Efficiently determine updated cells after workbook evaluation - apache-poi

Question

Is there an efficient way to determine which cells have changed after a call to FormulaEvaluator::evaluateAll?
I can see there is an interface IEvaluationListener that could be useful, but it's not available externally (seems to be used for testing only).

Answer 1

I looked for how evaluateAll works and found BaseFormulaEvaluator.evaluateAllFormulaCells.
This code has access to all cells which are evaluated. And it simply can be used by copy/paste. Since it is the same as apache poi uses while evaluateAll it should be as performant as evaluateAll.
Before evaluation you can get the cell's old value and after the evaluation you can get the cell's new value. So you can determine if the evaluation has changed something. Of course those changings will influence the performance but there is no way to avoid this in my opinion.
import org.apache.poi.ss.usermodel.*;
import java.io.InputStream;
import java.io.FileInputStream;
class ExcelEvaluateAllFormulas {
public static void evaluateAllFormulaCells(Workbook wb) {
FormulaEvaluator evaluator = wb.getCreationHelper().createFormulaEvaluator();
evaluateAllFormulaCells(wb, evaluator);
}
protected static void evaluateAllFormulaCells(Workbook wb, FormulaEvaluator evaluator) {
for(int i=0; i<wb.getNumberOfSheets(); i++) {
Sheet sheet = wb.getSheetAt(i);
for(Row r : sheet) {
for (Cell c : r) {
if (c.getCellTypeEnum() == CellType.FORMULA) {
CellType celltype = c.getCachedFormulaResultTypeEnum();
Object oldvalue = null;
if (celltype == CellType.NUMERIC) {
oldvalue = c.getNumericCellValue();
} else if (celltype == CellType.BOOLEAN) {
oldvalue = c.getBooleanCellValue();
} else if (celltype == CellType.STRING) {
oldvalue = c.getStringCellValue();
} else if (celltype == CellType.ERROR) {
oldvalue = "Err: " + c.getErrorCellValue();
}
if (oldvalue == null) oldvalue = "no value";
System.out.print(c.getSheet().getSheetName() + "!" + c.getAddress() + ":old value:" + oldvalue);
celltype = evaluator.evaluateFormulaCellEnum(c);
Object newvalue = null;
if (celltype == CellType.NUMERIC) {
newvalue = c.getNumericCellValue();
} else if (celltype == CellType.BOOLEAN) {
newvalue = c.getBooleanCellValue();
} else if (celltype == CellType.STRING) {
newvalue = c.getStringCellValue();
} else if (celltype == CellType.ERROR) {
newvalue = "Err: " + c.getErrorCellValue();
}
if (newvalue == null) newvalue = "no value";
System.out.println("->new value:" + newvalue);
if (oldvalue.equals(newvalue)) {
System.out.println("Value has not changed.");
} else {
System.out.println("Value has changed.");
}
}
}
}
}
}
public static void main(String[] args) throws Exception{
InputStream inp = new FileInputStream("ExcelWithFormulas.xlsx");
Workbook workbook = WorkbookFactory.create(inp);
workbook.getSheetAt(0).getRow(1).getCell(0).setCellValue(0.5);
evaluateAllFormulaCells(workbook);
workbook.close();
}
}
This example expects that the value of A2 in the first sheet is used as reference in some formulas. So changing this value leads to new values in formula cells while evaluation.