用java语言，写一段easypoi导出为excel的示例，需要把a表的数据导出，a表有b字段，c字段， b字段叫小类，c字段叫部门，导出类为自定义类， c部门里含有多个b小类，表头的小类名称随着b字段的个数数量增多

Sure, I can assist you with an example of exporting data from table A to an Excel file using EasyPoi library in Java. Here's the code snippet:

// define a custom class for exporting data
public class CustomExportClass {
    @Excel(name = "Department")
    private String department;

    // use List to store multiple "b" field values
    @Excel(name = "Small Class")
    private List<String> smallClassList;

    // getters and setters
}

// use EasyPoi library to export data to Excel file
public void exportToExcel() {
    // get data from table A and store it in a List
    List<A> dataList = getDataFromTableA();

    // define a List to store CustomExportClass objects
    List<CustomExportClass> exportDataList = new ArrayList<>();

    // iterate through each A object and create a CustomExportClass object
    for (A a : dataList) {
        CustomExportClass exportObj = new CustomExportClass();
        exportObj.setDepartment(a.getC());

        // split the "b" field into multiple values and add them to the List
        List<String> smallClassList = Arrays.asList(a.getB().split(","));
        exportObj.setSmallClassList(smallClassList);

        exportDataList.add(exportObj);
    }

    // define column headers for exporting
    String[] headers = {"Department", "Small Class"};

    // use EasyPoi to export data to Excel file
    ExcelUtil.exportExcel(new ExportParams(), CustomExportClass.class, exportDataList, headers, response);
}

Please note that you need to customize this code according to your specific needs and field names in table A. Also, make sure to add EasyPoi library to your project's dependencies.