用泛型构建复杂容器模型

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泛型的一个重要好处是能够简单而安全地创建复杂的模型，比如组装多层容器时，其泛型就能保障其安全了。
代码例子：

//商品class Product{    private final int id;    private String description;    private double price;    public Product(int id, String description, double price)    {        super();        this.id = id;        this.description = description;        this.price = price;        System.out.println(toString());    }    @Override    public String toString()    {               return id+": "+description+", price: $"+price;    }    public void priceChange(double change)    {        price+=change;    }    //这里用静初始化不单单为方便调用，更重要的是共用这个对象减少内存开销    public static Generator<Product> generator=new Generator<Product>()    {        private Random random=new Random(47);        @Override        public Product next()        {            return new Product(random.nextInt(1000),"Test",Math.round(random.nextDouble()*1000.0)+0.99);        }    };}//货架,让其摆设多个商品@SuppressWarnings("serial")class Shelf extends ArrayList<Product>{    public Shelf(int nProducts)    {        //生产器工具类填充这个货架，其元素为多个Product商品         Generators.fill(this, Product.generator, nProducts);    }} //走廊摆设多个货架@SuppressWarnings("serial")class Aisle extends ArrayList<Shelf>{    public Aisle(int nShelves,int nProducts)    {        //添加多个货架        for(int i=0;i<nShelves;i++)            add(new Shelf(nProducts));    }}//商店摆设多个走量@SuppressWarnings("serial")public class Store extends ArrayList<Aisle>{    public Store(int nAisles,int nShelves,int nProducts)    {         //添加多个走廊        for(int i=0;i<nAisles;i++)            add(new Aisle(nShelves, nProducts));    }    @Override    public String toString()    {        //用字符容器装其内容        StringBuilder builder=new StringBuilder();        for(Aisle a:this)            for(Shelf s:a)                for(Product p:s)                {                    builder.append(p);                    builder.append("\n");                }                           return builder.toString();    }    public static void main(String[] args)    {        System.out.println(new Store(2, 3, 2));    }}

运行结果：
258: Test, price: 400.99861:Test,price:160.99
868: Test, price: 417.99207:Test,price:268.99
551: Test, price: 114.99278:Test,price:804.99
520: Test, price: 554.99140:Test,price:530.99
704: Test, price: 250.99575:Test,price:24.99
674: Test, price: 440.99826:Test,price:484.99
258: Test, price: 400.99861:Test,price:160.99
868: Test, price: 417.99207:Test,price:268.99
551: Test, price: 114.99278:Test,price:804.99
520: Test, price: 554.99140:Test,price:530.99
704: Test, price: 250.99575:Test,price:24.99
674: Test, price: 440.99826:Test,price:484.99

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