import java.util.*;public class Schedule { // Map 用于存储课程安排信息 private Map<String, List<String>> schedule; public Schedule() { this.schedule = new HashMap<>(); } // 添加课程信息 public void addSchedule(String time, String course, String classroom) { if (schedule.containsKey(time)) { List<String> courses = schedule.get(time); if (courses.contains(course) || courses.contains(classroom)) { System.out.println("Error: Course or classroom already scheduled at this time."); } else { courses.add(course); courses.add(classroom); schedule.put(time, courses); System.out.println("Schedule added successfully."); } } else { List<String> courses = new ArrayList<>(); courses.add(course); courses.add(classroom); schedule.put(time, courses); System.out.println("Schedule added successfully."); } } // 获取课程安排信息 public List<String> getSchedule(String time) { if (schedule.containsKey(time)) { return schedule.get(time); } else { return null; } } public static void main(String[] args) { Schedule schedule = new Schedule(); schedule.addSchedule("Monday 9:00", "Math", "Room 101"); schedule.addSchedule("Monday 9:00", "Physics", "Room 102"); schedule.addSchedule("Monday 10:00", "English", "Room 103"); List<String> monday_9 = schedule.getSchedule("Monday 9:00"); System.out.println("Monday 9:00 schedule: " + monday_9.toString()); }}请将每一句代码解释一下

这是一个Java程序，名为Schedule，用于存储和获取课程安排信息。下面是代码的解释：

import java.util.*; // 导入Java工具包

public class Schedule { // 定义一个名为Schedule的类

    // Map 用于存储课程安排信息
    private Map<String, List<String>> schedule;

    // 类的构造函数，用于初始化schedule
    public Schedule() {
        this.schedule = new HashMap<>();
    }

    // 添加课程信息
    public void addSchedule(String time, String course, String classroom) {
        if (schedule.containsKey(time)) { // 判断时间是否已经被占用
            List<String> courses = schedule.get(time); // 获取该时间段已经占用的课程信息
            if (courses.contains(course) || courses.contains(classroom)) {
                // 判断该课程或教室是否已经被占用
                System.out.println("Error: Course or classroom already scheduled at this time.");
            } else {
                courses.add(course);
                courses.add(classroom);
                schedule.put(time, courses); // 将课程信息添加到schedule中
                System.out.println("Schedule added successfully.");
            }
        } else {
            List<String> courses = new ArrayList<>();
            courses.add(course);
            courses.add(classroom);
            schedule.put(time, courses); // 将课程信息添加到schedule中
            System.out.println("Schedule added successfully.");
        }
    }

    // 获取课程安排信息
    public List<String> getSchedule(String time) {
        if (schedule.containsKey(time)) {
            return schedule.get(time); // 返回该时间段的课程信息
        } else {
            return null; // 如果该时间段没有课程信息，则返回null
        }
    }

    // 主函数
    public static void main(String[] args) {
        Schedule schedule = new Schedule(); // 创建一个Schedule对象
        schedule.addSchedule("Monday 9:00", "Math", "Room 101"); // 添加课程信息
        schedule.addSchedule("Monday 9:00", "Physics", "Room 102");
        schedule.addSchedule("Monday 10:00", "English", "Room 103");
        List<String> monday_9 = schedule.getSchedule("Monday 9:00"); // 获取课程信息
        System.out.println("Monday 9:00 schedule: " + monday_9.toString()); // 打印课程信息
    }
}