程序代码：

package Homework;
import java.util.*;
public class Work9 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        try{
            System.out.println("请输入学生的总人数：");
            int size = sc.nextInt();
            Manage ma = new Manage(size);
            ma.addStu();              //问题从这里开始：跳到下面
            ma.getGrade();
            ma.getStuNumber();
            ma.count();
            ma.removeStu();
        }catch(Exception e){
            e.printStackTrace();
        }finally{
            try{
                sc.close();
            }catch(Exception e){
                e.printStackTrace();
            }
        }
    }
}
class Manage
{
    ArrayList<Student> al = null;
    public Manage(int size)
    {
        al = new ArrayList<Student>(size);
    }

    public void addStu()
    {
        Scanner sc = new Scanner(System.in);
        try{
            System.out.println("cuowu isnot here");
            for(int i=0; i<al.size(); i++)                 //for循环就没有被执行这是为什么呢？
            {
                System.out.println("cuowu isnot here");
                System.out.print("请输入第"+(i+1)+"个学生的学号：");    //另外想问的就是，若创建了多个Scanner输入流，怎么关闭，当关了一个就会抛出异常的，为什么啊？
                String number = sc.nextLine();
                System.out.print("请输入第"+(i+1)+"个学生的姓名：");
                String name = sc.nextLine();
                System.out.print("请输入第"+(i+1)+"个学生的成绩：");
                float grade = sc.nextFloat();
                System.out.println();
           
                Student st = new Student();
                st.setName(name);
                st.setNumber(number);
                st.setGrade(grade);
           
                al.add(st);
            }
        }catch(Exception e){
            e.printStackTrace();
        }
        System.out.println();
    }

    public void getGrade()
    {
        Scanner sc = new Scanner(System.in);
        try{
            System.out.println("请输入该学生的学号： ");
            String number = sc.nextLine();
       
            for(int i=0; i<al.size(); i++)
            {
                Student st = al.get(i);
                if(st.getNumber().equals(number))
                {
                    System.out.println("该学生的成绩为："+st.getGrade());
                }
            }
        }catch(Exception e){
            e.printStackTrace();
        }
        System.out.println();
    }

    public void getStuNumber()
    {
        Scanner sc = new Scanner(System.in);
        try{
            System.out.println("请输入该学生的成绩： ");
            float grade = sc.nextFloat();
       
            for(int i=0; i<al.size(); i++)
            {
                Student st = al.get(i);
                if(st.getGrade()==grade)
                {
                    System.out.println("得到该成绩的学生学号为："+st.getNumber());
                }
            }
        }catch(Exception e){
            e.printStackTrace();
        }
        System.out.println();
    }

    public void count()
    {
        int A = 0;
        int B = 0;
        int C = 0;
        int D = 0;
   
        for(int i=0; i<al.size(); i++)
        {
            Student st = al.get(i);
            float grade = st.getGrade();
       
            if(grade>=90)
            {
                A++;
            }
            else if(grade>=80 && grade<90)
            {
                B++;
            }
            else if(grade>=70 && grade<80)
            {
                C++;
            }
            else
            {
                D++;
            }
        }
   
        System.out.println("得到优的学生共有== "+A+" ==人");
        System.out.println("得到良的学生共有== "+B+" ==人");
        System.out.println("得到中的学生共有== "+C+" ==人");
        System.out.println("得到差的学生共有== "+D+" ==人");
        System.out.println();
    }

    public void removeStu()
    {
        Scanner sc = new Scanner(System.in);
        try{
            System.out.println("请输入想要删除的学生的学号");
            String number = sc.nextLine();
            for(int i=0; i<al.size(); i++)
            {
                Student st = al.get(i);
                if(st.getNumber() == number)
                {
                    al.remove(i);
                    break;
                }
            }
        }catch(Exception e){
            e.printStackTrace();
        }finally{
            try{
                sc.close();
            }catch(Exception e){
                e.printStackTrace();
            }
        }
   
    }
}
class Student
{
    private String number;
    private String name;
    private float grade;

    public float getGrade() {
        return grade;
    }
    public void setGrade(float grade) {
        this.grade = grade;
    }
    public String getNumber() {
        return number;
    }
    public void setNumber(String number) {
        this.number = number;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }

}