When working with generated spreadsheets, you may need to merge neighboring cells that hold the same value to improve clarity. Apache POI provides a straightforwrad way to accomplish this in Java. Below is a complete example demonstrating how to create an Excel file, populate it with data, and dynamically merge cells in a given column when consecutive rows share identical content.

Dependencies

Add the following Maven coordinates to your project:

<dependency>
    <groupId>org.apache.poi</groupId>
    <artifactId>poi-ooxml</artifactId>
    <version>5.2.5</version>
</dependency>

Implementation

Start by preparing sample data and a method to apply merges based on a specific column.

import org.apache.poi.ss.usermodel.*;
import org.apache.poi.ss.util.CellRangeAddress;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;

import java.io.FileOutputStream;
import java.io.IOException;

public class ExcelMergeExample {

    public static void main(String[] args) throws IOException {
        String[] header = {"Category"};
        String[] items = {"Fruit", "Fruit", "Vegetable", "Vegetable", "Vegetable", "Dairy"};

        try (Workbook wb = new XSSFWorkbook()) {
            Sheet sheet = wb.createSheet("Products");

            Row headerRow = sheet.createRow(0);
            headerRow.createCell(0).setCellValue(header[0]);

            for (int i = 0; i < items.length; i++) {
                Row row = sheet.createRow(i + 1);
                row.createCell(0).setCellValue(items[i]);
            }

            mergeDuplicateCellsInColumn(sheet, 0, 1);

            try (FileOutputStream fos = new FileOutputStream("merged_output.xlsx")) {
                wb.write(fos);
            }
        }
    }

    private static void mergeDuplicateCellsInColumn(Sheet sheet, int colIndex, int dataStartRow) {
        int lastRow = sheet.getLastRowNum();
        if (lastRow < dataStartRow) return;

        int mergeStart = dataStartRow;
        String currentValue = sheet.getRow(dataStartRow).getCell(colIndex).getStringCellValue();

        for (int r = dataStartRow + 1; r <= lastRow; r++) {
            Row row = sheet.getRow(r);
            if (row == null) continue;
            Cell cell = row.getCell(colIndex);
            if (cell == null) continue;
            String cellValue = cell.getStringCellValue();

            if (!cellValue.equals(currentValue)) {
                if (r - 1 > mergeStart) {
                    sheet.addMergedRegion(new CellRangeAddress(mergeStart, r - 1, colIndex, colIndex));
                }
                mergeStart = r;
                currentValue = cellValue;
            }
        }

        if (lastRow > mergeStart) {
            sheet.addMergedRegion(new CellRangeAddress(mergeStart, lastRow, colIndex, colIndex));
        }
    }
}

Running this code produces an merged_output.xlsx file where consecutive "Fruit", "Vegetable", and "Dairy" cels are merged, leaving the first cell of each group12