学习Java必备的基础知识09，要想学好必须扎实基本功（⭐建议收藏）#yyds干货盘点#

Posted 2021-12-03 java厂长

Day09-Java

@[toc]

关于作者

• 作者介绍

//电脑
class Computer{
    private View [];
    private Host;

}
//显示器
class View{

}
//主机
class Host{
    private Board;

}
//主板
class Board{
    private Cpu [];
    private Memory[];
    private Disk[];
}
//CPU
class Cpu{

}
//内存
class Memory{

}
//硬盘
class Disk{

}
public class TestDemo3{
    public static void main(String args[]){

    }
}

重点

数据表与简单Java类（一对多）

利用此关系模型，表示出emp和dept的关系，使用字段：

​ emp表：empno、ename、job、sal、comm、mgr、deptno；

​ dept表：deptno、dname、loc。

第一步 编写实体类

class Emp{
    private int empno;
    private String ename;
    private String job;
    private double sal;
    private double comm;
    public Emp(int empno,String ename,String job,double sal,double comm){
        this.empno = empno;
        this.ename = ename;
        this.job = job;
        this.sal = sal;
        this.comm = comm;
    }
    public String getEmpInfo(){
        return "【Emp】 empno = " + this.empno +
                ",ename = " + this.ename +
                ",job； = " + this.job +
                ",sal = " + this.sal +
                ",comm = " + this.comm;
    }
}
class Dept{
    private int deptno;
    private String dname;
    private String loc;
    public Dept(int deptno, String dname, String loc){
            this.deptno = deptno;
            this.dname = dname;
            this.loc = loc;
    }
    public String getDept(){
        return "deptno = " + this.deptno +
                ",dname = " + this.dname +
                ",loc = " + this.loc;
    }
}

public class Exam1_7{
    public static void main(String args[]){

    }
}

第二步 进行关系设计

一个雇员属于一个部门，需要追加部门引用

一个雇员有一个领导，领导一定是自身关联

一个部门有一个雇员，需要一个对象数组来描述多个雇员信息

class Emp{
    private int empno;
    private String ename;
    private String job;
    private double sal;
    private double comm;
    private Emp mgr; //描述雇员的领导
    private Dept dept;  //描述雇员的部门
    public Emp(int empno,String ename,String job,double sal,double comm){
        this.empno = empno;
        this.ename = ename;
        this.job = job;
        this.sal = sal;
        this.comm = comm;
    }
    public void setMgr(Emp mgr){
        this.mgr  = mgr;
    }
    public Emp getMgr(){
        return this.mgr;
    }
    public void setDept(Dept dept){
        this.dept = dept;
    }
    public Dept getDept(){
        return this.dept;
    }
    public String getEmpInfo(){
        return "【Emp】 empno = " + this.empno +
                ",ename = " + this.ename +
                ",job； = " + this.job +
                ",sal = " + this.sal +
                ",comm = " + this.comm;
    }
}
class Dept{
    private int deptno;
    private String dname;
    private String loc;
    private Emp [] emps;    //一个部门有多个雇员
    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 this.emps;
    }
    public String getDept(){
        return "deptno = " + this.deptno +
                ",dname = " + this.dname +
                ",loc = " + this.loc;
    }
}

public class Exam1_7{
    public static void main(String args[]){

    }
}

此时基本类定义完成。

第三步 开发需求

主函数main

public class Exam1_7{
    public static void main(String args[]){
        //第一步、设置类对象的关系
        //1.分别创建各自对象实例化
        Dept dept = new Dept(10,"市场部","New York");
        Emp ea = new Emp(7345,"Rock","CLERK",800.0,0);
        Emp eb = new Emp(7567,"Joker","MANAGER",3050.0,0);
        Emp ec = new Emp(7825,"Ben","PRESIDENT",10000.0,0);
        //2.设置雇员和领导的关系
        ea.setMgr(eb);
        eb.setMgr(ec);  //ec没有领导 自己就是最大的领导
        //3.设置雇员和部门的关系
        ea.setDept(dept);
        eb.setDept(dept);
        ec.setDept(dept);
        //4.设置部门的雇员
        dept.setEmps(new Emp[]{ea, eb, ec});
        //第二步、进行数据的取得
        //一个部门有多个雇员
        /* 
            【Emp】 empno = 7345,ename = Rock,job = CLERK,sal = 800.0,comm = 0.0
            【Emp】 empno = 7567,ename = Joker,job = MANAGER,sal = 3050.0,comm = 0.0
            【Emp】 empno = 7825,ename = Ben,job = PRESIDENT,sal = 10000.0,comm = 0.0
        */
        for(int i = 0; i <  dept.getEmps().length; i++){
            System.out.println(dept.getEmps()[i].getEmpInfo());
        }
        System.out.println();

