用java实现:文件StudentDB.java：单列类，模拟数据库中的课程，初始化后至少封装5名学生信息；

好的，以下是用 Java 实现的 StudentDB 单例类，模拟数据库中的课程，初始化后封装了5名学生的信息：

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

public class StudentDB {
    private static StudentDB instance = null;
    private List<Student> students;

    private StudentDB() {
        students = new ArrayList<>();
        students.add(new Student("John", "Doe", 1));
        students.add(new Student("Jane", "Doe", 2));
        students.add(new Student("Bob", "Smith", 3));
        students.add(new Student("Alice", "Johnson", 4));
        students.add(new Student("Tom", "Lee", 5));
    }

    public static synchronized StudentDB getInstance() {
        if (instance == null) {
            instance = new StudentDB();
        }
        return instance;
    }

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

class Student {
    private String firstName;
    private String lastName;
    private int id;

    public Student(String firstName, String lastName, int id) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.id = id;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }
}

这个类实现了单例模式，保证整个应用程序中只有一个 StudentDB 实例。在构造函数中初始化了5个学生，每个学生有名、姓和 ID 三个属性。可以通过 `getInstance()` 方法获取 StudentDB 实例，然后通过 `getStudents()` 方法获取学生列表。