设计Appliances类，包含品牌brand和电器的价格price。定义getter/setter方法，brand要求品牌至少4位，否则抛出异常，价格price要求价格必须大于等于0, 否则抛出异常，定义测试类Test, 利用File类建立goodsInfo.xls文件，利用jxl.jar包将商品信息存入goodsInfo.xls文件，实现数据的增删改查。

以下是一个完整的 Java 实现，并实现了数据的增删改查：

import java.io.File;
import java.util.ArrayList;
import java.util.List;
import jxl.Cell;
import jxl.Workbook;
import jxl.write.Label;
import jxl.write.WritableSheet;
import jxl.write.WritableWorkbook;
import jxl.write.WriteException;

public class Appliances {
    private String brand;
    private double price;

    public String getBrand() {
        return brand;
    }

    public void setBrand(String brand) throws Exception {
        if (brand.length() < 4) {
            throw new Exception("Brand must be at least 4 characters.");
        }
        this.brand = brand;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) throws Exception {
        if (price < 0) {
            throw new Exception("Price cannot be negative.");
        }
        this.price = price;
    }

    public static void main(String[] args) {
        List<Appliances> goods = new ArrayList<>();
        Appliances a1 = new Appliances();
        try {
            a1.setBrand("Brand1");
            a1.setPrice(100.0);
        } catch (Exception e) {
            e.printStackTrace();
        }
        goods.add(a1);

        Appliances a2 = new Appliances();
        try {
            a2.setBrand("Brand2");
            a2.setPrice(200.0);
        } catch (Exception e) {
            e.printStackTrace();
        }
        goods.add(a2);

        File file = new File("goodsInfo.xls");
        try {
            WritableWorkbook workbook = Workbook.createWorkbook(file);
            WritableSheet sheet = workbook.createSheet("Goods", 0);
            sheet.addCell(new Label(0, 0, "Brand"));
            sheet.addCell(new Label(1, 0, "Price"));
            int row = 1;
            for (Appliances a : goods) {
                sheet.addCell(new Label(0, row, a.getBrand()));
                sheet.addCell(new Label(1, row, String.valueOf(a.getPrice())));
                row++;
            }
            workbook.write();
            workbook.close();
        } catch (Exception e) {
            e.printStackTrace();
        }

        // Read and modify data using jxl.jar
        try {
            Workbook workbook = Workbook.getWorkbook(file);
            WritableWorkbook writableWorkbook = Workbook.createWorkbook(file, workbook);
            WritableSheet sheet = writableWorkbook.getSheet(0);

            // Add a new appliance
            Appliances a3 = new Appliances();
            a3.setBrand("Brand3");
            a3.setPrice(300.0);
            goods.add(a3);
            int row = sheet.getRows();
            sheet.addCell(new Label(0, row, a3.getBrand()));
            sheet.addCell(new Label(1, row, String.valueOf(a3.getPrice())));

            // Modify an existing appliance
            for (int i = 1; i < sheet.getRows(); i++) {
                Cell brandCell = sheet.getCell(0, i);
                if (brandCell.getContents().equals("Brand2")) {
                    Cell priceCell = sheet.getCell(1, i);
                    double price = Double.parseDouble(priceCell.getContents());
                    price *= 1.1;
                    sheet.addCell(new Label(1, i, String.valueOf(price)));
                    a2.setPrice(price);
                    break;
                }
            }

            // Delete an appliance
            for (int i = 1; i < sheet.getRows(); i++) {
                Cell brandCell = sheet.getCell(0, i);
                if (brandCell.getContents().equals("Brand1")) {
                    sheet.removeRow(i);
                    goods.remove(a1);
                    break;
                }
            }

            writableWorkbook.write();
            writableWorkbook.close();
        } catch (Exception e) {
            e.printStackTrace();
        }

        // Print all appliances
        for (Appliances a : goods) {
            System.out.println(a.getBrand() + ": " + a.getPrice());
        }
    }
}

这个实现在 `main` 方法中，首先创建了两个 `Appliances` 对象，并将它们添加到一个 `ArrayList` 中，然后使用 `jxl.jar` 库将商品信息写入 Excel 文件 `goodsInfo.xls` 中。接着，使用 `jxl.jar` 库读取并修改了文件中的数据，包括增加一个新的电器、修改一个现有的电器和删除一个电器。最后，打印了所有的电器。