This example demonstrate how to use HSSFCellStyle and HSSFFont to format the cell style in Excel document. Using this class we can define cell border, foreground and background color. We can also define the font we used to display the cell value.

package org.kodejava.poi;

import org.apache.poi.hssf.usermodel.*;
import org.apache.poi.hssf.util.HSSFColor;
import org.apache.poi.ss.usermodel.BorderStyle;
import org.apache.poi.ss.usermodel.FillPatternType;

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

public class ExcelCellFormat {
    public static void main(String[] args) {
        // Create an instance of workbook and sheet
        try (HSSFWorkbook workbook = new HSSFWorkbook()) {
            HSSFSheet sheet = workbook.createSheet();

            // Create an instance of HSSFCellStyle which will be used to format the
            // cell. Here we define the cell top and bottom border, and we also
            // define the background color.
            HSSFCellStyle style = workbook.createCellStyle();
            style.setBorderTop(BorderStyle.DOUBLE);
            style.setBorderBottom(BorderStyle.THIN);
            style.setFillForegroundColor(
                    HSSFColor.HSSFColorPredefined.GREY_25_PERCENT.getIndex());
            style.setFillPattern(FillPatternType.SOLID_FOREGROUND);

            // We also define the font that we are going to use for displaying the
            // data of the cell. We set the font to ARIAL with 20pt in size and
            // make it BOLD and give blue as the color.
            HSSFFont font = workbook.createFont();
            font.setFontName(HSSFFont.FONT_ARIAL);
            font.setFontHeightInPoints((short) 20);
            font.setBold(true);
            font.setColor(HSSFColor.HSSFColorPredefined.BLUE.getIndex());
            style.setFont(font);

            // We create a simple cell, set its value and apply the cell style.
            HSSFRow row = sheet.createRow(1);
            HSSFCell cell = row.createCell(1);
            cell.setCellValue(new HSSFRichTextString("Hi there... It's me again!"));
            cell.setCellStyle(style);
            sheet.autoSizeColumn((short) 1);

            // Finally, we write out the workbook into an Excel file.
            try (FileOutputStream fos = new FileOutputStream("ExcelDemo.xls")) {
                workbook.write(fos);
            } catch (IOException e) {
                e.printStackTrace();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Maven Dependencies

<dependency>
    <groupId>org.apache.poi</groupId>
    <artifactId>poi</artifactId>
    <version>5.2.3</version>
</dependency>