Java设计模式之迪米特法则(Law of Demeter, LoD)

    迪米特法则（Law of Demeter），Demeter是古希腊神话中的农业、谷物和丰收的女神，奥林匹斯十二主神之一。它的名字源于迪米特计划， 该项目是为纪念Demeter，“distribution-mother”和希腊农业女神而命名的，以表示自下而上的编程哲学。

    迪米特法具体含义可以从以下几句话中理解：

• Each unit should have only limited knowledge about other units: only units "closely" related to the current unit.
• Each unit should only talk to its friends; don't talk to strangers.
• Only talk to your immediate friends.

即：

• 每个单元对其他单元的了解应该是非常有限的：只和当前单元关系密切的单元联系。
• 每个单元只和朋友讲话，而不合陌生人交谈。
• 只和你的直接朋友讲话。

   我们引用例子来看一下：

 //总公司员工class Employee{private String id;public void setId(String id){this.id = id;}public String getId(){return id;}}//分公司员工class SubEmployee{private String id;public void setId(String id){this.id = id;}public String getId(){return id;}}class SubCompanyManager{public List getAllEmployee(){List list = new ArrayList();for(int i=0; i<100; i++){SubEmployee emp = new SubEmployee();//为分公司人员按顺序分配一个IDemp.setId("分公司"+i);list.add(emp);}return list;}}class CompanyManager{public List getAllEmployee(){List list = new ArrayList();for(int i=0; i<30; i++){Employee emp = new Employee();//为总公司人员按顺序分配一个IDemp.setId("总公司"+i);list.add(emp);}return list;}public void printAllEmployee(SubCompanyManager sub){List list1 = sub.getAllEmployee();for(SubEmployee e:list1){System.out.println(e.getId());}List list2 = this.getAllEmployee();for(Employee e:list2){System.out.println(e.getId());}}}public class Client{public static void main(String[] args){CompanyManager e = new CompanyManager();e.printAllEmployee(new SubCompanyManager());}}

    修改后如下：

class SubCompanyManager{  public List<SubEmployee> getAllEmployee(){  List<SubEmployee> list = new ArrayList<SubEmployee>();  for(int i=0; i<100; i++){  SubEmployee emp = new SubEmployee();  //为分公司人员按顺序分配一个ID  emp.setId("分公司"+i);  list.add(emp);  }  return list;  }  public void printEmployee(){  List<SubEmployee> list = this.getAllEmployee();  for(SubEmployee e:list){  System.out.println(e.getId());  }  }  
}  class CompanyManager{  public List<Employee> getAllEmployee(){  List<Employee> list = new ArrayList<Employee>();  for(int i=0; i<30; i++){  Employee emp = new Employee();  //为总公司人员按顺序分配一个ID  emp.setId("总公司"+i);  list.add(emp);  }  return list;  }  public void printAllEmployee(SubCompanyManager sub){  sub.printEmployee();  List<Employee> list2 = this.getAllEmployee();  for(Employee e:list2){  System.out.println(e.getId());  }  }  
}

    遵循迪米特法则可以降低类之间的耦合，每个类减少不必要的依赖。但是如果过度使用，会有大量的中介类，所以应该权衡利弊，不可盲目。

引用：

http://wiki.jikexueyuan.com/project/java-design-pattern-principle/principle-5.html

https://en.wikipedia.org/wiki/Law_of_Demeter