ArrayList 的用法

定义一个学生类Student,属性有age，name,score,classNum

有如下：

list.add(new Student("Tom",18,100,"class05"));
list.add(new Student("Jerry",22,70,"class04"));
list.add(new Student("Owen",25,90,"class05"));
list.add(new Student("Jim",30,80,"class05"));
list.add(new Student("Steve",28,66,"class06"));
list.add(new Student("Kevin",24,100,"class04"));

在这个list的基础上，完成下列要求：

1、统计出总班级数

2、统计每个班的平均分和人数

3、按学生的成绩排名（成绩相同的按年龄排名）

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class Student {
     int age;
     String name;
     int score;
     String classNum;
     Student(){
         
     }
     Student(String name,int age,int score,String classNum)
     {
         this.age=age;
         this.name=name;
         this.score=score;
         this.classNum=classNum;
     }
     
     public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getScore() {
        return score;
    }
    public void setScore(int score) {
        this.score = score;
    }
    public String getClassNum() {
        return classNum;
    }
    public void setClassNum(String classNum) {
        this.classNum = classNum;
    }
    
    public static void main(String[] args) {
        List <Student>list=new ArrayList<>();
        list.add(new Student("Tom",18,100,"class05"));
        list.add(new Student("Jerry",22,70,"class04"));
        list.add(new Student("Owen",25,90,"class05"));
        list.add(new Student("Jim",30,80,"class05"));
        list.add(new Student("Steve",28,66,"class06"));
        list.add(new Student("Kevin",24,100,"class04"));
        //统计班级
        Set <String> set=new HashSet<>();
        for(int i=0;i<list.size();i++)
        {  
            
            set.add(list.get(i).getClassNum());
            
            
        }
        System.out.println("总共有："+set.size());
        //每班平均分、人数
        Object[] array=set.toArray();
        
        for(int j=0;j<set.size();j++)
        {   int sum=0;
            int count=0;
            for(int i=0;i<list.size();i++)
            {
              if(list.get(i).getClassNum().equals(array[j])){
                  sum+=list.get(i).getScore();
                  count++;
              }
            }
            System.out.println(array[j]+" 平均分："+(sum/count)+"  人数："+count);
        }
        //排序
        int tmp=0;
        for(int i=0;i<list.size();i++)
        {
            for(int j=i+1;j<list.size();j++)
            {
                if(list.get(i).score>list.get(j).score)
                {
                    tmp=list.get(i).score;
                    list.get(i).score=list.get(j).score;
                    list.get(j).score=tmp;
                }
            }
            
        }
        
        for(int i=0;i<list.size();i++)
        {
            for(int j=i+1;j<list.size();j++)
            {
                if(list.get(i).score==list.get(j).score)
                {
                    if(list.get(i).age>list.get(j).age)
                    {
                        tmp=list.get(i).age;
                        list.get(i).age=list.get(j).age;
                        list.get(j).age=tmp;
                    }
                }
            }
            
        }
        for(int i=0;i<list.size();i++)
        {
            System.out.println(list.get(i).getName()+" "+list.get(i).age+":  "+list.get(i).getScore());
        }
        
    }

}

 ArrayList的用法和数组差不多，只是有很多的API，更方便我们解决问题