Java中數據表的建立

class Emp{
    private int empno;//職工編號
    private String ename;//姓名
    private String job;//職位
    private double sal;//基本工資
    private double comm;
    private Emp mgr;//所屬領導
    private  Dept dept;//所在部門

    public Emp(){}//無參構造

    public Emp(int empno,String ename,String job,double sal,double comm){//有參構造
        this.empno = empno;
        this.ename = ename;
        this.job = job;
        this.sal = sal;
        this.comm = comm;
    }

    public void setMgr(Emp mgr) {//傳遞領導信息
        this.mgr = mgr;
    }
    public Emp getMgr(){//獲取領導信息
        return this.mgr;
    }

    public void setDept(Dept dept) {//設置部門信息
        this.dept = dept;
    }

    public Dept getDept() {//讀取部門信息
        return dept;
    }

    public String getInfo(){
        return "職工編號："+this.empno +"，職工姓名：" + this.ename + "，職工職位：" + this.job + "，基本工資：" + this.sal
                +"，提成：" + this.comm;
    }
}
class Dept{
    private int deptno;
    private String dname;//部門名稱
    private String loc;//部門地址
    private Emp[] emps;//所有員工
    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 "部門編號：" + this.deptno +"，部門名稱：" + this.dname +",所在地址：" + this.loc;
    }
}
public class Main {
    public static void main(String[] args) {
        // 創建各自的實例化對象
        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);
        //設置領導關系
        ea.setMgr(eb);
        eb.setMgr(ec);//ec對象沒有領導，因為他是頭
        //設置員工和部門關系
        ea.setDept(dept);
        eb.setDept(dept);
        ec.setDept(dept);
        //設置部門和員工的關系
        dept.setEmps(new Emp[]{ea,eb,ec});

        //讀取數據
        System.out.println(dept.getInfo());//輸出部門信息
        for(int x=0;x<dept.getEmps().length;x++){
            System.out.println("\t"+dept.getEmps()[x].getInfo());
            if(dept.getEmps()[x].getMgr()!=null){ //若有領導
                System.out.println("\t"+dept.getEmps()[x].getMgr().getInfo());
            }
        }
        System.out.println("************************");
        //根據員工找到對應的領導信息和部門信息
        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());
    }
}

上面代碼是實現一對多的數據表，基於公司人事管理的例子（根據員工可以輸出其基本信息及所在部門信息和所屬領導信息，根據部門可以輸出所有員工及領導），在Java一對多的數據關系中，需要遵循以下設計原則：

簡單Java類設計原則：Java類與數據表的關系

　　1、Java的名稱 = 實體表的名稱

　　2、Java類的屬性 = 實體表的字段

　　3、Java類的一個對象 = 表的一行記錄

　　4、對象數組 = 表的多行記錄

　　5、外鍵關系 = 引用配置

 

多對多的數據表（學生成績管理）：根據學生可以輸出所修課程信息及成績，根據課程可以輸出學習該課程的學生信息及成績

class Student{//學生表
    private int stuid;
    private String sname;
    private int age;
    private StudentCourse studentCourse[];//學生成績信息
    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 "學號：" + this.stuid + "，學生姓名：" + this.sname + "，年齡：" + this.age;
    }
}
class Course{//課程表
    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 "課號：" + this.credit + "，名稱：" + this.name + ",學分：" + this.credit;
    }
}
class StudentCourse{//學生選課表
    private Student student;
    private Course course;
    private double score;//成績
    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) {
        //創建各自的獨立對象
        Student stu1 = new Student(1,"張三",18);
        Student stu2 = new Student(1,"李四",20);
        Student stu3 = new Student(1,"王五",19);
        Course ca = new Course("高等數學",1001,5);
        Course cb = new Course("線性代數",1002,4);
        //設置各自的關系
        //設置學生和課程的關系
        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)
        });
        //設置課程和學生的關系
        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)
        });
        //找到一門課程，並且輸出學習此課程的所有學生的信息及成績
        System.out.println(ca.getInfo());
        for(int x = 0;x < ca.getStudentCourse().length;x++){
            System.out.println("\t"+ca.getStudentCourse()[x].getStudent().getInfo()
                    +",成績："+ca.getStudentCourse()[x].getScore());
        };
        System.out.println("***********");
        //根據學生輸出其信息及選修的課程信息和所得成績
        System.out.println(stu1.getInfo());
        for(int x = 0;x < stu1.getStudentCourse().length;x++){
            System.out.println("\t選修課程：" + stu1.getStudentCourse()[x].getCourse().getInfo()
                    + "，所得成績：" + stu1.getStudentCourse()[x].getScore());
        }

    }
}