如何在Java中使用Apache POI向Excel插入空日期值

// 假设 studentSheet 和 listOfStudent 已经初始化
// Row studentRow = studentSheet.createRow(0); // 标题行已创建

int studentCount = 1;

for(Student student : listOfStudent) {
    Row studentRow = studentSheet.createRow(studentCount);
    studentRow.createCell(0).setCellValue(student.getId());
    studentRow.createCell(1).setCellValue(student.getName());
    // 当 student.getAdmissionDate() 返回 null 时，这里会抛出 NullPointerException
    studentRow.createCell(2).setCellValue(student.getAdmissionDate());
    studentCount++;
}

import org.apache.poi.ss.usermodel.Row;
import org.apache.poi.ss.usermodel.Sheet;
import org.apache.poi.ss.usermodel.Workbook;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;

import java.io.FileOutputStream;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;

// 示例 Student 类
class Student {
    private int id;
    private String name;
    private Date admissionDate;

    public Student(int id, String name, Date admissionDate) {
        this.id = id;
        this.name = name;
        this.admissionDate = admissionDate;
    }

    public int getId() { return id; }
    public String getName() { return name; }
    public Date getAdmissionDate() { return admissionDate; }
}

public class ExcelDateNullHandler {

    public static void generateExcelFile(List<Student> listOfStudent, String filePath) {
        Workbook workbook = new XSSFWorkbook();
        Sheet studentSheet = workbook.createSheet("Students");

        // 创建标题行
        Row headerRow = studentSheet.createRow(0);
        headerRow.createCell(0).setCellValue("Id");
        headerRow.createCell(1).setCellValue("Name");
        headerRow.createCell(2).setCellValue("Admission_Date");

        int studentCount = 1;

        for(Student student : listOfStudent) {
            Row studentRow = studentSheet.createRow(studentCount);
            studentRow.createCell(0).setCellValue(student.getId());
            studentRow.createCell(1).setCellValue(student.getName());

            // 关键：在设置日期值之前进行 null 检查
            if (student.getAdmissionDate() != null) {
                studentRow.createCell(2).setCellValue(student.getAdmissionDate());
            }
            // 如果 student.getAdmissionDate() 为 null，则不对 Cell(2) 调用 setCellValue，
            // 该单元格将保持为空白状态，这在 Excel 中等同于“空”或“无值”
            studentCount++;
        }

        // 写入文件
        try (FileOutputStream fileOut = new FileOutputStream(filePath)) {
            workbook.write(fileOut);
            System.out.println("Excel file generated successfully at: " + filePath);
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                workbook.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        List<Student> students = new ArrayList<>();
        students.add(new Student(1, "Alice", new Date())); // 有日期
        students.add(new Student(2, "Bob", null)); // 日期为空
        students.add(new Student(3, "Charlie", new Date(System.currentTimeMillis() - 86400000L * 30))); // 30天前

        generateExcelFile(students, "students_data.xlsx");
    }
}