XSSFSheet: Sheet index is out of range
<p>I am tring to write the contents of xlsx file with <code>POI</code> but got an <code>IllegalArgumentException</code>.</p>

<p>Following is my code:</p>

<pre><code>public static void ConvertToCSVOrgtoOrg(String Scenario) throws Exception {

    String IntermediateXLSXPath=GetFileDetailsSIF(Scenario,"ExcelPath");
    String IntermediateCSVPath=GetFileDetailsSIF(Scenario,"CSVPathOrgtoOrg");

    File inputFile = new File(IntermediateXLSXPath);
    File outputFile = new File(IntermediateCSVPath);
    // For storing data into CSV files
    StringBuffer data = new StringBuffer();

    try {
        FileOutputStream fos = new FileOutputStream(outputFile);
        // Get the workbook object for XLSX file
        XSSFWorkbook wBook = new XSSFWorkbook(new FileInputStream(inputFile));
        XSSFFormulaEvaluator.evaluateAllFormulaCells(wBook);
        // Get first sheet from the workbook
        XSSFSheet sheet = wBook.getSheetAt(18);
        XSSFFormulaEvaluator.evaluateAllFormulaCells(wBook);

        Row row;
        Cell cell;
        // Iterate through each rows from first sheet
        Iterator&lt;Row&gt; rowIterator = sheet.iterator();

        while (rowIterator.hasNext()) {
            row = rowIterator.next();
            data.append("\n");

            // For each row, iterate through each columns
            Iterator&lt;Cell&gt; cellIterator = row.cellIterator();
            while (cellIterator.hasNext()) {
                cell = cellIterator.next();

                switch (cell.getCellType()) {
                    case Cell.CELL_TYPE_BOOLEAN:
                        data.append(cell.getBooleanCellValue());
                        break;

                    case Cell.CELL_TYPE_NUMERIC:
                        data.append(cell.getNumericCellValue());
                        break;

                    case Cell.CELL_TYPE_STRING:
                        data.append(cell.getStringCellValue());
                        break;

                    case Cell.CELL_TYPE_FORMULA:
                        data.append(cell.getStringCellValue());

                    case Cell.CELL_TYPE_BLANK:
                        break;
                    default:
                }
            }
        }

        fos.write(data.toString().getBytes());
        Thread.sleep(1000);
        fos.close();

    } catch (Exception ioe) {
        ioe.printStackTrace();
    }

}
</code></pre>

<p>The exception I am getting is:</p>

<blockquote>
<pre><code>java.lang.IllegalArgumentException: Sheet index (18) is out of range (0..17)
  at org.apache.poi.xssf.usermodel.XSSFWorkbook.validateSheetIndex(XSSFWorkbook.java:1205)
  at org.apache.poi.xssf.usermodel.XSSFWorkbook.getSheetAt(XSSFWorkbook.java:960)
  at BasicCoreFramework.ObjectUtilities.ConvertToCSVOrgtoOrg(ObjectUtilities.java:1625)
  at TestUtilities.FileManipulation.PostAggregationScenario1Functionality(FileManipulation.java:40)
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
  at java.lang.reflect.Method.invoke(Unknown Source)
  at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:85)
  at org.testng.internal.Invoker.invokeMethod(Invoker.java:639)
  at org.testng.internal.Invoker.invokeTestMethod(Invoker.java:816)
  at org.testng.internal.Invoker.invokeTestMethods(Invoker.java:1124)
  at org.testng.internal.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:125)
  at org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:108)
  at org.testng.TestRunner.privateRun(TestRunner.java:774)
  at org.testng.TestRunner.run(TestRunner.java:624)
  at org.testng.SuiteRunner.runTest(SuiteRunner.java:359)
  at org.testng.SuiteRunner.runSequentially(SuiteRunner.java:354)
  at org.testng.SuiteRunner.privateRun(SuiteRunner.java:312)
  at org.testng.SuiteRunner.run(SuiteRunner.java:261)
  at org.testng.SuiteRunnerWorker.runSuite(SuiteRunnerWorker.java:52)
  at org.testng.SuiteRunnerWorker.run(SuiteRunnerWorker.java:86)
  at org.testng.TestNG.runSuitesSequentially(TestNG.java:1215)
  at org.testng.TestNG.runSuitesLocally(TestNG.java:1140)
  at org.testng.TestNG.run(TestNG.java:1048)
  at org.testng.remote.AbstractRemoteTestNG.run(AbstractRemoteTestNG.java:126)
  at org.testng.remote.RemoteTestNG.initAndRun(RemoteTestNG.java:137)
  at org.testng.remote.RemoteTestNG.main(RemoteTestNG.java:58)
</code></pre>
</blockquote>

<p>I do have a 18th sheet. Any help would be greatly appreciated.</p>