hashtable插入vector异常

写了一个hashtable插入vector的方法..感觉和别人写的一样啊
可是却报错..我把代码发一下：
public Vector QueryVet(String Sql) {
Vector vet = null;
try {
stmt = dbcon.createStatement();
rs = stmt.executeQuery(Sql);
ResultSetMetaData meta = rs.getMetaData();
while (rs.next()) {
Hashtable hash = new Hashtable();
for (int i = 0; i < meta.getColumnCount(); i++) {
hash.put(meta.getColumnName(i + 1).toLowerCase(), rs.getString(i + 1) == null ? "" : rs .getString(i + 1));
}
vet.add(hash);//单步这边就错了
System.out.print(vet.capacity());//为了检验hash有没有插入成功，我加了这一行，发现不能向控制台输出，就是到上一步就错了
}
} catch (Exception e) {
System.out.print("VET异常！");
System.out.print(e.toString());
vet=null;
}
return vet;
}下面是异常的情况：
VET异常！java.lang.NullPointerException2008-11-11 22:05:15 org.apache.catalina.core.StandardWrapperValve invoke
严重: Servlet.service() for servlet jsp threw exception
java.lang.NullPointerException
at org.apache.jsp.list_jsp._jspService(list_jsp.java:87)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:98)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:328)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:315)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:265)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:269)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:210)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:174)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:108)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:151)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:870)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:665)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:528)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:81)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:685)
at java.lang.Thread.run(Unknown Source)请问各位能告诉我哪边错么？