迪米特法則 Low of Demeter (LoD)

定義#

對於物件 Obj 的一個方法 fun() 而言， fun() 只能存取以下類型的物件：

自己或父類別的方法（to itself (Obj itself). You can play with yourself.）自己或父類屬性中的其他物件（to objects contained in attributes of itself or a superclass (Obj’s direct component objects). You can play with your own toys.）方法可以使用參數傳進來的物件（to an object that is passed as a parameter to the method (fun’s parameters). You can play with toys that were given to you.）使用自己所創造出來的物件（to an object that is created by the method (Any objects created/instantiated within fun). And you can play with toys you’ve made yourself.）全域變數中的物件（to an object that is stored in a global variable.）

範例#

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/* Class A */
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public class A {
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  private int a1;
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  C c;
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  F f;
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  G g = new G();
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  public A(C c) {               // LoD 1
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    this.c = c;
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  }
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  public void useSelf() {       // LoD 1
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    go();
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  }
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  public void go() {            // 沒有符合任何LoD，因為僅print且無使用object。
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    System.out.println("A go");
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  }
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  public void setA1(int a1) {   // LoD 1
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    this.a1 = a1;
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  }
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  public int getA1() {          // 沒有符合任何LoD，因為僅回傳而已。
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    return this.a1;
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  }
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  public void setC(C c) {       // LoD 1
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    this.c = c;
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  }
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  public C getC() {             // 沒有符合任何LoD，因為僅回傳而已。
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    return c;
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  }
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  private void useC() {         // LoD 2
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    System.out.println(c);
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    c.go();
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  }
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  public void useE(E e) {       // LoD 3
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    e.go();
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  }
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  public void useD() {          // LoD 4
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    D d = new D();
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    d.go();
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  }
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  public void useF() {          // LoD 4
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    f = new F();
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    f.go()
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  }
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  public void useG() {          // LoD 4
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    g.go();
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  }
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}
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/* Class B */
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public class B extends A {
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  public B(C c) {
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    super(c);
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  }
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  public void useSuperclassAttributeC() {  // LoD 2
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    c.go();
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  }
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  public void useSuperclassAttributeG() {  // LoD 2
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    g.go();
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  }
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}
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/* Class C */
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public class C {
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  public void go() {
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    System.out.println("C go");
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  }
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}
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/* Class D */
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public class D {
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  public void go() {
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    System.out.println("D go");
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  }
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}
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/* Class E */
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public class E {
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  public void go() {
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    System.out.println("E go");
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  }
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}
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/* Class F */
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public class F {
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  public void go() {
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    System.out.println("F go");
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  }
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}
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/* Class G */
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public class G {
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  public void go() {
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    System.out.println("G go");
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  }
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}

定義#

範例#

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