java.lang.IndexOutOfBoundsException: Index: 38, Size: 38

Question

I'm currently working on a project, and I got this error. I do not know why it stops at Index 38. The error is

org.apache.jasper.JasperException: javax.el.ELException: java.lang.IndexOutOfBoundsException: Index: 38, Size: 38
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:413)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:326)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:253)
javax.servlet.http.HttpServlet.service(HttpServlet.java:847)

The code that I had used to display was a while loop

var count = ${count};
 while (--count) {
      var val = ${valueList.get(count)}; //get value from her code
      var xValue = ${XValueList.get(count)};
      var yValue = ${YValueList.get(count)};
      //max = Math.max(max, val);
      //
      min = 0;
      var point = {
        x: xValue,
        y: yValue,
        value: val
      };
      points.push(point);
    }

   // var data = { max: max, min:min, data: points };
   var data = {data: points };
    return data;
  };

And I have 41 records in my database. Any help?

Answer 1

Arrays start counting at 0, but their size is given in number of fields. An array of size 38 actually has fields 0 through 37. Accessing field 38 throws an java.lang.IndexOutOfBoundsException.

Answer 2

You somehow missed the fact that JSP is a HTML/CSS/JS code generator and you expected that JSP EL expressions run "in sync" with JavaScript code embedded in JSP file. This is untrue. JSP/EL runs in webserver, produces HTML/CSS/JS output, basically as one large String which get sent from webserver to webbrowser, who in turn runs the JSP/EL-produced HTML/CSS/JS output.

An easy way to realize your mistake is doing a rightclick and View Source in webbrowser (on a JSP page containing tags and EL expressions which doesn't throw a server side exception like this, of course). You'll notice that it does actually not contain any single line of JSP/EL code.

Basically, the --count in your code snippet has only effect in JavaScript, not in JSP, because you basically printed the ${count} as a JavaScript variable var count during generating the HTML output. But the value of var count in JavaScript does in no way affect the value of ${count} which is used further down in ${valueList.get(count)}. The ${count} is still 38 there and has not become 37 or so.

---

That was the problem. Now we can advance to the solution. I won't post an answer to fix specifically your attempted solution, for the simple reason that this is from high level seen a wrong solution to the underlying problem you tried to solve: converting a Java model object to a JavaScript object.

The right solution to that is converting the Java model object to a string in JSON format in Java side and then letting JSP print it as if it's a JavaScript variable. Your Java model object has also another issue: you seem to have decoupled the X and Y values in two separate lists instead of using a single list with entities in turn having X and Y values.

First create a decent model object:

public class Point {
    private int x;
    private int y;
    // Add/generate constructor+getter+setter+equals+hashcode.
}

Then replace your XValueList and YValueList as below:

List<Point> points = new ArrayList<>();
points.add(new Point(1, 2));
points.add(new Point(3, 4));
points.add(new Point(5, 6));
// ...

Then use one of Java JSON APIs to convert this to a JSON string. I'll pick Gson in below example:

String pointsAsJson = new Gson().toJson(points);

Now let JSP print it as if it's a JS variable, the right way:

var data = {data: ${pointsAsJson} };

No need for a clumsy loop here massaging data forth and back.