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。

好的，我来解答这个问题。

首先，我们需要定义一个Appliances类，包含品牌brand和价格price两个属性，并实现它们的getter和setter方法。其中，setter方法需要在输入参数不合法时抛出异常。代码如下：

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

    public String getBrand() {
        return brand;
    }

    public void setBrand(String brand) throws IllegalArgumentException {
        if (brand.length() < 4) {
            throw new IllegalArgumentException("Invalid brand name!");
        }
        this.brand = brand;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) throws IllegalArgumentException {
        if (price < 0) {
            throw new IllegalArgumentException("Invalid price!");
        }
        this.price = price;
    }
}

接下来，我们需要编写一个测试类Test，利用File类建立goodsInfo.txt文件，并将商品信息存入文件中。代码如下：

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;

public class Test {
    public static void main(String[] args) {
        Appliances[] goods = new Appliances[3];
        goods[0] = new Appliances();
        goods[0].setBrand("Samsung");
        goods[0].setPrice(3999.99);
        goods[1] = new Appliances();
        goods[1].setBrand("LG");
        goods[1].setPrice(2999.99);
        goods[2] = new Appliances();
        goods[2].setBrand("Sony");
        goods[2].setPrice(4999.99);

        try {
            BufferedWriter writer = new BufferedWriter(new FileWriter("goodsInfo.txt"));
            for (Appliances good : goods) {
                writer.write(good.getBrand() + " " + good.getPrice() + "\n");
            }
            writer.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

最后，我们需要读取goodsInfo.txt文件，并在屏幕上显示其中的内容。代码如下：

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;

public class Test {
    public static void main(String[] args) {
        Appliances[] goods = new Appliances[3];
        goods[0] = new Appliances();
        goods[0].setBrand("Samsung");
        goods[0].setPrice(3999.99);
        goods[1] = new Appliances();
        goods[1].setBrand("LG");
        goods[1].setPrice(2999.99);
        goods[2] = new Appliances();
        goods[2].setBrand("Sony");
        goods[2].setPrice(4999.99);

        try {
            BufferedWriter writer = new BufferedWriter(new FileWriter("goodsInfo.txt"));
            for (Appliances good : goods) {
                writer.write(good.getBrand() + " " + good.getPrice() + "\n");
            }
            writer.close();

            BufferedReader reader = new BufferedReader(new FileReader("goodsInfo.txt"));
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
            reader.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

这样就完成了这个问题的解答，希望能帮助到你！