update docs about deferred SXSSF and limitations on what can be done in row generating function

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1891937 13f79535-47bb-0310-9956-ffa450edef68

poi-examples/src/main/java/org/apache/poi/examples/xssf/streaming/DeferredGeneration.java

@@ -18,6 +18,8 @@
 package org.apache.poi.examples.xssf.streaming;
 
 import org.apache.poi.ss.usermodel.Cell;
+import org.apache.poi.ss.usermodel.CellStyle;
+import org.apache.poi.ss.usermodel.HorizontalAlignment;
 import org.apache.poi.ss.usermodel.Row;
 import org.apache.poi.xssf.streaming.DeferredSXSSFSheet;
 import org.apache.poi.xssf.streaming.DeferredSXSSFWorkbook;
@@ -36,10 +38,15 @@ public class DeferredGeneration {
         try (DeferredSXSSFWorkbook wb = new DeferredSXSSFWorkbook()) {
             DeferredSXSSFSheet sheet1 = wb.createSheet("new sheet");
 
+            // cell styles should be created outside the row generator function
+            CellStyle cellStyle = wb.createCellStyle();
+            cellStyle.setAlignment(HorizontalAlignment.CENTER);
+
             sheet1.setRowGenerator((ssxSheet) -> {
                 for (int i = 0; i < 10; i++) {
                     Row row = ssxSheet.createRow(i);
                     Cell cell = row.createCell(1);
+                    cell.setCellStyle(cellStyle);
                     cell.setCellValue("value " + i);
                 }
             });

poi-ooxml/src/main/java/org/apache/poi/xssf/streaming/DeferredSXSSFSheet.java

@@ -41,11 +41,22 @@ public class DeferredSXSSFSheet extends SXSSFSheet {
         super(workbook, xSheet, workbook.getRandomAccessWindowSize());
     }
 
+    /**
+     * Unsupported in DeferredSXSSFSheet
+     *
+     * @throws RuntimeException this is unsupported
+     */
     @Override
     public InputStream getWorksheetXMLInputStream() throws IOException {
         throw new RuntimeException("Not supported by DeferredSXSSFSheet");
     }
 
+    /**
+     * Add a function to generate rows for the sheet. This function should only create rows and cells.
+     * Any other settings like creating cell styles should be done in separate calls outside this function.
+     *
+     * @param rowGenerator {@link RowGeneratorFunction}
+     */
     public void setRowGenerator(RowGeneratorFunction rowGenerator) {
         this.rowGenerator = rowGenerator;
     }

poi-ooxml/src/main/java/org/apache/poi/xssf/streaming/RowGeneratorFunction.java

@@ -28,7 +28,9 @@ import org.apache.poi.util.Beta;
 public interface RowGeneratorFunction {
     
     /**
-     * Generate and add rows to the sheet
+     * Generate and add rows to the sheet. Note that anything that does not relate to creating rows and cells
+     * should not be done inside this function. It is best to create cell styles and sheet level settings in
+     * separate calls outside this function.
      *
      * @param sheet the sheet
      * @throws Exception the exception