The Law of Demeter

Posted on |

更詳細的說法是“Law of Demeter for Functions/Methods” (LoD-F)，簡單化Object之間的互動，讓多個Object不要互相相依。

定義：

The Law of Demeter for functions requires that a method M of an object O may only invoke the methods of the following kinds of objects:

  1. O itself
  2. M’s parameters
  3. Any objects created/instantiated within M
  4. O’s direct component objects
  5. A global variable, accessible by O, in the scope of M

範例：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
/**
 * A Law of Demeter example in Java.
 * Created by Alvin Alexander, <a href="http://devdaily.com" title="http://devdaily.com">http://devdaily.com</a>.
 * This is an adaptation of the source code example from the book
 * The Pragmatic Programmer.
 */
public class LawOfDemeterInJava
{
  private Topping cheeseTopping;
   
  /**
   * Good examples of following the Law of Demeter.
   */
  public void goodExamples(Pizza pizza)
  {
    Foo foo = new Foo();
     
    // (1) it's okay to call our own methods
    doSomething();
     
    // (2) it's okay to call methods on objects passed in to our method
    int price = pizza.getPrice();
     
    // (3) it's okay to call methods on any objects we create
    cheeseTopping = new CheeseTopping();
    float weight = cheeseTopping.getWeightUsed();
     
    // (4) any directly held component objects
    foo.doBar();
  }
   
  private void doSomething()
  {
    // do something here ...
  }
}

參考資料：