fix for the Issue #186 -

src/main/java/com/monitorjbl/xlsx/sst/FileBackedList.java

@@ -29,17 +29,20 @@
  */
 public class FileBackedList implements AutoCloseable {
 
+  private static final String EMPTY_STRING = "";
   private final List<Long> pointers = new ArrayList<>();
   private final RandomAccessFile raf;
   private final FileChannel channel;
   private final Map<Integer, String> cache;
+  private final int cacheSize;
 
   private long filesize;
 
   public FileBackedList(File file, final int cacheSize) throws IOException {
     this.raf = new RandomAccessFile(file, "rw");
     this.channel = raf.getChannel();
     this.filesize = raf.length();
+    this.cacheSize = cacheSize;
     this.cache = new LinkedHashMap<Integer, String>(cacheSize, 0.75f, true) {
       public boolean removeEldestEntry(Map.Entry eldest) {
         return size() > cacheSize;
@@ -49,9 +52,10 @@ public boolean removeEldestEntry(Map.Entry eldest) {
 
   public void add(String str) {
     try {
-      if (str!=null && str.length()>0) {
+      if (cacheSize > 0 || (str!=null && str.length()>0)) {
         writeToFile(str);
       }
+
     } catch(IOException e) {
       throw new RuntimeException(e);
     }
@@ -73,6 +77,9 @@ public String getAt(int index) {
 
   private void writeToFile(String str) throws IOException {
     synchronized (channel) {
+      if (str == null) {
+        str = EMPTY_STRING;
+      }
       ByteBuffer bytes = ByteBuffer.wrap(str.getBytes(StandardCharsets.UTF_8));
       ByteBuffer length = ByteBuffer.allocate(4).putInt(bytes.array().length);
 

src/test/java/com/monitorjbl/xlsx/StreamingReaderTest.java

@@ -501,7 +501,7 @@ public void testSpecialStyles() throws Exception {
   @Test
   public void testBlankNumerics() throws Exception {
     File f = new File("src/test/resources/blank_cells.xlsx");
-    try(Workbook wb = StreamingReader.builder().open(f)) {
+    try(Workbook wb = StreamingReader.builder().sstCacheSize(2).open(f)) {
       Row row = wb.getSheetAt(0).iterator().next();
       assertThat(row.getCell(1).getStringCellValue(), equalTo(""));
       assertThat(row.getCell(1).getRichStringCellValue().getString(), equalTo(""));
@@ -510,10 +510,25 @@ public void testBlankNumerics() throws Exception {
     }
   }
 
+  @Test
+  public void testBlankCellWithSstCacheSize() throws Exception {
+    File f = new File("src/test/resources/blank_cell_to_test_sst_size.xlsx");
+    Map<Integer, List<Cell>> contents = new HashMap<>();
+    try(Workbook wb = StreamingReader.builder().sstCacheSize(8).open(f)) {
+      for(Row row : wb.getSheetAt(0)) {
+        contents.put(row.getRowNum(), new ArrayList<>());
+        for(Cell c : row) {
+            contents.get(row.getRowNum()).add(c);
+        }
+      }
+    }
+    assertThat(contents.get(1).get(2).getStringCellValue(), equalTo(""));
+  }
+
   @Test
   public void testFirstRowNumIs0() throws Exception {
     File f = new File("src/test/resources/data_types.xlsx");
-    try(Workbook wb = StreamingReader.builder().open(f)) {
+    try(Workbook wb = StreamingReader.builder().sstCacheSize(4).open(f)) {
       Row row = wb.getSheetAt(0).iterator().next();
       assertThat(row.getRowNum(), equalTo(0));
     }