下面的类设计的不合理,要求用户在调用18行Pring方法前,先为17行赋值。
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public class Space
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{
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public static void Main(string[] args)
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{
6![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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FilePrint pring = new FilePrint();
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//如果用户忘记调用FileName,系统将不可靠
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//pring.FileName = @"c:\text.txt";
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pring.Print();
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}
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}
13![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
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public class FilePrint
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{
16![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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public string FileName = null;
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public void Print()
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{
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System.Console.WriteLine("Do prining {0}", FileName);
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}
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}
把代码改为下面就合理的多![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
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public class Space
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{
3![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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public static void Main(string[] args)
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{
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FilePrint pring = new FilePrint(@"c:\text.txt");
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pring.Print();
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//代码可以更简约
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new FilePrint(@"c:\text.txt").Print();
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}
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}
14![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
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public class FilePrint
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{
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public FilePrint(string path)
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{
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FileName = path;
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}
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public readonly string FileName = null;
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public void Print()
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{
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System.Console.WriteLine("Do prining {0}", FileName);
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}
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}
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通过构造函数,强制用户在实例化的时候给与FileName的值,调用更安全更简约。
第23行对数据成员FileName修饰为readonly表示该数据成员为只读,只读的意思是,该成员只有一次赋值的机会(声明时立即赋值或在构造函数时赋值)。readonly和const的区别是
readonly有两种赋值的机会:声明时立即赋值或在构造函数时赋值
const只有在声明时赋值
另外一个很值得注意的是,const其实就是static数据修饰,不过不能直接用static来修饰。const包括了static的含义,所以const需要通过类来访问。
我们再来看一个构造函数的运用
请仔细考虑一下代码
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public class Space
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{
3![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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public static void Main(string[] args)
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{
6![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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System.Console.WriteLine(new SalesContract(100).SalePrice);
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System.Console.WriteLine(new SalesContract(100,120).SalePrice);
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System.Console.WriteLine(new SalesContract(100,120,180).SalePrice);
10![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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}
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}
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public class SalesContract : Contract//销售合同
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{
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public SalesContract(double costPrice)
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: this(costPrice, costPrice)
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{ }
20![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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public SalesContract(double costPrice, double minimumPrice)
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: this(costPrice, minimumPrice, minimumPrice)
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{ }
24![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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public SalesContract(double costPrice, double minimumPrice, double salePrice)
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{
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CostPrice = costPrice;
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MinimumPrice = minimumPrice;
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SalePrice = salePrice;
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}
31![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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private double MinimumPrice;//最低价
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public double SalePrice;//销售价格
34![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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public bool CheckPrice()//价格检查
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{
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return SalePrice < Math.Min(MinimumPrice, CostPrice);
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}
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internal double PreferentialPrice;//优惠价
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}
41![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
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public class Contract//合同
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{
44![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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public string Buyer;//买方
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public string Seller;//卖方
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protected double CostPrice;//成本价
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}
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通过3个构造函数,用户可以方便的对ConstPrice、SalePrice、MinimunPrice进行赋值。
我们再来看一个关于构造函数的用例。
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public class Space
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{
3![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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public static void Main(string[] args)
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{
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System.Console.WriteLine(new Order("123-12").OrderNumber);
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System.Console.WriteLine(new Order(Order.CreateOrderNumber()).OrderNumber);
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System.Console.WriteLine(new Order(Order.CreateOrderNumber()).OrderNumber);
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}
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}
11![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
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public class Order
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{
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public Order(string orderNumber)
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{
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this.OrderNumber = orderNumber;
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}
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private static int Count;
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public readonly string OrderNumber;
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public static string CreateOrderNumber()
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{
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System.DateTime d = System.DateTime.Now;
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return System.DateTime.Now.ToString("yMd-") + (++Count).ToString();
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}
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}
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上面的代码看似很合理,用户可以自己定义OrderNumber也可以通过CreateOrderNumber方法,Order在构造时总是可以得到Number。
但这样的设计,暴露了对CreateOrderNumber的调用。
我们看看经过修改后的设计
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public class Order
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{
3![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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public Order(string orderNumber)
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{
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this.OrderNumber = orderNumber;
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}
8![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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public Order():this(CreateOrderNumber())
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{
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}
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private static int Count;
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public readonly string OrderNumber;
17![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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private static string CreateOrderNumber()
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{
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System.DateTime d = System.DateTime.Now;
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return System.DateTime.Now.ToString("yMd-") + (++Count).ToString();
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}
23![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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}
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我们添加了一个默认的构造函数Order()在第9行,又把18行的CreateOrderNumber改为private,那么这会有什么效果呢?
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public class Space
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{
3![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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public static void Main(string[] args)
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{
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System.Console.WriteLine(new Order("123-12").OrderNumber);
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System.Console.WriteLine(new Order().OrderNumber);
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System.Console.WriteLine(new Order().OrderNumber);
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}
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}
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看到了吗?在这个代码中,如果用户给与了Number则使用用户给与的值,否则,Order能自动给与值。向用户封装了CreateOrderNumber的存在。这就是类设计的一个很重要的原则:自我维护。