菜鸡的Java笔记 - java 双向一对多映射

双向一对多映射
    two-way

    开发要求：
        根据数据表的结构进行简单java类的转换：
        要求实现如下的输出信息：
            可以根据课程取得全部参与此课程用户的信息
                输出课程信息：
                输出参与此课程用户的信息以及考试成绩
            用户可以取得自己所参加的课程信息
                输出某一个用户的信息
                输出该用户所参加的所有课程信息以及对应的考试成绩
        关系上来讲：一个用户可以参加多门课程，一门课程可以有多个用户参加，每个用户在每个课程内都会有一个成绩
        此时最麻烦的问题在于用户-课程关系表中除了关联字段之外，还包含有其他字段，这样的表一定要作为一个实体类出现
        所以现在需要定义有三个类
        第一步：先完成基本字段

class User{
    private String userid:
    private String name:
    public User(String userid,String name){
        this.userid = userid:
        this.name = name:
    }
    public String getlnfo(){
        return     "用户编号："+this.userid
                +"，姓名"+this.name:
    }
}
class Course{
    private int cid:
    private String title:
    private int num:
    private String note:
    public Course(int cid,String title,int num,String note){
        this.cid = cid:
        this.title = title:
        this.num = num:
        this.note = note:
    }
    public String getlnfo(){
        return     "课程编号："+this.cid
                +"，名称："+this.title
                +"，课时："+this.num
                +"，简介："+this.note:
    }
}
public class TwoWay{
    public static void main(String args[]){

    }
}

第二步：进行字段关联的时候都是以外键为主
            为了可以进行关联，需要引入一个新的类：要保存用户，课程等信息的联系

class User{
    private String userid;
    private String name;
    public User(String userid,String name){
        this.userid = userid;
        this.name = name;
    }
    public String getlnfo(){
        return     "用户编号："+this.userid
                +"，姓名"+this.name;
    }
}
class Course{
    private int cid;
    private String title;
    private int num;
    private String note;
    public Course(int cid,String title,int num,String note){
        this.cid = cid;
        this.title = title;
        this.num = num;
        this.note = note;
    }
    public String getlnfo(){
        return     "课程编号："+this.cid
                +"，名称："+this.title
                +"，课时："+this.num
                +"，简介："+this.note;
    }
    public Course getCourse(){
        return this.course;
    }
    public User getUser(){
        return this.user;
    }
}
class UserCourse{
    private User user;
    private Course course;
    private String note;
    private double score;
    public UserCourse(User user,Course course,String note,double score){
        this.user = user;
        this.course = course;
        this.note = note;
        this.score = score;
    }
}
public class TwoWay{
    public static void main(String args[]){

    }
}

第三步：程序测试

class User{
    private String userid;
    private String name;
    private UserCourse ucs[];
    public User(String userid,String name){
        this.userid = userid;
        this.name = name;
    }
    public void setUcs(UserCourse ucs[]){
        this.ucs = ucs;
    }
    public UserCourse[] getUcs(){
        return this.ucs;
    }
    public String getlnfo(){
        return     "用户编号："+this.userid
                +"，姓名"+this.name;
    }
}
class Course{
    private int cid;
    private String title;
    private int num;
    private String note;
    private UserCourse ucs[];
    public Course(int cid,String title,int num,String note){
        this.cid = cid;
        this.title = title;
        this.num = num;
        this.note = note;
    }
    public void setUcs(UserCourse ucs[]){
        this.ucs = ucs;
    }
    public UserCourse[] getUcs(){
        return this.ucs;
    }
    public String getlnfo(){
        return     "课程编号："+this.cid
                +"，名称："+this.title
                +"，课时："+this.num
                +"，简介："+this.note;
    }

}
class UserCourse{
    private User user;
    private Course course;
    private String note;
    private double score;
    public UserCourse(User user,Course course,String note,double score){
        this.user = user;
        this.course = course;
        this.note = note;
        this.score = score;
    }
    public double getScore(){
        return this.score;
    }
    public Course getCourse(){
        return this.course;
    }
    public User getUser(){
        return this.user;
    }
}
public class TwoWay{
    public static void main(String args[]){
        //第一步：设置类与类之间的关系
        //1.定义单独的类对象
        User ua = new User("zhangsan","张三");
        User ub = new User("lisi","李四");
        User uc = new User("wangwu","王五");
        Course c1 = new Course(1,"Oracle",50,"-");
        Course c2 = new Course(2,"java",300,"-");
        //2.设置彼此的关系
        UserCourse uca = new UserCourse(ua,c1,"暂无评价",90.0);
        UserCourse ucb = new UserCourse(ua,c2,"暂无评价",91.0);
        UserCourse ucc = new UserCourse(ub,c1,"暂无评价",92.0);
        UserCourse ucd = new UserCourse(uc,c1,"暂无评价",93.0);
        UserCourse uce = new UserCourse(uc,c2,"暂无评价",94.0);
        //
        ua.setUcs(new UserCourse[]{uca,ucb});
        ub.setUcs(new UserCourse[]{ucc});
        uc.setUcs(new UserCourse[]{ucd,uce});
        c1.setUcs(new UserCourse[]{uca,ucc,ucd});
        c2.setUcs(new UserCourse[]{ucb,uce});
        // 第二步：取得数据
        System.out.println(c1.getlnfo()); // 输出一个课程信息
        for(int x = 0;x<c1.getUcs().length;x++){ // 该门课程的用户信息
            System.out.println("\t|-【参与用户】 "+c1.getUcs()[x].getUser().getlnfo()+"，考试成绩"+c1.getUcs()[x].getScore());
        }
        System.out.println("*******************************************");
        System.out.println(ua.getlnfo());
        for(int x = 0;x<ua.getUcs().length;x++){// 都是UserCourse对象
            System.out.println("\t|-【参与用户】 "+ua.getUcs()[x].getCourse().getlnfo()+"，考试成绩"+ua.getUcs()[x].getScore());
        }
    }
}

本程序与之前的代码相比，唯一麻烦的地方在于中间的关系表上的其他字段
            
            代码链是本次讲解的重点所在
            
            不晕的方法（笨方法容易理解的方法）

    System.out.println(ua.getlnfo());
     UserCourse uct[] = ua.getUcs();
        for(int x = 0;x<uct.length;x++){// 都是UserCourse对象
        Course c = uct[x].getCourse();
            System.out.println("\t|-【参与用户】 "+c.getlnfo()+"，考试成绩"+uct[x].getScore());
        }