        //一个雇员有一个领导 
        //【Emp】 empno = 7567,ename = Joker,job = MANAGER,sal = 3050.0,comm = 0.0
        System.out.println(ea.getMgr().getEmpInfo());
        //【Emp】 empno = 7825,ename = Ben,job = PRESIDENT,sal = 10000.0,comm = 0.0
        System.out.println(eb.getMgr().getEmpInfo());
        //Exception in thread "main" java.lang.NullPointerException 
        //没有设置关系 所以自己是老板 显示空指针异常
        //System.out.println(ec.getMgr().getEmpInfo());
        System.out.println();

        //一个雇员属于同个部门
        //deptno = 10,dname = 市场部,loc = New York
        System.out.println(ea.getDept().getDeptInfo());
        //deptno = 10,dname = 市场部,loc = New York
        System.out.println(eb.getDept().getDeptInfo());
        //deptno = 10,dname = 市场部,loc = New York
        System.out.println(ec.getDept().getDeptInfo());

    }
}

在上面的代码基础上进行改进

//一个部门有多个雇员,一个雇员有一个领导,一个雇员属于同个部门
         /* 
【Emp】 empno = 7345,ename = Rock,job = CLERK,sal = 800.0,comm = 0.0
该雇员所属的领导【Emp】 empno = 7567,ename = Joker,job = MANAGER,sal = 3050.0,comm = 0.0
该雇员所属的部门deptno = 10,dname = 市场部,loc = New York
------------------------------------------------------------------
【Emp】 empno = 7567,ename = Joker,job = MANAGER,sal = 3050.0,comm = 0.0
该雇员所属的领导【Emp】 empno = 7825,ename = Ben,job = PRESIDENT,sal = 10000.0,comm = 0.0
该雇员所属的部门deptno = 10,dname = 市场部,loc = New York
------------------------------------------------------------------
【Emp】 empno = 7825,ename = Ben,job = PRESIDENT,sal = 10000.0,comm = 0.0
自己就是领导
------------------------------------------------------------------
         */
        for(int i = 0; i <  dept.getEmps().length; i++){
            System.out.println(dept.getEmps()[i].getEmpInfo());         
            if(dept.getEmps()[i].getMgr() != null){
                System.out.println("该雇员所属的领导" +
                dept.getEmps()[i].getMgr().getEmpInfo());
                System.out.println("该雇员所属的部门" +
                dept.getEmps()[i].getDept().getDeptInfo());
                System.out.println("------------------------------------------------------------------");
            }
            else{
                System.out.println("自己就是领导");
                System.out.println("------------------------------------------------------------------");
            }

        }

数据表与简单Java类（多对多）

利用此关系模型，表示出student和course的关系，使用字段：

​ student表：stuid、name、age。

​ course表：cid、name、credit。

​ 关系表：学生编号、课程编号、成绩

要求：

可以找到一门课程，参加此次课程的所有学生信息和成绩

可以根据一个学生，找到所参加的所有课程和没门课程的一个成绩

第一步 编写实体类

class Student{
    private int stuid;
    private String name;
    private int age;
    public Student(int stuid, String name, int age){
        this.stuid = stuid;
        this.name = name;
        this.age = age;
    }
    public String getStuInfo(){
        return "【Student】stuid = " + this.stuid +
                ",name = " + this.name +
                ",age = " +this.age ;
    }
}
class Course{
    private int cid;
    private String name;
    private double credit;
    public Course(int cid, String name, double credit){
        this.cid = cid;
        this.name = name;
        this.credit = credit;
    }
    public String getCouInfo(){
        return "【Course】cid = " + this.cid +
                ",name = " + this.name +
                ",credit = " + this.credit;
    }

}
class StudentCourse{//学生选课
    private Student student;
    private Course course;
    private double score;
    public StudentCourse(Student student, Course course, double score){
        this.student =student;
        this.course = course;
        this.score = score;
    }
    public Student getStudent(){
        return this.student;
    }
    public Course getCrouse(){
        return this.course;
    }
    public double getScore(){
        return this.score;
    }

