容器分类

对容器中的对象按属性分类，用面向对象的思路看看如何实现。

案例：学生类Student中，包含属性（成绩，班级，姓名）。生成若干学生对象，求出各个班级的平均分。

这个问题若干直接使用循环判断，找到所有的班级种类再按班级找学生，再存储，过程非常复杂。

面向对象的思路：建立一个班级类，用于储存属于这个班级的学生；再用一个Map容器储存（班级名称，班级对象）。

代码：

package com.hlyc.Stream;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;

public class Demo1 {
    public static void main(String args[]){
        List list = new ArrayList();
        generate(list);
        Map<String, ClassRoom> map = new HashMap<String, ClassRoom>(); 
        count(list, map);
        Set<Entry<String, ClassRoom>> entrySet = map.entrySet();
        for(Entry<String, ClassRoom> entry : entrySet)
            System.out.println(entry.getKey() + "班" + "	总分：" 
                    + entry.getValue().getTotal() + "	平均分：" 
                                + entry.getValue().getTotal()/entry.getValue().getList().size());
    }
    public static void generate(List<Student> list){
        list.add(new Student("1", 120, "c"));
        list.add(new Student("2", 130, "x"));
        list.add(new Student("3", 140, "y"));
        list.add(new Student("3", 105, "z"));
        list.add(new Student("2", 107, "a"));
        list.add(new Student("1", 110, "b"));
    }
    public static void count(List<Student> list, Map<String, ClassRoom> map){
        for(Student student : list){
            String cid = student.getCid();
            double grade = student.getGrade();
            if(!map.containsKey(cid))
                map.put(cid, new ClassRoom());
            map.get(cid).setTotal(map.get(cid).getTotal() + grade);
            map.get(cid).getList().add(student);
        }
    }
}
class Student{
    private String cid;
    private double grade;
    private String name;
    public String getCid() {
        return cid;
    }
    public void setCid(String cid) {
        this.cid = cid;
    }
    public double getGrade() {
        return grade;
    }
    public void setGrade(double grade) {
        this.grade = grade;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        try {
            this.name = name;
        } finally {
            // TODO: handle finally clause
        }
    }
    public Student(String cid, double grade, String name) {
        this.cid = cid;
        this.grade = grade;
        this.name = name;
    }
    public Student(){
        
    }
    @Override
    public String toString() {
        return "Student [cid=" + cid + ", grade=" + grade + ", name=" + name + "]";
    }
}
class ClassRoom{
    private String cid;
    private double total;
    List<Student> list;
    public String getCid() {
        return cid;
    }
    public void setCid(String cid) {
        this.cid = cid;
    }
    public double getTotal() {
        return total;
    }
    public void setTotal(double total) {
        this.total = total;
    }
    public List<Student> getList() {
        return list;
    }
    public void setList(List<Student> list) {
        this.list = list;
    }
    public ClassRoom(String cid, double total, List<Student> list) {
        this.cid = cid;
        this.total = total;
        this.list = list;
    }
    public ClassRoom(){
        this.list = new ArrayList();
    }
    @Override
    public String toString() {
        return "ClassRoom [cid=" + cid + ", total=" + total + ", list=" + list + "]";
    }
}

这样就很简单了。不相信的同学可以试试使用循环遍历加判断的方法做。