Introduce Null Object (1)
- 使用時機
- 作法
- 建立null object
- 用isNull()取代原有的條件判斷
- 移除條件判斷
- 原始程式
當程式中一再出現，檢查某物件是否為null時，可考慮引入null物件。
為source class建立一個子類別，使其行為像source class的null版本，在source class及 子類別中都加入isNull() method，source class的isNull()傳回false，子類別的isNull() 傳回true。
找出所有程式中，檢查source object是否為null的程式碼，將它們都改為使用isNull()檢查。
假設有個客戶管理系統，系統裡需要記錄客戶的姓名和電子郵件信箱，系統可以列出所有客戶的 資料，在這樣的假設前提下，建立了如下的程式:
using System;
namespace NullObjectPrj
{
public class Customer
{
private readonly Label _name;
private readonly Label _mail;
public Customer(Label name, Label mail)
{
_name = name;
_mail = mail;
}
public Customer(Label name) : this(name, null)
{
}
public void display()
{
if (_name != null)
{
_name.display();
}
if (_mail != null)
{
_mail.display();
}
}
public override string ToString()
{
string result = "[ Customer:";
result += " name=";
if (_name == null)
{
result += "\"(none)\"";
}
else
{
result += _name;
}
result += " mail=";
if (_mail == null)
{
result += "\"(none)\"";
}
else
{
result += _mail;
}
result += " ]";
return result;
}
}
public class Label
{
private readonly string _label;
public Label(string label)
{
_label = label;
}
public void display()
{
Console.WriteLine("display: " + _label);
}
public override string ToString()
{
return "\"" + _label + "\"";
}
}
class Program
{
static void Main(string[] args)
{
Customer[] customer = {
new Customer(new Label("Steven"), new Label("steven@idealist.idv.tw")),
new Customer(new Label("Shelley"), new Label("shelley@idealist.idv.tw")),
new Customer(new Label("Alice"))
};
foreach (Customer cstm in customer)
{
Console.WriteLine(cstm.ToString());
cstm.display();
Console.WriteLine("");
}
Console.ReadLine();
}
}
}
上面程式，Customer class的display() method及ToString() method中，
一再重複的檢查Label object是否為null，程式看起來相當雜亂，這正是可考慮使用null object以簡
化程式的地方，透過導入Null Object，可讓程式看起來更簡單明瞭。在開始重構前，我們先看一下執行的結果，接下來的重構過程，每完成一個步驟 ，都應編譯並測試一下，確定不管程式碼如何改變，執行結果都不變。
- 建立null object
- 用isNull()取代原有的條件判斷
- 移除條件判斷
如下程式，為source class - Label建立一個子類別NullLabel，接著在Label及NullLabel中都加入 isNull() method，Label的isNull()傳回false，NullLabel的isNull()傳回true。改好後編譯並測試 一下，看結果是否正確。
using System;
namespace NullObjectPrj
{
public class Customer
{
private readonly Label _name;
private readonly Label _mail;
public Customer(Label name, Label mail)
{
_name = name;
_mail = mail;
}
public Customer(Label name)
: this(name, null)
{
}
public void display()
{
if (_name != null)
{
_name.display();
}
if (_mail != null)
{
_mail.display();
}
}
public override string ToString()
{
string result = "[ Customer:";
result += " name=";
if (_name == null)
{
result += "\"(none)\"";
}
else
{
result += _name;
}
result += " mail=";
if (_mail == null)
{
result += "\"(none)\"";
}
else
{
result += _mail;
}
result += " ]";
return result;
}
}
public class Label
{
private readonly string _label;
public Label(string label)
{
_label = label;
}
// 新增
virtual public bool isNull()
{
return false;
}
//修改, 加上virtual讓子類別可以override。
virtual public void display()
{
Console.WriteLine("display: " + _label);
}
public override string ToString()
{
return "\"" + _label + "\"";
}
}
// 新增
public class NullLabel : Label
{
public NullLabel() : base("(none)") { }
public override bool isNull()
{
return true;
}
override public void display()
{
//因為是空的，所以不列印出任何東西。
}
}
class Program
{
static void Main(string[] args)
{
Customer[] customer = {
