.out.println("Contact Saved !");
}
/**
*
* The function UpdateXMLFile updates the existing XML file with the new user data.
* The new set of user data gets a unique id for the element "Contact" which is calculated and generated by UpdateXMLFile.
*
*/
void UpdateXMLFile() throws Exception{
doc = docBuilder.parse(filePathString);
int AttrValue=1;
Node ContactInfo = doc.getFirstChild();
NodeList NumberChildNodes = ContactInfo.getChildNodes();
for(int i=0;i<NumberChildNodes.getLength();i++)
{
Node n = NumberChildNodes.item(i);
if ("Contact".equals(n.getNodeName()))
{
AttrValue+=1;
}
}
Element contact = doc.createElement("Contact");
ContactInfo.appendChild(contact);
Attr attr = doc.createAttribute("id");
attr.setValue(Integer.toString(AttrValue));
contact.setAttributeNode(attr);
Element firstname = doc.createElement("FirstName");
firstname.appendChild(doc.createTextNode(FirstName));
contact.appendChild(firstname);
Element lastname = doc.createElement("LastName");
lastname.appendChild(doc.createTextNode(LastName));
contact.appendChild(lastname);
Element PN = doc.createElement("PhoneNumber");
PN.appendChild(doc.createTextNode(java.lang.Long.toString(PhoneNumber)));
contact.appendChild(PN);
}
}
Class ErrorHandler：
package xmlphonebook;
import javax.xml.parsers.ParserConfigurationException;
import java.lang.NumberFormatException;
import javax.xml.transform.TransformerException;
import org.xml.sax.SAXException;
import java.io.IOException;
public class ErrorHandler extends Exception{
ErrorHandler(Exception e){
if(e.toString().contains("NumberFormatException"))
{ System.out.println("Enter information corractly");
}else if(e.toString().contains("ParserConfigurationException"))
{ System.out.println("Doc builder Error");
e.printStackTrace();
}else if(e.toString().contains("TransformerException"))
{ System.out.println("Transformation Error between source and result ");
e.printStackTrace();
}else if(e.toString().contains("SAXException"))
{ System.out.println("Doc Builder Error in UpdateXMLFile function");
e.printStackTrace();
}else if(e.toString().contains("IOException"))
{ System.out.println("IO Exception Error");
e.printStackTrace();
}else{
e.printStackTrace();
}
}
}
输出
Enter Your FirstName =
xxxxxxxx
Enter Your LastName =
yyyyyyyy
Enter Your PhoneNumber =
7896456123693
错误：
java.lang.NullPointerException
at xmlphonebook.StoreContactInformation.WriteToXMLFile(StoreContactInformation.java:105)
at xmlphonebook.StoreContactInformation.<init>(StoreContactInformation.java:65)
at xmlphonebook.XMLPhoneBook.storeContactInfo(XMLPhoneBook.java:35)
at xmlphonebook.XMLPhoneBook.main(XMLPhoneBook.java:43)
导致错误的函数：
void WriteToXMLFile() throws Exception{
transformer.setOutputProperty(OutputKeys.INDENT, "yes");
transformer.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "2");
source = new DOMSource(doc);
result = new StreamResult(new File(filePathString));
transformer.transform(source, result);
System.out.println("Contact Saved !");
}
请告诉我我在做什么错误。
答案 0 :(得分：0)
我在工作区中复制了所有课程，确实有NPE，但正如我在之前的评论中提到的那样，
transformer.setOutputProperty(OutputKeys.INDENT, "yes");
transformer是NULL，我通过调试检查了它并保留了一个断点并使用相同的解决方案来修复它，如我的评论所述。
这是我在构造函数
中解决它的方法 StoreContactInformation(String fn, String ln, long pn) throws Exception {
filePathString="PhoneBook.xml"; //File Path is the current working directory.
f = new File(filePathString);
FirstName=fn;
LastName=ln;
PhoneNumber=pn;
docFactory = DocumentBuilderFactory.newInstance();
docBuilder = docFactory.newDocumentBuilder();
transformerFactory = TransformerFactory.newInstance();
/******* added this line ******************/
transformer=transformerFactory.newTransformer();
/******* added this line ******************/
// check if the XML file exist already !
if(f.exists() && !f.isDirectory()) {
//if exist
UpdateXMLFile();
WriteToXMLFile();
}else{
//if do not exist
CreateXMLFile();
WriteToXMLFile();
}
}
得到了这个输出，
输入您的FirstName = xxxx
输入您的LastName = yyyy
输入您的PhoneNumber = 123456
联系已保存！