ROW Duplication

[KalemKutu]

Background:
1500 columns ( calendar of ca. 3 years = ~1500 days)
1000 rows

in Total ~1.5 Mio cells

ROW = 0
skip

ROW = 1
I am creating for row=1, 1500 columns, i.e. 1500 cells
Each cell is empty.
I am just changing the bg-color.
- Weekend cells are grey background
- Weekdays cells are white background.

So far so good! If I save the Excel file, I got what I need.

ROW = 2...1000 HELP
Now I wanna apply this for the next 1000 rows aswell.
But WHAT is the fastest way to do this?

I tried followinig copy-approach, but not successfully:

final CellCopyPolicy defaultCopyPolicy = new CellCopyPolicy();
defaultCopyPolicy.createBuilder();
        
defaultCopyPolicy.setCondenseRows(false);
defaultCopyPolicy.setCopyCellFormula(false); 
defaultCopyPolicy.setCopyCellStyle(true);
defaultCopyPolicy.setCopyCellValue(false); 
defaultCopyPolicy.setCopyHyperlink(false); 
defaultCopyPolicy.setCopyMergedRegions(false);
defaultCopyPolicy.setCopyRowHeight(false);
defaultCopyPolicy.setMergeHyperlink(false); 
        
for (int j = 2; j <=1000; j++) {            
         sheet.copyRows(1, 1, j, defaultCopyPolicy);                
}

The for-loop takes 45 seconds.
Afterwards it freezes during saving the file:

FileOutputStream out = new FileOutputStream(new File(targetFile));
			
workbook.write(out);
out.close();

WHAT is the fastest way to do this?

(Taskmanager shows 2GB)

[SurfMan]

Two observations:
1) You are using the builder wrong. It won't influence the outcome, but in the future it may affect any builder you create:

  CellCopyPolicy policy = new CellCopyPolicy().createBuilder()
                                                    .cellFormula(false)
                                                    .cellStyle(true)
                                                    .cellValue(false)
                                                    .condenseRows(false)
                                                    .copyHyperlink(false)
                                                    .mergedRegions(false)
                                                    .mergeHyperlink(false)
                                                    .rowHeight(false)
                                                    .build();

2) This smells like a OutOfMemoryError. Crank it up to the most RAM you can spare and try again.

Can you provide the full source, so I can try it myself?

[Tolls]

For POI (and Excel, to be honest) you want to keep the number of styles down. There is a limit.

Since all you are doing is colouring a column of cells, then select that column and apply the colour to all of them at once.
That way you only have 1500 things to do.
In fact, since you know weekends are two days next to each other, you can cut that down again...and finally, just select all 1500 columns at the start and apply white to them in one go.

[KalemKutu]

@Tolls

Thank you for the approach, this really speed up my code!