怎么用JAVA来写一个系统，代码怎么写

写一个系统的代码需要根据具体的需求和功能来进行设计和实现。下面是一个简单的Java系统代码示例，以医生管理为例：

1.首先需要定义一个Doctor类，包含医生的基本信息。

public class Doctor {
    private int id;
    private String name;
    private int age;
    private String gender;
    private String department;
    private String position;

    // 构造函数
    public Doctor(int id, String name, int age, String gender, String department, String position) {
        this.id = id;
        this.name = name;
        this.age = age;
        this.gender = gender;
        this.department = department;
        this.position = position;
    }

    // getter和setter方法
    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;
    }

    public String getDepartment() {
        return department;
    }

    public void setDepartment(String department) {
        this.department = department;
    }

    public String getPosition() {
        return position;
    }

    public void setPosition(String position) {
        this.position = position;
    }
}

2.然后需要定义一个DoctorDao接口，包含医生信息的增删改查等操作。

public interface DoctorDao {
    // 添加医生
    public void addDoctor(Doctor doctor);

    // 删除医生
    public void deleteDoctor(int id);

    // 更新医生信息
    public void updateDoctor(Doctor doctor);

    // 根据id查找医生
    public Doctor findDoctorById(int id);

    // 获取所有医生信息
    public List<Doctor> getAllDoctors();
}

3.接着实现DoctorDao接口，使用MySQL数据库进行数据存储。

public class DoctorDaoImpl implements DoctorDao {
    // 数据库连接
    private Connection conn;

    // 构造函数
    public DoctorDaoImpl(Connection conn) {
        this.conn = conn;
    }

    @Override
    public void addDoctor(Doctor doctor) {
        String sql = "insert into doctor (id, name, age, gender, department, position) values (?, ?, ?, ?, ?, ?)";
        PreparedStatement pstmt;
        try {
            pstmt = conn.prepareStatement(sql);
            pstmt.setInt(1, doctor.getId());
            pstmt.setString(2, doctor.getName());
            pstmt.setInt(3, doctor.getAge());
            pstmt.setString(4, doctor.getGender());
            pstmt.setString(5, doctor.getDepartment());
            pstmt.setString(6, doctor.getPosition());
            pstmt.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void deleteDoctor(int id) {
        String sql = "delete from doctor where id = ?";
        PreparedStatement pstmt;
        try {
            pstmt = conn.prepareStatement(sql);
            pstmt.setInt(1, id);
            pstmt.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void updateDoctor(Doctor doctor) {
        String sql = "update doctor set name=?, age=?, gender=?, department=?, position=? where id=?";
        PreparedStatement pstmt;
        try {
            pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, doctor.getName());
            pstmt.setInt(2, doctor.getAge());
            pstmt.setString(3, doctor.getGender());
            pstmt.setString(4, doctor.getDepartment());
            pstmt.setString(5, doctor.getPosition());
            pstmt.setInt(6, doctor.getId());
            pstmt.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    @Override
    public Doctor findDoctorById(int id) {
        String sql = "select * from doctor where id=?";
        Doctor doctor = null;
        PreparedStatement pstmt;
        ResultSet rs;
        try {
            pstmt = conn.prepareStatement(sql);
            pstmt.setInt(1, id);
            rs = pstmt.executeQuery();
            if (rs.next()) {
                doctor = new Doctor(rs.getInt("id"), rs.getString("name"), rs.getInt("age"), rs.getString("gender"), rs.getString("department"), rs.getString("position"));
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return doctor;
    }

    @Override
    public List<Doctor> getAllDoctors() {
        String sql = "select * from doctor";
        List<Doctor> doctors = new ArrayList<Doctor>();
        PreparedStatement pstmt;
        ResultSet rs;
        try {
            pstmt = conn.prepareStatement(sql);
            rs = pstmt.executeQuery();
            while (rs.next()) {
                Doctor doctor = new Doctor(rs.getInt("id"), rs.getString("name"), rs.getInt("age"), rs.getString("gender"), rs.getString("department"), rs.getString("position"));
                doctors.add(doctor);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return doctors;
    }
}

4.最后在主函数中进行测试。

public class Test {
    public static void main(String[] args) {
        Connection conn = DBUtil.getConnection(); // 连接数据库
        DoctorDao dao = new DoctorDaoImpl(conn); // 创建DoctorDaoImpl实例
        // 添加医生
        Doctor doctor1 = new Doctor(1, "张三", 30, "男", "内科", "主治医师");
        dao.addDoctor(doctor1);
        Doctor doctor2 = new Doctor(2, "李四", 35, "女", "外科", "副主任医师");
        dao.addDoctor(doctor2);
        // 获取所有医生信息
        List<Doctor> doctors = dao.getAllDoctors();
        for (Doctor doctor : doctors) {
            System.out.println(doctor.getName());
        }
        // 更新医生信息
        Doctor doctor3 = new Doctor(1, "张三", 30, "男", "外科", "主任医师");
        dao.updateDoctor(doctor3);
        // 根据id查找医生
        Doctor doctor4 = dao.findDoctorById(1);
        System.out.println(doctor4.getName());
        // 删除医生
        dao.deleteDoctor(1);
        dao.deleteDoctor(2);
    }
}

以上是一个简单的医生管理系统的Java代码实现示例。具体的系统开发需要根据具体需求进行设计和实现。