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写Android时总是遇到空指针问题，主要的原因是因为好多时候类没有初始化的缘故。

public class XMLParserHandler extends DefaultHandler {

private Book book=new Book(); private ArrayList<Book> booklist=new ArrayList<Book>(); private String nodeName; private StringBuilder value; @Override public void startDocument() throws SAXException { // TODO Auto-generated method stub super.startDocument(); value=new StringBuilder(); } @Override public void endDocument() throws SAXException { // TODO Auto-generated method stub super.endDocument(); } @Override public void startElement(String uri, String localName, String qName, Attributes attributes) throws SAXException { // TODO Auto-generated method stub super.startElement(uri, localName, qName, attributes); nodeName=localName; value.setLength(0); } @Override public void endElement(String uri, String localName, String qName) throws SAXException { // TODO Auto-generated method stub super.endElement(uri, localName, qName); if(qName.equals("data")){ booklist.add(book); } } @Override public void characters(char[] ch, int start, int length) throws SAXException { // TODO Auto-generated method stub super.characters(ch, start, length); value.append(ch,start,length); Log.d("mainactivity",value.toString().trim()); if(nodeName.equals("id")){ book.setId(Integer.parseInt(value.toString().trim())); }else if(nodeName.equals("study_number")){ book.setStudy_number(Integer.parseInt(value.toString().trim())); }else if(nodeName.equals("name")){ book.setName(value.toString().trim()); }else if(nodeName.equals("sex")){ book.setSex(value.toString().trim()); }else if(nodeName.equals("major")){ book.setMajor(value.toString().trim()); }else if(nodeName.equals("enroll_year")){     book.setEnroll_year(Integer.parseInt(value.toString().trim())); }else if(nodeName.equals("class")){ book.setClassw(value.toString().trim()); }else if(nodeName.equals("telephone")){ book.setTelephone(value.toString().trim()); }else if(nodeName.equals("qq")){ book.setQq(value.toString().trim()); }else if(nodeName.equals("email")){ book.setEmail(value.toString().trim()); }else if(nodeName.equals("remark")){ book.setRemark(value.toString().trim()); } } public ArrayList<Book> getBooklist() { return booklist; } public void setBooklist(ArrayList<Book> booklist) { this.booklist = booklist; }

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