0926-----homework（3）

/**
  *homework0926
  *@author:kai li
  */
package com.kai.li.homework0926;
import java.util.List;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Map;
import static java.util.stream.Collectors.groupingBy;
/**
  *following class is client 
  */
public class HomeWork0926{
    public static void main(String[] args)throws Exception{
        
        /**
          *question three
          */

        /*create data source*/

    List<Student> students=Arrays.asList(new Student("小明",18,100,"class04"),new Student("小黄",22,70,"class04"),new Student("小真",25,90,"class03"),new Student("小花",30,80,"class02"),new Student("tom",28,66,"class02"),new Student("jerry",24,100,"class02")); 
    
    /*operate*/
    int sumAge=students.stream().mapToInt(Student::getAge).sum();
    int sumScore=students.stream().mapToInt(Student::getScore).sum();
    System.out.println("学生的平均年龄是："+sumAge/students.size()+"，平均分数是："+sumScore/students.size());

    Map<String,List<Student>> studentsByClass=students.stream().collect(groupingBy(Student::getClassNum));
    System.out.println("class average score is fllowing:");

    studentsByClass.keySet().stream()                        //get stream
            .mapToInt(i->(studentsByClass.get(i).stream()        //map to int
                        .mapToInt(Student::getScore)    //map to int from the level 2
                        .sum())                //sum
                        /studentsByClass.get(i).size()) //average
            .forEach(System.out::println);                //iterator and println

    }
}
/**
  *following class for question three
  *create data source class
  *student class
  */
class Student{
    private String name;
    private int age;
    private int score;
    private String classNum;
    Student(String name,int age,int score,String classNum){
        this.name=name;
        this.age=age;
        this.score=score;
        this.classNum=classNum;
    }
    public String getName(){
        return name;
    }
    public int getAge(){
        return age;
    }
    public int getScore(){
        return score;
    }
    public String getClassNum(){
        return classNum;
    }
}