Demeter's Law~

Demeter's law is a principle in object-oriented design. Its goal is to reduce the direct dependence between objects . It emphasizes 一个对象应该尽量减少与其他对象之间的直接交互that, 只与自己的直接朋友进行通信(the so-called friend relationship between objects means that 耦合关系there are many ways of coupling: dependence, association, Combination, aggregation, etc. We call it 出现成员变量，方法参数，方法返回值中的类为直接朋友, while 出现在局部变量中的类不是直接朋友), this can prevent an object from overly understanding the internal structure of other objects, thereby reducing the coupling and dependence of the code , improving the maintainability of the code, reducing the complexity of the system, and being convenient Perform unit testing and refactoring.

Example:

We create student, class, school respectively, establish a direct friend relationship between student and class, and establish a direct friend relationship between school and class. However, there is no interactive relationship between student and school, which reduces the coupling between classes.

package druidResources;

import java.util.ArrayList;
import java.util.List;

// 学生类
class Student {
    
    
    private String name;

    public Student(String name) {
    
    
        this.name = name;
    }

    public String getName() {
    
    
        return name;
    }
}

// 班级类
class Class {
    
    
	//实现班级和学生交互
    private List<Student> students;

    public Class() {
    
    
        students = new ArrayList<>();
    }

    public void addStudent(Student student) {
    
    
        students.add(student);
    }

    public List<Student> getStudents() {
    
    
        return students;
    }
}

// 学校类
class School {
    
    
	//实现班级和学校交互
    private List<Class> classes;

    public School() {
    
    
        classes = new ArrayList<>();
    }

    public void addClass(Class clazz) {
    
    
        classes.add(clazz);
    }

    // 获取学校中所有学生的姓名
    public List<String> getAllStudentNames() {
    
    
        List<String> names = new ArrayList<>();
        for (Class clazz : classes) {
    
    
            for (Student student : clazz.getStudents()) {
    
    
                names.add(student.getName());
            }
        }
        return names;
    }
}

public class LawOfDemeter {
    
    
    public static void main(String[] args) {
    
    
        // 创建学校对象
        School school = new School();
        // 创建班级对象
        Class class1 = new Class();
        Class class2 = new Class();
        // 创建学生对象
        Student student1 = new Student("张三");
        Student student2 = new Student("李四");
        Student student3 = new Student("王五");
        // 添加学生到班级
        class1.addStudent(student1);
        class1.addStudent(student2);
        class2.addStudent(student3);
        // 添加班级到学校
        school.addClass(class1);
        school.addClass(class2);
        // 获取学校中所有学生的姓名
        List<String> names = school.getAllStudentNames();
        // 打印学生姓名
        for (String name : names) {
    
    
            System.out.println(name);
        }
    }
}

Output:

张三
李四
王五