new Customer(new Label("Steven"), new Label("steven@idealist.idv.tw")),
new Customer(new Label("Shelley"), new Label("shelley@idealist.idv.tw")),
new Customer(new Label("Alice"))
};
foreach (Customer cstm in customer)
{
Console.WriteLine(cstm.ToString());
cstm.display();
Console.WriteLine("");
}
Console.ReadLine();
}
}
}
用isNull()取代Customer中的條件判斷，得到如下的程式，此時程式透過isNull()來判斷， 語意上是比較清晰了，但仍到處都是判斷式。
using System;
namespace NullObjectPrj
{
public class Customer
{
private readonly Label _name;
private readonly Label _mail;
public Customer(Label name, Label mail)
{
_name = name;
_mail = mail;
}
public Customer(Label name)
: this(name, new NullLabel()) //修改㺿
{
}
public void display()
{
if (!_name.isNull()) //修改㺿
{
_name.display();
}
if (!_mail.isNull()) //修改㺿
{
_mail.display();
}
}
public override string ToString()
{
string result = "[ Customer:";
result += " name=";
if (_name.isNull()) //修改㺿
{
result += "\"(none)\"";
_name.display();
}
else
{
result += _name;
}
result += " mail=";
if (_mail.isNull()) //修改㺿
{
result += "\"(none)\"";
}
else
{
result += _mail;
}
result += " ]";
return result;
}
}
public class Label
{
private readonly string _label;
public Label(string label)
{
_label = label;
}
virtual public bool isNull()
{
return false;
}
virtual public void display()
{
Console.WriteLine("display: " + _label);
}
public override string ToString()
{
return "\"" + _label + "\"";
}
}
public class NullLabel : Label
{
public NullLabel() : base("(none)") { }
public override bool isNull()
{
return true;
}
override public void display()
{
}
}
class Program
{
static void Main(string[] args)
{
Customer[] customer = {
new Customer(new Label("Steven"), new Label("steven@idealist.idv.tw")),
new Customer(new Label("Shelley"), new Label("shelley@idealist.idv.tw")),
new Customer(new Label("Alice"))
};
foreach (Customer cstm in customer)
{
Console.WriteLine(cstm.ToString());
cstm.display();
Console.WriteLine("");
}
Console.ReadLine();
}
}
}
移除條件判斷後，程式變得清晰明瞭，重構大功告成! 更進階的手法，在 Introduce Null Object (2)再介紹。
using System;
namespace NullObjectPrj
{
public class Customer
{
private readonly Label _name;
private readonly Label _mail;
public Customer(Label name, Label mail)
{
_name = name;
_mail = mail;
}
public Customer(Label name)
: this(name, new NullLabel())
{
}
public void display()
{
//修改
_name.display();
_mail.display();
}
//修改㺿
public override string ToString()
{
string result = "[ Customer: name=" + _name + " mail=" + _mail + " ]";
return result;
}
}
public class Label
{
private readonly string _label;
public Label(string label)
{
_label = label;
}
virtual public bool isNull()
{
return false;
}
virtual public void display()
{
Console.WriteLine("display: " + _label);
}
public override string ToString()
{
return "\"" + _label + "\"";
}
}
public class NullLabel : Label
{
public NullLabel() : base("(none)") { }
public override bool isNull()
{
return true;
}
override public void display()
{
}
}
class Program
{
static void Main(string[] args)
{
Customer[] customer = {
new Customer(new Label("Steven"), new Label("steven@idealist.idv.tw")),
new Customer(new Label("Shelley"), new Label("shelley@idealist.idv.tw")),
new Customer(new Label("Alice"))
};
foreach (Customer cstm in customer)
{
Console.WriteLine(cstm.ToString());
cstm.display();
Console.WriteLine("");
}
Console.ReadLine();
}
}
}