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.txt文件，利用BufferedWriter(缓冲流)+ FileWriter（字符流）将商品信息存入goodsInfo.txt文件，利用BufferedReader+FileReader读取文件内容，在屏幕显示。

下面是Appliances类的代码实现：

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("品牌至少4位！");
        }
        this.brand = brand;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) throws Exception {
        if (price < 0) {
            throw new Exception("价格必须大于等于0！");
        }
        this.price = price;
    }
}

下面是Test类的代码实现：

import java.io.*;

public class Test {
    public static void main(String[] args) {
        Appliances[] appliances = new Appliances[3];
        appliances[0] = new Appliances();
        appliances[1] = new Appliances();
        appliances[2] = new Appliances();

        try {
            appliances[0].setBrand("Haier");
            appliances[0].setPrice(2399.00);
            appliances[1].setBrand("Midea");
            appliances[1].setPrice(1999.00);
            appliances[2].setBrand("Gree");
            appliances[2].setPrice(2899.00);
        } catch (Exception e) {
            e.printStackTrace();
        }

        File file = new File("goodsInfo.txt");
        try {
            if (!file.exists()) {
                file.createNewFile();
            }
            BufferedWriter bw = new BufferedWriter(new FileWriter(file));
            for (Appliances appliance : appliances) {
                bw.write(appliance.getBrand() + "," + appliance.getPrice());
                bw.newLine();
            }
            bw.close();
        } catch (IOException e) {
            e.printStackTrace();
        }

        try {
            BufferedReader br = new BufferedReader(new FileReader(file));
            String line = null;
            while ((line = br.readLine()) != null) {
                System.out.println(line);
            }
            br.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

运行Test类后，可以在控制台上看到输出结果：

Haier,2399.0
Midea,1999.0
Gree,2899.0

同时，也会在项目目录下生成一个名为“goodsInfo.txt”的文件，其中存储了商品信息。