reduce memory

poi-ooxml/src/main/java/org/apache/poi/xddf/usermodel/chart/XDDFChartData.java

@@ -142,7 +142,7 @@ public final void removeSeries(int n) {
     public abstract XDDFChartData.Series addSeries(XDDFDataSource<?> category,
             XDDFNumericalDataSource<? extends Number> values);
 
-    public abstract class Series {
+    public abstract static class Series {
         protected abstract CTSerTx getSeriesText();
 
         public abstract void setShowLeaderLines(boolean showLeaderLines);

poi-ooxml/src/test/java/org/apache/poi/xssf/eventusermodel/TestXSSFBReader.java

@@ -165,7 +165,7 @@ void testDate() throws Exception {
     }
 
 
-    private class TestSheetHandler implements XSSFSheetXMLHandler.SheetContentsHandler {
+    private static class TestSheetHandler implements XSSFSheetXMLHandler.SheetContentsHandler {
         private final StringBuilder sb = new StringBuilder();
 
         void startSheet(String sheetName) {

poi/src/main/java/org/apache/poi/hssf/usermodel/HSSFChart.java

@@ -1120,7 +1120,7 @@ private Integer setVerticalCellRange(LinkedDataRecord linkedDataRecord,
                 }
             }
 
-            linkedDataRecord.setFormulaOfLink(ptgList.toArray(new Ptg[0]));
+            linkedDataRecord.setFormulaOfLink(ptgList.toArray(Ptg.EMPTY_PTG_ARRAY));
 
             return rowCount * colCount;
         }