Java中数据表的建立

class Emp{
    private int empno;//职工编号
    private String ename;//姓名
    private String job;//职位
    private double sal;//基本工资
    private double comm;
    private Emp mgr;//所属领导
    private  Dept dept;//所在部门

    public Emp(){}//无参构造

    public Emp(int empno,String ename,String job,double sal,double comm){//有参构造
        this.empno = empno;
        this.ename = ename;
        this.job = job;
        this.sal = sal;
        this.comm = comm;
    }

    public void setMgr(Emp mgr) {//传递领导信息
        this.mgr = mgr;
    }
    public Emp getMgr(){//获取领导信息
        return this.mgr;
    }

    public void setDept(Dept dept) {//设置部门信息
        this.dept = dept;
    }

    public Dept getDept() {//读取部门信息
        return dept;
    }

    public String getInfo(){
        return "职工编号："+this.empno +"，职工姓名：" + this.ename + "，职工职位：" + this.job + "，基本工资：" + this.sal
                +"，提成：" + this.comm;
    }
}
class Dept{
    private int deptno;
    private String dname;//部门名称
    private String loc;//部门地址
    private Emp[] emps;//所有员工
    public Dept(){ }
    public Dept(int deptno,String dname,String loc){
        this.deptno = deptno;
        this.dname = dname;
        this.loc = loc;
    }

    public void setEmps(Emp[] emps) {
        this.emps = emps;
    }

    public Emp[] getEmps() {
        return emps;
    }

    public String getInfo(){
        return "部门编号：" + this.deptno +"，部门名称：" + this.dname +",所在地址：" + this.loc;
    }
}
public class Main {
    public static void main(String[] args) {
        // 创建各自的实例化对象
        Dept dept = new Dept(10,"ACCOUNTING","NEW YORK");
        Emp ea = new Emp(7369,"SMITH","CLERK",800.0,0.0);
        Emp eb = new Emp(7466,"ALLEN","MANAGER",2450.0,0.0);
        Emp ec = new Emp(7839,"KING","PRESIDENT",5000.0,0.0);
        //设置领导关系
        ea.setMgr(eb);
        eb.setMgr(ec);//ec对象没有领导，因为他是头
        //设置员工和部门关系
        ea.setDept(dept);
        eb.setDept(dept);
        ec.setDept(dept);
        //设置部门和员工的关系
        dept.setEmps(new Emp[]{ea,eb,ec});

        //读取数据
        System.out.println(dept.getInfo());//输出部门信息
        for(int x=0;x<dept.getEmps().length;x++){
            System.out.println("\t"+dept.getEmps()[x].getInfo());
            if(dept.getEmps()[x].getMgr()!=null){ //若有领导
                System.out.println("\t"+dept.getEmps()[x].getMgr().getInfo());
            }
        }
        System.out.println("************************");
        //根据员工找到对应的领导信息和部门信息
        System.out.println(eb.getInfo());
        if(eb.getMgr()!=null)
            System.out.println("\t"+eb.getMgr().getInfo());
        if(eb.getDept()!=null)
            System.out.println("\t"+eb.getDept().getInfo());
    }
}

上面代码是实现一对多的数据表，基于公司人事管理的例子（根据员工可以输出其基本信息及所在部门信息和所属领导信息，根据部门可以输出所有员工及领导），在Java一对多的数据关系中，需要遵循以下设计原则：

简单Java类设计原则：Java类与数据表的关系

　　1、Java的名称 = 实体表的名称

　　2、Java类的属性 = 实体表的字段

　　3、Java类的一个对象 = 表的一行记录

　　4、对象数组 = 表的多行记录

　　5、外键关系 = 引用配置

多对多的数据表（学生成绩管理）：根据学生可以输出所修课程信息及成绩，根据课程可以输出学习该课程的学生信息及成绩

class Student{//学生表
    private int stuid;
    private String sname;
    private int age;
    private StudentCourse studentCourse[];//学生成绩信息
    public Student(){}
    public Student(int stuid,String sname,int age){
        this.stuid = stuid;
        this.sname = sname;
        this.age = age;
    }
    public void setStudentCourse(StudentCourse[] studentCourse) {
        this.studentCourse = studentCourse;
    }

    public StudentCourse[] getStudentCourse() {
        return studentCourse;
    }

    public String getInfo(){
        return "学号：" + this.stuid + "，学生姓名：" + this.sname + "，年龄：" + this.age;
    }
}
class Course{//课程表
    private String name;
    private int cid;
    private int credit;
    private StudentCourse studentCourse[];
    public Course(){}
    public Course(String name,int cid,int credit){
        this.cid = cid;
        this.name = name;
        this.credit = credit;
    }

    public StudentCourse[] getStudentCourse() {
        return studentCourse;
    }

    public void setStudentCourse(StudentCourse[] studentCourse) {
        this.studentCourse = studentCourse;
    }

    public String getInfo(){
        return "课号：" + this.credit + "，名称：" + this.name + ",学分：" + this.credit;
    }
}
class StudentCourse{//学生选课表
    private Student student;
    private Course course;
    private double score;//成绩
    public StudentCourse(){
    }
    public StudentCourse(Student student,Course course,double score){
        this.course = course;
        this.score = score;
        this.student = student;
    }

    public Course getCourse() {
        return course;
    }

    public Student getStudent() {
        return student;
    }

    public double getScore() {
        return score;
    }
}
public class Main {

    public static void main(String[] args) {
        //创建各自的独立对象
        Student stu1 = new Student(1,"张三",18);
        Student stu2 = new Student(1,"李四",20);
        Student stu3 = new Student(1,"王五",19);
        Course ca = new Course("高等数学",1001,5);
        Course cb = new Course("线性代数",1002,4);
        //设置各自的关系
        //设置学生和课程的关系
        stu1.setStudentCourse(new StudentCourse[]{
                new StudentCourse(stu1,ca,92.5),
                new StudentCourse(stu1,cb,94.0)
        });
        stu2.setStudentCourse(new StudentCourse[]{
                new StudentCourse(stu2,ca,89.0)
        });
        stu3.setStudentCourse(new StudentCourse[]{
                new StudentCourse(stu3,cb,95.0),
                new StudentCourse(stu3,ca,90.5)
        });
        //设置课程和学生的关系
        ca.setStudentCourse(new StudentCourse[]{
                new StudentCourse(stu1,ca,92.5),
                new StudentCourse(stu2,ca,89.0)
                new StudentCourse(stu3,ca,90.5)
        });
        cb.setStudentCourse(new StudentCourse[]{
                new StudentCourse(stu1,cb,94.0),
                new StudentCourse(stu3,cb,95.0)
        });
        //找到一门课程，并且输出学习此课程的所有学生的信息及成绩
        System.out.println(ca.getInfo());
        for(int x = 0;x < ca.getStudentCourse().length;x++){
            System.out.println("\t"+ca.getStudentCourse()[x].getStudent().getInfo()
                    +",成绩："+ca.getStudentCourse()[x].getScore());
        };
        System.out.println("***********");
        //根据学生输出其信息及选修的课程信息和所得成绩
        System.out.println(stu1.getInfo());
        for(int x = 0;x < stu1.getStudentCourse().length;x++){
            System.out.println("\t选修课程：" + stu1.getStudentCourse()[x].getCourse().getInfo()
                    + "，所得成绩：" + stu1.getStudentCourse()[x].getScore());
        }

    }
}