编写文件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，下面是一个简单的 StudentDB.java 的单例类示例，其中包含了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("S001", "Tom", "Male", "Computer Science"));
        students.add(new Student("S002", "Jane", "Female", "Mathematics"));
        students.add(new Student("S003", "Bob", "Male", "Electrical Engineering"));
        students.add(new Student("S004", "Alice", "Female", "Mechanical Engineering"));
        students.add(new Student("S005", "John", "Male", "Physics"));
    }

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

    public Student getStudent(String studentId){
        for(Student student : students){
            if(student.getId().equals(studentId)){
                return student;
            }
        }
        return null;
    }

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

以上代码实现了一个单例的 StudentDB 类，其中包含了5名学生的信息，包括学生编号、姓名、性别和专业。你可以通过调用 `getInstance()` 方法来获取该单例对象，并通过 `getStudent()` 和 `getStudents()` 方法来获取特定学生和所有学生列表。