========================ClassRoom.java==============================

public class ClassRoom

{
 private Student[] students = new Student[10];
    private int size;
 
 
 
 
 
 public void printStudent(){
        for(int i= 0; i<size; i++){
   students[i].printStudent();
  }

 }

 public void addStudent(Student st){

  students[size++] = st;  //++size 먼저 1이 증가함. // 이것은 먼저 0을 저장하고 1을 더함

 }

 public int getSize(){
  return size;
 }

 public void setClassRank(){
         Student[] temp = new Student[size];

   for(int i=0; i<size; i++){
    temp[i] =students[i];
   }

   for(int i= 0; i<size-1; i++){
    for(int j=i+1; j<size; j++){
     if (temp[i].getTot() < temp[j].getTot()){
      Student t = temp[i];
      temp[i] = temp[j];
      temp[j] = t;
     }
    }
   }
   for(int i=0; i<size; i++){
                 temp[i].setClassRank(i+1);
   }
 }

     
     public Student getStudent(int index){
 
  return students[index];
 }

 
}
================ScoreManager.java=========================================================

import java.util.*;

public class ScoreManager {
 
 private Scanner sc;
 private boolean bContinue=true;
 private Student[] classRoom;
 private int no = 0;
 int classNum = 0;
 ClassRoom[] classes;

 public static void main(String[] args) {
  new ScoreManager().go();
 }

 private void go() {
  //정의
  sc = new Scanner(System.in);
  classRoom = new Student[10];
  
  
  showTitle();
  classNum = howManyClasses();
  classes = new ClassRoom[classNum];
  for( int i = 0; i<classNum; i++){
   classes[i] = new ClassRoom();
  }
  
  while(bContinue){
   int menu = showMenu();
   
   switch(menu){
   case 0:
    System.out.println("프로그램을 종료합니다....");
    bContinue=false;
    break;
   case 1:
    inputStudent();
    break;
   case 2:
    showScoreTable();
    break;
   default:
    System.out.println("메뉴는 (0-2) 사이의 값만 선택가능합니다.");
    break;
   }
  }
  
 }

 private void showScoreTable() {
  System.out.println();
  System.out.println("=====================성적 처리 결과========================");
  System.out.println("반 번호 이름   국어 영어 수학 총점 평균 학점 반석차 전체석차");
  System.out.println("===========================================================");
  setTotalRank();
  
  
  for(int i=0; i<classNum; i++){
    classes[i].setClassRank();
          classes[i].printStudent(); //classes[0] 종료후 --------------출력, classes[1] 종료후ㅠ -------출력 classes[classnum-1] 종료후====== 출력
   
    
    
    
    System.out.println("----------------------------------------------------------");
    
   }
    
    
   
   
   System.out.println("===========================================================");
   
  
 }
  

 private void setTotalRank(){

        int totalSize = 0;

  for(int i=0; i<classNum; i++){
   totalSize += classes[i].getSize();
  }
  
  Student[] temp = new Student[totalSize];
        int count = 0;
        for(int i=0; i<classNum; i++){
   for(int j=0; j<classes[i].getSize(); j++){
    temp[count++] = classes[i].getStudent(j);
   }
  }

  //정렬과정
        for(int i=0; i<totalSize-1; i++){
     for(int j=i+1; j<totalSize; j++){
    if(temp[i].getTot() < temp[j].getTot())
    {
      Student t = temp[i];
      temp[i] = temp[j];
      temp[j] = t;
    }
   }
  }
     for(int i=0; i<totalSize; i++){
                 temp[i].setTotalRank(i+1);
   }
 
 

 }

 private void inputStudent() {
  try{
  
  

  System.out.println("======성적 입력=====");
  System.out.print("몇 반 학생을 추가합니까?(1~"+classNum+")");
  int ban = sc.nextInt();
  int no = classes[ban-1].getSize()+1;
   System.out.print(no+"번 학생 이름 = ");
   sc.reset();
   String name = sc.next();
   sc.reset();
   System.out.print("국어 = ");
   int kor = sc.nextInt();
   System.out.print("영어 = ");
   int eng = sc.nextInt();
   System.out.print("수학 = ");
   int mat = sc.nextInt();
            //학생 생성;
            Student st = new Student(ban, no, name, kor, eng, mat);
   
   classes[ban-1].addStudent(st);  //student[size++] = st;  size=0  size = size + 1;

  } catch (Exception e){
   System.out.println("잘못된 정보 입력으로 메뉴로 돌아갑니다.");
   sc.nextLine();
   return;
  
  
  
  }
  
   
 }

 private void showTitle() {
  System.out.println();
  System.out.println("==========================================");
  System.out.println("=            성적 관리 프로그램          =");
  System.out.println("=                              ver 0.2   =");
  System.out.println("=                        created by JI   =");
  System.out.println("= 설명:반별로 성적을 관리할 수 있습니다. =");
  System.out.println("==========================================");
  
 }

 private int showMenu() {
  int menu=-1;
  System.out.println();
  System.out.println("===================메 뉴===================");
  System.out.println("0 : 프로그램 종료");
  System.out.println("1 : 성적 입력");
  System.out.println("2 : 성적 출력");
  System.out.println("===========================================");
  System.out.print("선택 >> ");
  try{
   menu = sc.nextInt();
  } catch (Exception e){
   System.out.println("숫자만 입력 가능합니다.");
   sc.nextLine();   
  }
  return menu;
 }

   private int howManyClasses() {
     System.out.print("몇 개의 학급을 생성하겠습니다?");
  int num = sc.nextInt();
  return num;
   }

}
===========================================Student.java==========================================
public class Student {
 private int classNum;
 private String name;
 private int no;
 private int kor;
 private int eng;
 private int mat;
 private int tot;
 private double avg;
 private String grade;
 private int classRank;
 private int totalRank;
 
 
    public Student(int ban, int no, String name, int kor, int eng, int mat){

       classNum = ban;
    this.no = no;//(class에 있는 변수를 사용)  this.no는 class에 , no은 Student 생성자)
       this.name = name;
    this.kor = kor;
    this.eng = eng;
    this.mat = mat;
    calcTot();
    calcAvg();
    calcGrade();

 }
    public void calcGrade(){
  if(avg >=90) grade = "A";
  else if(avg >=80) grade = "B";
  else if(avg >=70) grade = "C";
  else if(avg >=60) grade = "D";
  else grade = "F";

 }

 public void calcTot(){
  tot = kor+eng+mat;  
 }
 
 public void calcAvg(){
  avg = tot/3.0f;
 }

    public void printStudent(){
  System.out.printf("%2d %4d %3s %4d %4d %4d %4d %4.1f %2s %6d %8d \\n",
        classNum, no, name, kor, eng, mat, tot, avg, grade, classRank, totalRank);
  
 }

 public int getTot(){
  return tot;
 }

 public void setClassRank(int rank){
  classRank = rank;
 }

 public void setTotalRank(int rank){
  totalRank = rank;
 }

}