1. 概念
- 迪米特法则(Demeter Principle)又叫最少知道原则,即一个类对自己依赖的类知道的越少越好。
- 迪米特法则还有个更简单的定义:只与直接的朋友通信
- 朋友:有耦合(依赖,关联,组合,聚合)关系的对象
- 直接的朋友:成员变量,方法参数,方法返回值中的类
2. 优点
- 降低耦合性,提高模块功能的独立性
- 非常实用,能直接用于开发环境
3. 代码示例
示例1(违背迪米特法则的案例)
package com.bz.design.principle.demeter;
import java.util.ArrayList;
import java.util.List;
public class Demeter001 {
public static void main(String[] args) {
SchoolManager schoolManager = new SchoolManager();
schoolManager.printAllEmployee(new CollegeManager());
}
}
class Employee {
private String id;
public void setId(String id) {
this.id = id;
}
public String getId() {
return this.id;
}
}
class CollegeEmployee {
private String id;
public void setId(String id) {
this.id = id;
}
public String getId() {
return this.id;
}
}
class CollegeManager {
public List<CollegeEmployee> getAllEmployee() {
List<CollegeEmployee> list = new ArrayList<>();
for (int i = 0; i < 10; i++) {
CollegeEmployee emp = new CollegeEmployee();
emp.setId("学院员工id=" + i);
list.add(emp);
}
return list;
}
}
class SchoolManager {
public List<Employee> getAllEmployee() {
List<Employee> list = new ArrayList<>();
for (int i = 0; i < 5; i++) {
Employee emp = new Employee();
emp.setId("学校总部员工id=" + i);
list.add(emp);
}
return list;
}
void printAllEmployee(CollegeManager sub) {
List<CollegeEmployee> list1 = sub.getAllEmployee();
System.out.println("----------学院员工----------");
for (CollegeEmployee e : list1) {
System.out.println(e.getId());
}
List<Employee> list2 = this.getAllEmployee();
System.out.println("------------学校总部员工------------");
for (Employee e : list2) {
System.out.println(e.getId());
}
}
}
示例2
- 应用实例改进
- 前面设计的问题在于SchoolManager中,CollegeEmployee类并不不是SchoolManager类的直接朋友(分析)
- 按照迪米特法则,应该避免类中出现这样非直接朋友关系的耦合
- 对代码按照迪米特法则进行改进
package com.bz.design.principle.demeter.improve;
import java.util.ArrayList;
import java.util.List;
public class DemeterImprove {
public static void main(String[] args) {
System.out.println("---使用迪米特法则改进---");
SchoolManager schoolManager = new SchoolManager();
schoolManager.printAllEmployee(new CollegeManager());
}
}
class Employee {
private String id;
public void setId(String id) {
this.id = id;
}
public String getId() {
return this.id;
}
}
class CollegeEmployee {
private String id;
public void setId(String id) {
this.id = id;
}
public String getId() {
return this.id;
}
}
class CollegeManager {
public List<CollegeEmployee> getAllEmployee() {
List<CollegeEmployee> list = new ArrayList<>();
for (int i = 0; i < 10; i++) {
CollegeEmployee emp = new CollegeEmployee();
emp.setId("学院员工id=" + i);
list.add(emp);
}
return list;
}
public void printEmployee() {
List<CollegeEmployee> list1 = this.getAllEmployee();
System.out.println("----------学院员工----------");
for (CollegeEmployee e : list1) {
System.out.println(e.getId());
}
}
}
class SchoolManager {
public List<Employee> getAllEmployee() {
List<Employee> list = new ArrayList<>();
for (int i = 0; i < 5; i++) {
Employee emp = new Employee();
emp.setId("学校总部员工id=" + i);
list.add(emp);
}
return list;
}
void printAllEmployee(CollegeManager sub) {
sub.printEmployee();
List<Employee> list2 = this.getAllEmployee();
System.out.println("------------学校总部员工------------");
for (Employee e : list2) {
System.out.println(e.getId());
}
}
}
4. 应用注意事项
- 不要干
- objectA.getObjectB().doSomething();
- objectA.getObjectB().getObjectC().doSomething();
- 遵循迪米特法则的设计模式
- 注意事项