}

第二步 进行关系设计

class Student{
    private int stuid;
    private String name;
    private int age;
    private StudentCourse studentCourses [];
    public Student(int stuid, String name, int age){
        this.stuid = stuid;
        this.name = name;
        this.age = age;
    }
    public void setStudentCourses(StudentCourse []studentCourses){
        this.studentCourses = studentCourses;
    }
    public StudentCourse[] getStudentCourses(){
        return this.studentCourses;
    }
    public String getStuInfo(){
        return "【Student】stuid = " + this.stuid +
                ",name = " + this.name +
                ",age = " +this.age ;
    }
}
class Course{
    private int cid;
    private String name;
    private double credit;
    private StudentCourse studentCourses[];
    public Course(int cid, String name, double credit){
        this.cid = cid;
        this.name = name;
        this.credit = credit;
    }
    public void setStudentCourses(StudentCourse []studentCourses){
        this.studentCourses = studentCourses;
    }
    public StudentCourse[] getstudentCourses(){
        return this.studentCourses;
    }
    public String getCouInfo(){
        return "【Course】cid = " + this.cid +
                ",name = " + this.name +
                ",credit = " + this.credit;
    }

}
class StudentCourse{//学生选课
    private Student student;
    private Course course;
    private double score;
    public StudentCourse(Student student, Course course, double score){
        this.student =student;
        this.course = course;
        this.score = score;
    }
    public Student getStudent(){
        return this.student;
    }
    public Course getCrouse(){
        return this.course;
    }
    public double getScore(){
        return this.score;
    }

}

第三步 开发需求

主函数main

public class Exam1_8{
    public static void main(String args[]){
        //第一步、设置类对象的关系
        //1.分别创建各自对象实例化
        Student stu1 = new Student(107,"Rock",13);
        Student stu2 = new Student(108,"Joker",18);
        Student stu3 = new Student(109,"Perke",22);
        Course ca = new Course(1,"数据结构",4.0);
        Course cb = new Course(2,"计算机操作系统",2.0);
        Course cc = new Course(3,"SSM框架集合",3.0);

        //2.设置学生和课程的关系
        stu1.setStudentCourses(new StudentCourse[]{
            new StudentCourse(stu1,ca,78.0),
        });
        stu2.setStudentCourses(new StudentCourse[]{
            new StudentCourse(stu2,ca,87.0),
            new StudentCourse(stu2,cb,79.0)
        });
        stu3.setStudentCourses(new StudentCourse[]{
            new StudentCourse(stu3,ca,90.0),
            new StudentCourse(stu3,cb,95.0),
            new StudentCourse(stu3,cc,99.0)
        });

        //3.设置课程和学生的关系
        ca.setStudentCourses(new StudentCourse[]{
            new StudentCourse(stu1,ca,78.0),
            new StudentCourse(stu2,ca,87.0),
            new StudentCourse(stu3,ca,90.0)
        });
        cb.setStudentCourses(new StudentCourse[]{
            new StudentCourse(stu2,cb,79.0),
            new StudentCourse(stu3,ca,90.0)
        });
        cc.setStudentCourses(new StudentCourse[]{
            new StudentCourse(stu3,cc,99.0)
        });

        //第二步、进行数据的取得
        //可以找到一门课程，参加此次课程的所有学生信息和成绩
        /* 
【Course】cid = 1,name = 数据结构,credit = 4.0
学生名单：【Student】stuid = 107,name = Rock,age = 13,成绩：78.0
学生名单：【Student】stuid = 108,name = Joker,age = 18,成绩：87.0
学生名单：【Student】stuid = 109,name = Perke,age = 22,成绩：90.0
        */
        System.out.println(ca.getCouInfo());
        for(int i = 0 ;i < ca.getStudentCourses().length ; i++){
            System.out.print("学生名单："+ ca.getStudentCourses()[i].getStudent().getStuInfo());
            System.out.println(",score = "+ ca.getStudentCourses()[i].getScore());  
        }

        System.out.println("--------------------------------------------------------------------");

         //可以根据一个学生，找到所参加的所有课程和没门课程的一个成绩
         /* 
【Student】stuid = 109,name = Perke,age = 22
选课列表：【Course】cid = 1,name = 数据结构,credit = 4.0,score = 90.0
选课列表：【Course】cid = 2,name = 计算机操作系统,credit = 2.0,score = 95.0
选课列表：【Course】cid = 3,name = SSM框架集合,credit = 3.0,score = 99.0
         */
         System.out.println(stu3.getStuInfo());
        for(int i = 0; i < stu3.getStudentCourses().length ; i++){
            System.out.print("选课列表：" + stu3.getStudentCourses()[i].getCourse().getCouInfo());
            System.out.println(",score = "+ stu3.getStudentCourses()[i].getScore());
        }   
    }
}