Change Value to Reference(将值对象改为引用对象)

做法

• 使用Replace Constructor with Factory Method

• 编译，测试

• 决定由什么对象负责数据访问刷新对象的途径

  可能是一个静态字典或一个注册表对象

你也可以使用多个对象作为新对象的访问点

• 决定这些引用对象应该预先创建好，或是应该动态创建

  如果这些引用对象是预先创建好的，而你必须从内存中将它们读取出来，那么就得确保它们在被需要的时候能够被及时加载

• 修改工厂函数，令它返回引用对象

  如果对象是预先创建好的，你就需要考虑：万一有人索求一个其实并不存在的对象，要如何处理错误？

你可能希望对工厂函数使用Rename Method，使其传达这样的信息：它返回的是一个既存对象

• 编译，测试

范例

class Customer{
  public Customer(String name){
    _name = name;
  }
  public String getName(){
    return _name;
  }
  private final String _name;
}
class Order...
  public Order(String customerName){
      _customer = new Customer(customerName); 
    }
    public String getCustomerName(){
    return _customer.getName();
  }
    public void setCustomer(String customerName){
    _customer = new Customer(customerName);
  }
    private Customer _customer;
}
private static int numberOfOrderFor(Collection orders,String customer){
  int result = 0;
  Iterator iter = orders.iterator();
  while (iter.hasNext()){
    Order each = (Order) iter.next();
    if (each.getCustomerName().equals(customer)){
      result ++;
    }
  }
  return result;
}

修改后：

class Customer{
  public static Customer getNamed(String name){
    return (Customer) _instances.get(name);
  }

  private static Dictionary _instances  = New Hashtable();

  static void loadCustomers(){
    new Customer("lemon Car Hire").store();
    new Customer("Associated Coddee Machines").store();
    new Customer("Bilston Gasworks").store();
  }

  private void store(){
    _instances.put(this.getName(), this);
  }

  private Customer(String name){
    _name = name;
  }

  public String getName(){
    return _name;
  }

  private final String _name;
}

class Order...
  public Order(String customerName){
      _customer = Customer.create(customerName); 
    }

    public String getCustomerName(){
    return _customer.getName();
  }

    public void setCustomer(String customerName){
    _customer = new Customer(customerName);
  }

    private Customer _customer;
}