java substring?

Question

I have the following code:

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){}

It is throwing an error on duedate.substring(1,10)

I'm trying to understand why this is incorrect? thanks in advance

error:

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

Like @danben says, you need to post the error to get meaningful help. Looking at your code, though, I'd imagine that duedate at the time of the error is null . You probably need to move the code that's actually using duedate inside your while loop.

Answer 2

First of all, I suppose rs should be rs1 on line 10 of your code sample. Even so, I guess that by that time rs1 has no more lines left, so getString returns null , which results in a NullPointerException on the next line.

Even if that were not the case, the string returned may not be 10 characters long, in which case substring(1,10) fails with an ArrayIndexOutOfBoundsException.

Answer 3

You are declaring String duedate inside your While Loop. So once rs1.next() returns false, it breaks from the While Loop and duedate goes away (out of scope of While Loop). No big deal since you declare it again outside the scope of the While Loop. However, by then, rs1 contains no more entries so rs1.getString("duedate") returns null.

You need to either declare String duedate outside before the scope of the While Loop OR, like HankGay mentioned, move the table+= into the While Loop.

Answer 4

In case of a NullPointerException - duedate is null , which is equal to a NULL value in the record

In case of a IndexOutOfBoundsException - duedate is less then 10 characters long, so the value in the database is either incorrect (if you expect entries with more than 10 chars) or even empty.

Anyway - expect, that duedate may be null , empty or too short and just add some code behind String duedate = rs.getString("duedate"); to handle those cases.

Answer 5

I see the potential for two possible issues, the first is a NullPointerException in

duedate.substring(1,10)

because you don't really know if

String duedate = rs1.getString("duedate");

was set to a value or null. Perhaps there is no returned String in the rs1.getString(...) call.

The other potential issue is that you never check that duedate is more than 10 characters long. If you ask for a substring, then it should be within the returned String. For example, I would expect the code:

String abc = "abc";
abc.substring(4, 24);

to throw an exception, probably an ArrayOutOfBoundsException, but as long as it threw some exception, it would be proper.

Answer 6

The first String duedate is superfluous. The variable is redeclared inside the loop.

But the part causing the exception is likely that you don't check for duedate coming back as null. Try something like:

try{  
while(rs1.next()){ 
String duedate = rs1.getString("duedate"); 
if (duedate==null && duedate.length()>=10)
  table+="<tr><td>Date missing or invalid</td></tr>"; // or whatever text is appropriate
else
  table+="<tr><td>"+duedate.substring(1,10)+"</td></tr>";  
} catch(java.sql.SQLException e){}

I also wonder why you extracting positions 1 - 9 of duedate. Do you realize that substring considers the first position to be zero, and that the second parameter is the first character to NOT include? If duedate is, say, "2010-07-05", this will give "010-07-05". If you're trying to get the first 10 characters, you want substring(0,10). Of course I don't know what's really in your duedate field.