新手上路之List,HashMap遍历二(1对多) 笔记总结

Student 类,用于单个学生的信息(1):

public class Student {    // 单个学生    // 姓名    private String name;    // 成绩     private Double score;    // 课程    private String courseEve;    public Student() {        super();        // TODO Auto-generated constructor stub    }    public Student(String name, Double score, String courseEve) {        super();        this.name = name;        this.score = score;        this.courseEve = courseEve;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public Double getScore() {        return score;    }    public void setScore(Double score) {        this.score = score;    }    public String getCourseEve() {        return courseEve;    }    public void setCourseEve(String courseEve) {        this.courseEve = courseEve;    }    @Override    public int hashCode() {        final int prime = 31;        int result = 1;        result = prime * result + ((courseEve == null) ? 0 : courseEve.hashCode());        result = prime * result + ((name == null) ? 0 : name.hashCode());        result = prime * result + ((score == null) ? 0 : score.hashCode());        return result;    }    @Override    public boolean equals(Object obj) {        if (this == obj)            return true;        if (obj == null)            return false;        if (getClass() != obj.getClass())            return false;        Student other = (Student) obj;        if (courseEve == null) {            if (other.courseEve != null)                return false;        } else if (!courseEve.equals(other.courseEve))            return false;        if (name == null) {            if (other.name != null)                return false;        } else if (!name.equals(other.name))            return false;        if (score == null) {            if (other.score != null)                return false;        } else if (!score.equals(other.score))            return false;        return true;    }}

学生列表(set容器添加),Course类(N):

import java.util.HashSet;import java.util.Set;public class Course {    // 学生列表     // 課程名    private String title;    //存储学生类    private Set<Student> students;    // 统计总分数    // 引用类型默认为null,加减 会出异常    private Double total = 0.0;    public Course() {        students = new HashSet<>();    }    public Course(String title, Student stu) {        this();        this.title = title;        this.students.add(stu);        this.total += stu.getScore();    }    public String getTitle() {        return title;    }    public void setTitle(String title) {        this.title = title;    }    public Set<Student> getStudents() {        return students;    }    public void setStudents(Set<Student> students) {        this.students = students;    }    public Double getTotal() {        return total;    }    public void setTotal(Double total) {        this.total = total;    }}

实例代码TestStudentList 类:

import java.util.ArrayList;import java.util.HashMap;import java.util.Iterator;import java.util.List;import java.util.Map;public class TestStudentList {    public static void main(String[] args) {        // 实例一些学生的信息        Student student = new Student("徐凤年", 99.0,"Java");        Student student1 = new Student("徐胭脂", 89.5, "JavaEE");        Student student2 = new Student("张还生", 65.4, "Java");        Student student3 = new Student("徐凤年", 99.0,"c#");        Student student4 = new Student("李观虾", 35.5, ".NET");        Student student5 = new Student("林二", 89.4, "orcal");        // 创建list,并把这些学生信息添加进去        List<Student> list = new ArrayList<>();        list.add(student);        list.add(student1);        list.add(student2);        list.add(student3);        list.add(student4);        list.add(student5);        // 用于存放学生列表,通过课程名,索引        Map<String, Course> map = new HashMap<>();         Iterator<Student> it = list.iterator();        while (it.hasNext()) {            Student stu = it.next();            String course = stu.getCourseEve();            double score = stu.getScore();            Course value = map.get(course);            if (null == value) {                // 不存在, 添加到容器里                //else里的操作交由Course类做了                map.put(course, new Course(course, stu));            } else {                // 存在                value.getStudents().add(stu);                value.setTotal(value.getTotal() + score);                //System.out.println("*****" + value.getTotal());            }        }        Course course = map.get("Java");        // 打印相同课程的平均分        System.out.println(course.getTotal()+ "=======" + course.getStudents().size());        System.out.println(course.getTitle() + "---平均分:--->" + course.getTotal()/course.getStudents().size());        System.out.println("学生列表:");        for (Student student6 : course.getStudents()) {            System.out.println(student6.getCourseEve() + "********" + student6.getName() + "********" + student6.getScore());        }    }}

结果如下:

164.4=======2**************[com.shsxtTest.Student@8b06bf9d, com.shsxtTest.Student@f0dd5981]Java---平均分:--->82.2学生列表:Java********张还生********65.4Java********徐凤年********99.0

心语

常用的容器:    1、ArrayList: 集中统一处理多个对象 ，底层:数组        增: add(E) add(int,E)        遍历: 迭代器 、for+get(索引)  foreach        删:remove(int) remove(E)        改:set(int,E)        查:size() get(int) 2、HashMap： 从一堆中快速定位一个对象,底层:数组+链表       增: put(k,v)       遍历: keySet +get  entrySet+getKey|getValue        删:remove       改: put(k,v)       查: size  get(K)