java 子串？

Question

我有以下代码:

String table;
private DB.ConnectMAS mas=new DB.ConnectMAS();
java.sql.ResultSet rs1 = mas.DBquery("select * from products");


try{ 
String duedate = "";
while(rs1.next()){
    String duedate = rs1.getString("duedate");
table+="<tr><td>"+duedate.substring(1,10)+"</td></tr>"; 
} catch(java.sql.SQLException e){}

它在 duedate.substring(1,10) 上抛出错误

我试图理解为什么这是不正确的？提前致谢

错误:

SEVERE: Servlet.service() for servlet POList threw exception
java.lang.NullPointerException
    at POList.getDetails(POList.java:126)
    at POList.getTable(POList.java:165)
    at POList.processRequest(POList.java:55)
    at POList.doGet(POList.java:177)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:690)
    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.Http11AprProcessor.process(Http11AprProcessor.java:834)
    at org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:640)
    at org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1286)
    at java.lang.Thread.run(Unknown Source)

Answer 1

就像 @danben 所说，您需要发布错误才能获得有意义的帮助。不过，看看您的代码，我想错误发生时的 duedate 是 null。您可能需要将实际使用duedate的代码移动到while循环中。