一个对象应该对其他对象保持最少的了解，又叫最少知道原则,尽量降低类与类之间的耦合度.迪米特原则主要强调:只和朋友交流,不和陌生人说话.出现在成员变量、方法的输入、输出参数中的类都可以成为成员朋友类,而出现在方法体内部的类不属于朋友类

Boss让TeamLeader统计线上课程数量,TeamLeader把统计结果告诉Boss.

public class Course {}

public class TeamLeader {public void checkNumberOfCourses(List<Course> courseList){System.out.println(courseList.size());}
}

public class Boss {public void commandCheckNumber(TeamLeader teamLeader){ArrayList<Course> courseList = new ArrayList<Course>();for (int i = 0; i < 20; i++) {courseList.add(new Course());}teamLeader.checkNumberOfCourses(courseList);}public static void main(String[] args) {Boss boss = new Boss();TeamLeader teamLeader = new TeamLeader();boss.commandCheckNumber(teamLeader);}}

根据迪米特原则,Boss只想要结果，不需要和Course直接交流.而TeamLeader统计需要引用Course对象.Boss和Course并不是朋友

public class TeamLeader {public void commandCheckNumber(){ArrayList<Course> courseList = new ArrayList<Course>();for (int i = 0; i < 20; i++) {courseList.add(new Course());}System.out.println(courseList.size());}}

public class Boss {public static void main(String[] args) {Boss boss = new Boss();boss.checkNumberOfCourses(new TeamLeader());}public void checkNumberOfCourses(TeamLeader teamLeader){System.out.println(teamLeader);}}