05. Law of Demeter LOD

1. Definition

• It means that an object should have the least knowledge of other objects , also To know the principle at least （Least Knowledge Principle,LKP）, Minimize the coupling between classes .
• Dimitar's principle emphasizes only communicating with friends , Don't talk to strangers . Appears in the member variable 、 Method input 、 All classes in the output parameters can be called member friend classes , The class that appears inside the method body does not belong to the friend class .

2. Case study

Case scenario ： Now let's design a permission system ,Boss Need to see the number of courses currently posted online . Now ,Boss To find the TeamLeader Go and count ,TeamLeader Tell the statistics to Boss.

2.1 Initial code

• Course

public class Course {
    
}

• TeamLeader

public class TeamLeader {
    

    public void checkNumberOfCourse(List<Course> courseList){
    
        System.out.println(" The number of courses published so far is ："+courseList.size());
    }
}

• Boss

public class Boss {
    
    public void commandCheckNumber(TeamLeader teamLeader){
    
        //  simulation Boss Page by page, page by page ,TeamLeader Real time statistics 
        List<Course> courseList = new ArrayList<Course>();
        for (int i = 0; i < 20; i++) {
    
            courseList.add(new Course());
        }
        teamLeader.checkNumberOfCourse(courseList);
    }
}

• LDLTest

public class LDLTest {
    
    public static void main(String[] args) {
    
        Boss boss = new Boss();
        TeamLeader teamLeader = new TeamLeader();
        boss.commandCheckNumber(teamLeader);
    }
}

From the above code functions have been implemented , The code doesn't seem to have a problem . But according to the Demeter principle ,Boss Just want results , You don't need to follow Course Generate direct communication . and TeamLeader Statistics need to quote Course object .Boss and Course It's not a friend , As you can see from the class diagram below

2.2 Optimize the code

• TeamLeader

public class TeamLeader {
    

    public void checkNumberOfCourse(){
    
        //  simulation Boss Page by page, page by page ,TeamLeader Real time statistics 
        List<Course> courseList = new ArrayList<Course>();
        for (int i = 0; i < 20; i++) {
    
            courseList.add(new Course());
        }
        System.out.println(" The number of courses published so far is ："+courseList.size());
    }
}

• Boss

public class Boss {
    
    public void commandCheckNumber(TeamLeader teamLeader){
    
        teamLeader.checkNumberOfCourse();
    }
}

Class diagram again ,Course and Boss There's no connection anymore .