Java遍历Employees对象数组写入Excel报ClassCastException异常

需要实现功能：定义Employees类，包含firstName、lastName、city三个属性，实例化多个该类对象存入数组后，遍历数组提取每个对象的三个属性值，写入Excel文件。
代码运行到第二个for循环块时触发中断，抛出异常：

Exception in thread "main" java.lang.ClassCastException: class http.Employees cannot be cast to class [Ljava.lang.Object; (http.Employees is in unnamed module of loader 'app'; [Ljava.lang.Object; is in module java.base of loader 'bootstrap') at http.App.main(App.java:64).

原始代码如下：

class Employees {
    String firstName;
    String lastName;
    String city;

    public Employees(String firstName, String lastName, String city) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.city = city;
    }
}

public class App {
    public static void main(String[] args) throws Exception {
        String excelPath = "C:/Work/01_TSM/java/test.xlsx";

        FileInputStream inputStream = new FileInputStream(new File(excelPath));

//      create workbook object
        XSSFWorkbook workbook = new XSSFWorkbook(inputStream);

//      create spreadsheet object
        XSSFSheet spreadsheet = workbook.getSheetAt(1);

//       create row object
        XSSFRow row;

//       This data needs to be written in (Object[])
        HashMap<Integer, Object> employeeData = new HashMap<Integer, Object>();

        Employees employee1 = new Employees("John", "Smith", "CT");
        Employees employee2 = new Employees("Jane", "Doe", "O");
        Employees[] employees = { employee1, employee2 };

//       Insert data
        for (int i = 0; i < employees.length; i++) {
            employeeData.put(i, employees[i]);
        }
        ;

        Set<Integer> keyid = employeeData.keySet();

        int rowid = 1;

//       writing the data into the sheets
        for (Integer key : keyid) {
            row = spreadsheet.createRow(rowid++);

            // here the code breaks
            Object[] objectArr = (Object[]) employeeData.get(key);
            int cellid = 0;
            
            
            for (Object obj : objectArr) {

                Cell cell = row.createCell(cellid++);
                cell.setCellValue((String) obj);
            }
        }

//       writing the workbook into the file
        FileOutputStream out = new FileOutputStream(new File("C:/Work/01_TSM/java/test.xlsx"));
        workbook.write(out);
        out.close();
    }
}

• 核心问题是类型不匹配：你往employeeData中存入的value是单个Employees类实例，不是Object[]数组类型，执行强制转换(Object[]) employeeData.get(key)时必然抛出类型转换异常。Java不会自动把自定义类的实例拆成由属性组成的数组，必须手动提取属性值。
• 逻辑冗余：这里完全不需要使用HashMap中转数据，你已经定义好了Employees类型的数组，直接遍历数组就能拿到所有员工数据，额外加一层Map只会增加出错概率。
• 资源泄漏风险：原始代码中FileInputStream没有关闭，操作完Excel后可能残留文件锁。

import org.apache.poi.ss.usermodel.Cell;
import org.apache.poi.xssf.usermodel.XSSFRow;
import org.apache.poi.xssf.usermodel.XSSFSheet;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;

class Employees {
    String firstName;
    String lastName;
    String city;

    public Employees(String firstName, String lastName, String city) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.city = city;
    }
}

public class App {
    public static void main(String[] args) throws Exception {
        String excelPath = "C:/Work/01_TSM/java/test.xlsx";
        
        // 用try-with-resources自动关闭流资源，避免泄漏
        try (FileInputStream inputStream = new FileInputStream(new File(excelPath));
             XSSFWorkbook workbook = new XSSFWorkbook(inputStream)) {

            XSSFSheet spreadsheet = workbook.getSheetAt(1);
            int rowid = 1;

            // 初始化员工数据
            Employees[] employees = {
                    new Employees("John", "Smith", "CT"),
                    new Employees("Jane", "Doe", "O")
            };

            // 直接遍历员工数组写入行
            for (Employees emp : employees) {
                XSSFRow row = spreadsheet.createRow(rowid++);
                int cellid = 0;
                // 逐个写入三个属性到对应单元格
                row.createCell(cellid++).setCellValue(emp.firstName);
                row.createCell(cellid++).setCellValue(emp.lastName);
                row.createCell(cellid++).setCellValue(emp.city);
            }

            // 写入文件并自动关闭输出流
            try (FileOutputStream out = new FileOutputStream(new File(excelPath))) {
                workbook.write(out);
            }
        }
    }
}

如果你一定要保留HashMap的写法，需要在往Map存数据时，就把每个员工的属性封装成Object数组，后续强转才不会报错，示例如下：

HashMap<Integer, Object[]> employeeData = new HashMap<>();
for (int i = 0; i < employees.length; i++) {
    Employees emp = employees[i];
    // 存的时候就组装成对象数组
    employeeData.put(i, new Object[]{emp.firstName, emp.lastName, emp.city});
}

这种写法没有实际价值，仅作原理演示，不推荐使用。

内容的提问来源于stack exchange，提问作者marincean adrian