ADD file via upload

StudentDAO.java

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+package com.studentmanagement.model;
+
+import com.studentmanagement.exception.DuplicateStudentException;
+import com.studentmanagement.exception.StudentNotFoundException;
+
+import java.util.*;
+import java.util.stream.Collectors;
+
+/**
+ * 学生数据访问对象
+ * 负责学生数据的存储和访问操作，使用单例模式
+ */
+public class StudentDAO {
+    private static StudentDAO instance;
+    private Map<String, Student> students; // 存储学生对象的集合
+
+    /**
+     * 私有构造函数，防止外部实例化
+     */
+    private StudentDAO() {
+        students = new HashMap<>();
+        // 添加一些示例数据
+        initSampleData();
+    }
+
+    /**
+     * 获取单例实例
+     * @return StudentDAO的唯一实例
+     */
+    public static synchronized StudentDAO getInstance() {
+        if (instance == null) {
+            instance = new StudentDAO();
+        }
+        return instance;
+    }
+
+    /**
+     * 初始化示例数据
+     */
+    private void initSampleData() {
+        Student s1 = new Student("001", "张三", 20, "男");
+        s1.addCourseGrade("数学", 85.5);
+        s1.addCourseGrade("英语", 92.0);
+        s1.addCourseGrade("计算机", 88.5);
+
+        Student s2 = new Student("002", "李四", 21, "女");
+        s2.addCourseGrade("数学", 90.0);
+        s2.addCourseGrade("英语", 88.0);
+        s2.addCourseGrade("计算机", 95.0);
+
+        Student s3 = new Student("003", "王五", 19, "男");
+        s3.addCourseGrade("数学", 78.5);
+        s3.addCourseGrade("英语", 82.0);
+        s3.addCourseGrade("计算机", 80.0);
+
+        try {
+            addStudent(s1);
+            addStudent(s2);
+            addStudent(s3);
+        } catch (DuplicateStudentException e) {
+            // 示例数据初始化不应抛出重复异常
+            System.err.println("示例数据初始化错误: " + e.getMessage());
+        }
+    }
+
+    /**
+     * 添加学生
+     * @param student 学生对象
+     * @throws DuplicateStudentException 如果学生ID已存在
+     */
+    public void addStudent(Student student) throws DuplicateStudentException {
+        if (students.containsKey(student.getId())) {
+            throw new DuplicateStudentException("学生ID已存在: " + student.getId(), student.getId());
+        }
+        students.put(student.getId(), student);
+    }
+
+    /**
+     * 获取所有学生
+     * @return 学生列表
+     */
+    public List<Student> getAllStudents() {
+        return new ArrayList<>(students.values());
+    }
+
+    /**
+     * 根据ID查找学生
+     * @param id 学生ID
+     * @return 学生对象
+     * @throws StudentNotFoundException 如果学生不存在
+     */
+    public Student getStudentById(String id) throws StudentNotFoundException {
+        Student student = students.get(id);
+        if (student == null) {
+            throw new StudentNotFoundException("未找到ID为" + id + "的学生", id);
+        }
+        return student;
+    }
+
+    /**
+     * 更新学生信息
+     * @param student 学生对象
+     * @throws StudentNotFoundException 如果学生不存在
+     */
+    public void updateStudent(Student student) throws StudentNotFoundException {
+        if (!students.containsKey(student.getId())) {
+            throw new StudentNotFoundException("未找到ID为" + student.getId() + "的学生", student.getId());
+        }
+        students.put(student.getId(), student);
+    }
+
+    /**
+     * 删除学生
+     * @param id 学生ID
+     * @throws StudentNotFoundException 如果学生不存在
+     */
+    public void deleteStudent(String id) throws StudentNotFoundException {
+        if (!students.containsKey(id)) {
+            throw new StudentNotFoundException("未找到ID为" + id + "的学生", id);
+        }
+        students.remove(id);
+    }
+
+    /**
+     * 根据课程筛选学生
+     * @param course 课程名称
+     * @return 选修该课程的学生列表
+     */
+    public List<Student> getStudentsByCourse(String course) {
+        List<Student> result = new ArrayList<>();
+        for (Student student : students.values()) {
+            if (student.getCourseGrades().containsKey(course)) {
+                result.add(student);
+            }
+        }
+        return result;
+    }
+
+    /**
+     * 根据成绩范围筛选学生
+     * @param minGrade 最低成绩
+     * @param maxGrade 最高成绩
+     * @return 平均成绩在范围内的学生列表
+     */
+    public List<Student> getStudentsByGradeRange(double minGrade, double maxGrade) {
+        List<Student> result = new ArrayList<>();
+        for (Student student : students.values()) {
+            double avgGrade = student.calculateAverageGrade();
+            if (avgGrade >= minGrade && avgGrade <= maxGrade) {
+                result.add(student);
+            }
+        }
+        return result;
+    }
+}