在Java中读写Excel文件，可以使用Apache POI库。以下是使用Apache POI读取和写入Excel文件的简单示例。

一、添加依赖（Maven）:

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

二、读取Excel文件:

import org.apache.poi.ss.usermodel.*;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;
 
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
 
public class ExcelReader {
    public static void main(String[] args) throws IOException {
        FileInputStream file = new FileInputStream(new File("example.xlsx"));
        Workbook workbook = new XSSFWorkbook(file);
        Sheet sheet = workbook.getSheetAt(0);
 
        for (Row row : sheet) {
            for (Cell cell : row) {
                
                System.out.print(cell.toString() + "\t");

                /*switch (cell.getCellType()) {
                    case STRING:
                        System.out.print(cell.getStringCellValue() + "\t");
                        break;
                    case NUMERIC:
                        System.out.print(cell.getNumericCellValue() + "\t");
                        break;
                    case BOOLEAN:
                        System.out.print(cell.getBooleanCellValue() + "\t");
                        break;
                    case FORMULA:
                        System.out.print(cell.getCellFormula() + "\t");
                        break;
                    default: break;
                }*/
            }
            System.out.println();
        }
        workbook.close();
    }
}

三、写入Excel文件:

import org.apache.poi.ss.usermodel.*;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;
 
import java.io.FileOutputStream;
import java.io.IOException;
 
public class ExcelWriter {
    public static void main(String[] args) throws IOException {
        Workbook workbook = new XSSFWorkbook();
        Sheet sheet = workbook.createSheet("Example Sheet");
 
        Row row = sheet.createRow(0);
        Cell cell = row.createCell(0);
        cell.setCellValue("Hello");
 
        row.createCell(1).setCellValue(123);
        row.createCell(2).setCellValue(true);
        row.createCell(3).setCellFormula("SUM(B1:C1)");
 
        FileOutputStream file = new FileOutputStream("example_output.xlsx");
        workbook.write(file);
        workbook.close();
        file.close();
    }
}